PATENT ABSTRACT
In a conventional switch circuit capable of bidirectional conductivity, there is the problem that latch-up occurs in a parasitic thyristor included in a transistor having a switching function. Therefore it is an object of the present invention to provide a switch circuit capable of bidirectional conductivity while suppressing the occurrence of latch-up due to a parasitic thyristor. The present invention provides a switch circuit that includes diodes connected in parallel with each of a MOS transistor having the switching function and parasitic diodes present at the source and the drain of the MOS transistor.

PATENT DESCRIPTION
This application is based on an application Ser. No. 2005-379474 filed in Japan, the content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to switch circuits with the ability to control latch-up due to a parasitic element. 
     (2) Description of the Related Art 
     Japanese Patent Application Publication Nos. 2003-224244, 2000-224298, and 2004-350127 each disclose a conventional switch circuit in which bidirectional conductivity is possible. 
     Japanese Patent Application Publication No. 2003-163589 discloses a semiconductor device in which a transistor and a Schottky barrier diode are provided on the same chip. In the Schottky barrier diode, the drain electrode of the transistor is connected to a cathode electrode, and the source electrode of the transistor is connected to an anode electrode. By providing this semiconductor device with a Schottky barrier diode, the occurrence of a minority carrier in a PN junction face between a drain diffusion layer and an N-well region of the transistor is suppressed, so that it is possible to prevent the operation of a parasitic transistor present on the substrate on which the semiconductor device is formed. 
     However, the transistors used to configure the switch circuit disclosed in Japanese Patent Application Publication Nos. 2003-224244, 2000-224298, and 2004-350127 have PN junction faces between each of the diffusion layers that function as the drain or the source, and the well regions in which these diffusion layers are formed, and when a forward bias voltage is applied to these PN junction faces, minority carrier injection occurs. When minority carrier injection occurs, there is a risk that latch-up will occur due to a parasitic thyristor present on the substrate on which the switch circuit is formed. 
     Also, with the semiconductor device disclosed in Japanese Patent Application Publication No. 2003-163589, it is possible to suppress the occurrence of a minority carrier by mitigating the forward bias current that occurs at the drain-side PN junction face, but when the direction of the current between the source and the drain has been reversed, it is not possible to suppress the occurrence of a minority carrier at the source-side PN junction face. 
     In order to address these problems, it is an object of the present invention to provide a switch circuit, a diode, and a transistor, in which bidirectional conductivity is possible, and with the ability to control the occurrence of latch-up due to a parasitic element. 
     SUMMARY OF THE INVENTION 
     In order to attain the above object, the present invention provides a switch circuit comprising first and second input-output terminals; a MOS transistor, whose source is connected to the first input-output terminal, and whose drain is connected to the second input-output terminal; a first rectifying unit, provided between the first input-output terminal and a back gate of the MOS transistor; a second rectifying unit, provided between the second input-output terminal and the back gate of the MOS transistor; and a control unit operable to control an on-off state of the MOS transistor based on a control signal. 
     The first rectifying unit is connected having the same rectifying direction as and in parallel with a parasitic diode that is parasitic between the source and the back gate of the MOS transistor; and the second rectifying unit is connected having the same rectifying direction as and in parallel with a parasitic diode that is parasitic between the drain and the back gate of the MOS transistor. 
     According to the above configuration, the first rectifying unit is connected having the same rectifying direction as and in parallel with a parasitic diode on the source-side of the MOS transistor, and reduces the current that flows to the source-side parasitic diode. Also, the second rectifying unit is connected having the same rectifying direction as and in parallel with a parasitic diode on the drain-side of the MOS transistor. Thus it is possible to reduce the forward bias current that flows to the two parasitic diodes, to suppress the occurrence of latch-up in both a parasitic element that includes the source-side parasitic diode and a parasitic element that includes the drain-side parasitic diode. Accordingly, in a case in which current is allowed to flow from the first input-output terminal to the second input-output terminal, and in a case in which current is allowed to flow from the second input-output terminal to the first input-output terminal, it is possible to attain the excellent effect of being able to suppress the occurrence of latch-up. 
     Also, the MOS transistor is a P-channel-type transistor; the first rectifying unit and the second rectifying unit are respectively provided with an anode terminal and a cathode terminal, the anode terminal receiving input of current and the cathode terminal outputting current; the anode terminal of the first rectifying unit is connected to the first input-output terminal, and the cathode terminal is connected to the back gate of the MOS transistor; and the anode terminal of the second rectifying unit is connected to the second input-output terminal, and the cathode terminal is connected to the back gate of the MOS transistor. 
     The switch circuit further comprises a third rectifying unit, in which the anode terminal is connected to a power source potential, and the cathode terminal is connected to the back gate of the MOS transistor; wherein the control unit acquires an operating potential from the back gate of the MOS transistor. 
     According to this configuration, the control unit acquires an operating potential from the back gate of the MOS transistor, so a separate power source is not required. 
     Also, according to the above configuration, a power source potential is always supplied to the back gate of the MOS transistor via the third rectifying unit. Thus, it is possible for the control unit to always operate normally, even when the potential of the first and second input-output terminals supplied to the back gate of the MOS transistor does not reach the potential necessary for operation of the control unit. 
     The switch circuit further comprises a supplemental transistor that is an N-channel-type MOS transistor, whose source is connected to the first input-output terminal, and whose drain is connected to the second input-output terminal, and whose back gate is connected to a ground potential; wherein the control unit further controls the on-off state of the supplemental transistor in synchronization with the on-off state of the MOS transistor. 
     According to this configuration, the switch circuit is provided with an N-type supplemental transistor whose on-off state is switched in synchronization with the on-off state of the MOS transistor, so it is possible to expand the range of potential that can be transmitted between the input-output terminals. 
     The switch circuit further comprises a first adjusting transistor that is a P-channel-type MOS transistor, whose source or drain is connected to the first input-output terminal, and whose back gate and the other among the source and the drain are connected to the back gate of the MOS transistor, and whose gate is connected to the second input-output terminal; and a second adjusting transistor that is a P-channel-type MOS transistor, whose source or drain is connected to the second input-output terminal, and whose back gate and the other among the source and the drain are connected to the back gate of the MOS transistor, and whose gate is connected to the first input-output terminal. 
     According to this configuration, when the potential of the first input-output terminal is higher than that of the second input-output terminal, the first adjusting transistor enters the on state, and the second adjusting transistor enters the off state. When in the on state, the first adjusting transistor, in parallel with the first rectifying unit, allows current to flow from the first input-output terminal to the back gate of the MOS transistor, and thus it is possible to further reduce the current that flows through the source-side parasitic diode of the MOS transistor. When, conversely, the potential of the second input-output terminal is higher than that of the first input-output terminal, the second adjusting transistor enters the on state, reducing the current that flows through the drain-side parasitic diode of the MOS transistor. 
     Also, when a high potential is applied to the first and second input-output terminals, the potential of the back gate of the MOS transistor also becomes a high potential. Then, when a low potential is applied to the first and second input-output terminals, regardless of the size of the potential of the first and second input-output terminals, the first and second adjusting transistors both enter the on state, and the potential of the back gate of the MOS transistor is reduced to the same potential as the higher potential among the potentials of the first and second input-output terminals. Thus, it is possible to prevent a reduction in current capacity due to a substrate bias effect of the MOS transistor. 
     Also, the MOS transistor used to configure the switch circuit is an N-channel-type transistor; the first rectifying unit and the second rectifying unit respectively comprise an anode terminal that receives input of current; a cathode terminal that outputs current; wherein the anode terminal of the first rectifying unit is connected to the back gate of the MOS transistor, and the cathode terminal of the first rectifying unit is connected to the first input-output terminal; and the anode terminal of the second rectifying unit is connected to the back gate of the MOS transistor, and the cathode terminal of the second rectifying unit is connected to the second input-output terminal. 
     The switch circuit further comprises a third rectifying unit, in which the anode terminal is connected to the back gate of the MOS transistor, and the cathode terminal is connected to a ground potential; wherein the control unit acquires an operating potential from the back gate of the MOS transistor. 
     According to this configuration, the control unit acquires an operating potential that is not greater than a predetermined threshold value from the back gate of the MOS transistor, so a separate power source is not necessary. 
     Also, with the above configuration, due to the third rectifying unit, a potential that is not greater than a ground potential is always maintained at the back gate of the MOS transistor. Thus, it is possible for the control unit to always operate normally, even when the potential of the first and second input-output terminals, which is supplied to the back gate of the MOS transistor via the first and second rectifying units, exceeds the threshold value of the control unit. 
     The switch circuit further comprises a supplemental transistor that is a P-channel-type MOS transistor, whose source is connected to the first input-output terminal, and whose drain is connected to the second input-output terminal, and whose back gate is connected to a power source potential; wherein the control unit further controls the on-off state of the supplemental transistor in synchronization with the on-off state of the MOS transistor. 
     According to this configuration, the switch circuit is provided with a P-type supplemental transistor whose on-off state is switched in synchronization with the on-off state of the MOS transistor, so it is possible to expand the range of potential that can be transmitted between the input-output terminals. 
     Also, the switch circuit further comprises a first adjusting transistor that is an N-channel-type MOS transistor, whose source or drain is connected to the first input-output terminal, and whose back gate and the other among the source and the drain are connected to the back gate of the MOS transistor, and whose gate is connected to the second input-output terminal; and a second adjusting transistor that is an N-channel-type MOS transistor, whose source or drain is connected to the second input-output terminal, and whose back gate and the other among the source and the drain is connected to the back gate of the MOS transistor, and whose gate is connected to the first input-output terminal. 
     According to this configuration, when the potential of the first input-output terminal is lower than that of the second input-output terminal, the first adjusting transistor enters the on state, and the second adjusting transistor enters the off state. When in the on state, the first adjusting transistor, in parallel with the first rectifying unit, allows current to flow from the back gate of the MOS transistor to the first input-output terminal, and thus it is possible to further reduce the current that flows through the source-side parasitic diode of the MOS transistor. Accordingly, it is possible to further reduce the risk of latch-up in a parasitic element that includes a source-side PN junction. 
     When the potential of the second input-output terminal is less than that of the first input-output terminal, conversely, the second adjusting transistor enters the on state, so it is possible to further reduce the current that flows to the drain-side parasitic diode of the MOS transistor. 
     Also, when a low potential is applied to the first and second input-output terminals, the potential of the back gate of the MOS transistor also becomes a low potential. Then, when a high potential is applied to the first and second input-output terminals, regardless of the size of the potential of the first and second input-output terminals, the first and second adjusting transistors both enter the on state, reducing the potential of the back gate of the MOS transistor. Thus, it is possible to prevent a reduction in current capacity due to a substrate bias effect of the MOS transistor. 
     Also, at least one among the first and second rectifying units comprises a first semiconductor region of a second conductivity-type, formed on a semiconductor substrate of a first conductivity-type; a first diffusion layer of the second conductivity-type, formed in the first semiconductor region; a second semiconductor region of the first conductivity-type, formed in the first semiconductor region; a second diffusion layer of the first conductivity-type, formed in the second semiconductor region; a third diffusion layer of the second conductivity-type, formed in the second semiconductor region; a first terminal, connected to the first diffusion layer and the second diffusion layer; and a second terminal, connected to the third diffusion layer. 
     In order for the first and second rectifying units to reduce the current that flows to the parasitic diodes of the MOS transistor, it is necessary to allow more current to flow, and thus it is desirable to adopt a configuration in which latch-up is not caused by the first and second rectifying units themselves. 
     According to this configuration, a parasitic thyristor is present in the diode, but because the first and second diffusion layers are connected, the adjacent first and second semiconductor regions have the same potential. Thus, current does not flow between the first and second semiconductor regions, so latch-up is not caused in the parasitic thyristor even if minority carrier injection occurs at another PN junction face. 
     Also, in the switch circuit, at least one among the first and second rectifying units comprises a first semiconductor region of a second conductivity-type, formed on a semiconductor substrate of a first conductivity-type; a first diffusion layer of the second conductivity-type, formed in the first semiconductor region; a second semiconductor region of the first conductivity-type, formed in the first semiconductor region; a second diffusion layer of the first conductivity-type, formed in the second semiconductor region; an internal MOS transistor of these second conductivity-type, formed in the second semiconductor region; a first terminal, connected to the first and second diffusion layers, either the source or the drain of the internal MOS transistor, and the gate of the internal MOS transistor; and a second terminal, connected to the other among the source and the drain of the internal MOS transistor. 
     In order for the first and second rectifying units to reduce the current that flows to the parasitic diodes of the MOS transistor, it is necessary to allow more current to flow, and thus it is desirable to adopt a configuration in which latch-up is not caused by the first and second rectifying units themselves. 
     According to this configuration, in the first or second rectifying unit, the first and second diffusion layers are connected, so the adjacent first and second semiconductor regions have the same potential, and thus current does not flow between the first and second semiconductor regions. Accordingly, latch-up is not caused in the parasitic thyristor including the first and second semiconductor regions. 
     Further, it is possible for a rectifying unit with the above configuration to reduce minority carrier injection at a PN junction face in the internal MOS transistor, because current is allowed to flow via a channel region formed in the internal MOS transistor. Accordingly, latch-up does not occur even in a parasitic thyristor including a PN junction face in the internal MOS transistor. 
     Also, in the switch circuit, a threshold voltage of the internal MOS transistor is lower than a built-in potential of the parasitic diode. 
     When, as described above, the threshold voltage of the internal MOS transistor is reduced, the current that passes through the channel region is increased in the internal MOS transistor, reducing the current that passes through the parasitic diode between the source or the drain and the back gate of the internal MOS transistor. Accordingly, it is possible to further reduce the risk that latch-up will occur in a parasitic thyristor including a PN junction face in the internal MOS transistor. 
     The switch circuit further comprises a voltage dividing unit that causes a predetermined voltage drop between the source of the MOS transistor and the first input-output terminal, and/or between the drain of the MOS transistor and the second input-output terminal. 
     According to the above configuration, it is possible to reduce the voltage applied to at least one of the parasitic diode present on the source side of the MOS transistor and the parasitic diode present on the drain side. The current that flows to a parasitic diode increases according to the voltage that is applied to the parasitic diode. Accordingly, by reducing the voltage that is applied to the parasitic diode, it is possible to reduce the current that flows to the parasitic diode. 
     The present invention provides a switch circuit comprising first and second input-output terminals; a P-channel-type MOS transistor, whose source is connected to the first input-output terminal, and whose drain is connected to the second input-output terminal; a first rectifying unit whose anode terminal is connected to the first input-output terminal, and whose cathode terminal is connected to the back gate of the P-channel-type MOS transistor, the anode terminal receiving input of current and the cathode terminal outputting current; a second rectifying unit whose anode terminal is connected to the second input-output terminal, and whose cathode terminal is connected to the back gate of the P-channel-type MOS transistor, a third rectifying unit whose anode terminal is connected to a power source potential, and whose cathode terminal is connected to the back gate of the P-channel-type MOS transistor; a first control unit that acquires an operating potential from the back gate of the P-channel-type MOS transistor, and controls the on-off state of the P-channel-type MOS transistor based on a control signal; an N-channel-type MOS transistor whose source is connected to the first input-output terminal, and whose drain is connected to the second input-output terminal; a fourth rectifying unit whose anode terminal is connected to the back gate of the N-channel-type MOS transistor, and whose cathode terminal is connected to the first input-output terminal; a fifth rectifying unit whose anode terminal is connected to the back gate of the N-channel-type MOS transistor, and whose cathode terminal is connected to the second input-output terminal; a sixth rectifying unit whose anode terminal is connected to the back gate of the N-channel-type MOS transistor, and whose cathode terminal is connected to a ground potential; and a second control unit that acquires an operating potential from the back gate of the N-channel-type MOS transistor, and controls the on-off state of the N-channel-type MOS transistor based on the control signal. 
     The structure of the P-channel-type MOS transistor and the N-channel-type MOS transistor used to configure the above switch circuit includes a parasitic diode configured with a PN junction between the back gate and the source and drain formed on the back gate. 
     The first rectifying unit is connected in parallel with the source-side parasitic diode of the P-channel-type MOS transistor, and reduces the current that flows to the source-side parasitic diode. Also, the second rectifying unit is connected in parallel with the drain-side parasitic diode of the P-channel-type MOS transistor, and reduces the current that flows to the drain-side parasitic diode. Thus, it is possible to suppress the occurrence of latch-up in both the parasitic thyristor that includes the source-side parasitic diode, and the parasitic thyristor that includes the drain-side parasitic diode. Accordingly, whether current is allowed to flow from the first input-output terminal to the second input-output terminal, or conversely current is allowed to flow from the second input-output terminal to the first input-output terminal, it is possible to attain the excellent effect of being able to suppress the occurrence of latch-up. 
     Also, the fourth rectifying unit and the fifth rectifying unit respectively reduce the current that flows to the parasitic diode between the source and back gate of the N-channel-type MOS transistor, and the current that flows to the parasitic diode between the drain and the back gate, thus controlling the occurrence of latch-up. 
     Further, the P-channel-type MOS transistor with the above configuration is capable of transmitting a positive potential, and the N-channel-type MOS transistor is capable of transmitting a negative potential. Thus, by connecting the two conductivity-type MOS transistors in parallel, it is possible to transmit a wide range of potential. 
     In addition, because the first control unit acquires an operating potential from the back gate of the P-channel MOS transistor, a separate external power source is not necessary. Also, the back gate of the P-channel-type MOS transistor is connected to a power source potential via the third rectifying unit. It is possible for the first control unit to operate normally even when the potential of the first and second input-output terminals transmitted to the back gate of the P-channel-type MOS transistor via the first and second rectifying units does not reach the potential necessary for operation of the first control unit. 
     Likewise, because the second control unit acquires an operating potential not more than a predetermined threshold value from the back gate of the MOS transistor, a separate external power source is not necessary. Also, due to the sixth rectifying unit, a potential that is not greater than a ground potential is always maintained at the back gate of the MOS transistor. Thus, it is possible for the second control unit to always operate normally, even when the potential of the first and second input-output terminals, which is supplied to the back gate of the MOS transistor via the fourth and fifth rectifying units, exceeds the threshold value of the second control unit. 
     Also, the present invention provides a diode, comprising a first semiconductor region of a second conductivity-type, formed on a semiconductor substrate of a first conductivity-type; a first diffusion layer of the second conductivity-type, formed in the first semiconductor region; a second semiconductor region of the first conductivity-type, formed in the first semiconductor region; a second diffusion layer of the first conductivity-type diffusion layer, formed in the second semiconductor region; a third diffusion layer of the second conductivity-type, formed in the second semiconductor region; a first terminal, connected to the first diffusion layer and the second diffusion layer; and a second terminal, connected to the third diffusion layer. 
     With this configuration, a parasitic thyristor is present in the diode of the present invention, but because the first and second diffusion layers are connected, the adjacent first and second semiconductor regions have the same potential. Thus, current does not flow between the first and second semiconductor regions, so latch-up is not caused in the parasitic thyristor even if minority carrier injection occurs at another PN junction face. 
     Also, the present invention provides a diode comprising a first semiconductor region of a second conductivity-type, formed on a semiconductor substrate of a first conductivity-type; a first diffusion layer of the second conductivity-type, formed in the first semiconductor region; a second semiconductor region of the first conductivity-type, formed in the first semiconductor region; a second diffusion layer of the first conductivity-type, formed in the second semiconductor region; an internal MOS transistor of the second conductivity-type, formed in the second semiconductor region; a first terminal, connected to the first and second diffusion layers, either the source or the drain of the internal MOS transistor, and the gate of the internal MOS transistor; and a second terminal, connected to the other among the source and the drain of the internal MOS transistor. 
     With this configuration, because the first and second diffusion layers are connected, the adjacent first and second semiconductor regions have the same potential, so current does not flow between the first and second semiconductor regions. Accordingly, in the diode of the present invention, latch-up does not occur in the parasitic thyristor including the first and second semiconductor regions. 
     Further, in the diode with the above configuration, because current is allowed to flow via the channel region formed in the internal MOS transistor, minority carrier injection at a PN junction face in the internal MOS transistor is reduced. Accordingly, latch-up is unlikely to occur even in the parasitic thyristor including a PN junction face in the internal MOS transistor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and the other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate a specific embodiment of the invention. 
       In the drawings: 
         FIG. 1  is a circuit diagram that shows the configuration of a switch circuit  1 ; 
         FIG. 2  is a cross-sectional diagram of a substrate on which the switch circuit  1  has been formed; 
         FIG. 3A  is a cross-sectional diagram that shows the cross-sectional structure of a diode  115 ; 
         FIG. 3B  is a circuit diagram that shows a parasitic thyristor  121  that is parasitic on the diode  115 ; 
         FIG. 3C  is a circuit diagram showing a parasitic thyristor  126  that is parasitic on the diode  115 ; 
         FIG. 4  is a cross-sectional diagram that shows the structure of a diode  130 ; 
         FIG. 5  is a circuit diagram that shows the configuration of a switch circuit  1   a;    
         FIG. 6  is a circuit diagram that shows the configuration of a switch circuit  1   b;    
         FIG. 7  is a cross-sectional diagram that shows the cross-sectional structure of a transistor  135 ; 
         FIG. 8  is a circuit diagram that shows the configuration of a switch circuit  1   c;    
         FIG. 9  is a cross-sectional diagram that shows the cross-sectional structure of a transistor  141 ; 
         FIG. 10  is a circuit diagram that shows the configuration of a switch circuit  2 ; 
         FIG. 11  is a cross-sectional diagram that shows the cross-sectional structure of the switch circuit  2 ; 
         FIG. 12  is a circuit diagram that shows the configuration of a switch circuit  2   a;    
         FIG. 13  is a circuit diagram that shows the configuration of a switch circuit  2   b;    
         FIG. 14  is a circuit diagram that shows the configuration of a switch circuit  2   c;    
         FIG. 15  is a circuit diagram that shows the configuration of a switch circuit  3 ; 
         FIG. 16  is a circuit diagram that shows the configuration of a switch circuit  3   a;    
         FIG. 17  is a circuit diagram that shows the configuration of a switch circuit  3   b;    
         FIG. 18  is a circuit diagram that shows the configuration of a switch circuit  3   c;    
         FIG. 19  is a circuit diagram that shows the configuration of a switch circuit  4 ; 
         FIG. 20  is a circuit diagram that shows the configuration of a switch circuit  4   a;    
         FIG. 21  is a circuit diagram that shows the configuration of a switch circuit  4   b;    
         FIG. 22  is a circuit diagram that shows the configuration of a switch circuit  4   c;    
         FIG. 23  is a circuit diagram that shows the configuration of a switch circuit  5 ; 
         FIG. 24  is a circuit diagram that shows the configuration of a switch circuit  5   a;    
         FIG. 25  is a circuit diagram that shows the configuration of a switch circuit  5   b;    
         FIG. 26  is a circuit diagram that shows the configuration of a switch circuit  5   c;    
         FIG. 27  is a circuit diagram that shows the configuration of a switch circuit  6 ; 
         FIG. 28  is a circuit diagram that shows the configuration of a switch circuit  6   a;    
         FIG. 29  is a circuit diagram that shows the configuration of a switch circuit  6   b;    
         FIG. 30  is a circuit diagram that shows the configuration of a switch circuit  6   c;    
         FIG. 31  is a circuit diagram that shows the configuration of a switch circuit  7 ; 
         FIG. 32  is a circuit diagram that shows the configuration of a switch circuit  8 ; 
         FIG. 33  is a circuit diagram that shows the configuration of a switch circuit  8   a;    
         FIG. 34  is a circuit diagram that shows the configuration of a switch circuit  8   b;    
         FIG. 35  is a circuit diagram that shows the configuration of a switch circuit  8   c;    
         FIG. 36  is a circuit diagram that shows the configuration of a switch circuit  9 ; 
         FIG. 37  is a circuit diagram that shows the configuration of a switch circuit  9   a;    
         FIG. 38  is a circuit diagram that shows the configuration of a switch circuit  9   b ; and 
         FIG. 39  is a circuit diagram that shows the configuration of a switch circuit  9   c.    
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     1. Embodiment 1 
     Following is a description of a switch circuit  1  according to Embodiment 1 of the present invention, with reference to the accompanying drawings. 
     1.1 Overview of Switch Circuit  1   
     Switch circuit  1  is provided with a PMOS transistor  113 , and two diodes that are respectively connected in parallel to a PN junction face between the back gate and the drain and the source of the PMOS transistor  113 . Forward bias current that flows into a parasitic diode between the back gate and the drain or the source is mitigated by these diodes, thus preventing latch-up due to a parasitic thyristor present on the substrate on which the switch circuit is formed. 
     In the present specification, in a MOS transistor, a semiconductor layer that faces a gate electrode, sandwiching insulating oxide film, is referred to as a back gate. 
     1.2 Configuration of Switch Circuit  1   
       FIG. 1  is a circuit diagram that shows the configuration of the switch circuit  1 . As shown in  FIG. 1 , the switch circuit  1  is configured from the PMOS transistor  113 , diodes  115  and  116 , a level shift circuit  114 , and input-output terminals  111  and  112 . Either the drain or the source of the PMOS transistor  113  is connected to the input-output terminal  111 , and the other is connected to the input-output terminal  112 . An input terminal of the level shift circuit  114  is connected to an external circuit that outputs a control signal, and an output terminal is connected to a gate electrode of the PMOS transistor  113 . A positive high voltage power terminal of the level shift circuit  114  is connected to the back gate of the PMOS transistor  113 . The anode terminal of the diode  115  is connected to the input-output terminal  111 , and the cathode terminal of the diode  115  is connected to the back gate of the PMOS transistor  113 . The anode terminal of the diode  116  is connected to the input-output terminal  112 , and the cathode terminal of the diode  116  is connected to the back gate of the PMOS transistor  113 . 
     Parasitic diodes  117  and  118  in  FIG. 1  are parasitic diodes included in the PMOS transistor  113 . 
       FIG. 2  shows the cross-sectional structure of a semiconductor substrate on which the switch circuit  1  has been formed. The level shift circuit  114  included in  FIG. 1  is omitted from  FIG. 2 . 
     (1) PMOS Transistor  113   
     As shown in  FIG. 2 , the PMOS transistor  113  is configured from an N-well region  1130  formed on a P-type substrate  1100 , P+ diffusion layers  1132  and  1133  formed in the N-well region  1130 , and a gate electrode  1160 . Although not shown in  FIG. 2 , an insulating oxide film is formed between the gate electrode  1160  and the N-well region  1130 . The P-type substrate  1100  is connected to a ground potential. 
     The PN junction between the P+ diffusion layer  1132  and the N-well region  1130  forms the parasitic diode  117 , and the PN junction between the P+ diffusion layer  1133  and the N-well region  1130  forms the parasitic diode  118 . 
     In the PMOS transistor  113 , when electric potential is transmitted bi-directionally and the potential of the input-output terminal  111  is higher than the potential of the input-output terminal  112 , the P+ diffusion layer  1132  functions as the source, and the P+ diffusion layer  1133  functions as the drain. Conversely, when the potential of the input-output terminal  112  is higher than the potential of the input-output terminal  111 , the P+ diffusion layer  1132  functions as the drain, and the P+ diffusion layer  1133  functions as the source. 
     As shown in  FIGS. 1 and 2 , the P+ diffusion layer  1132  of the PMOS transistor  113  is connected to the input-output terminal  111 , and the P+ diffusion layer  1133  is connected to the input-output terminal  112 . The PMOS transistor  113  is connected to the cathode terminal of the diodes  115  and  116  via N+ diffusion layers  1131  and  1134  formed in the N-well region  1130  (the back gate). Thus, the potential of the N-well region  1130  (the back gate) is the same as the higher potential among the potentials of the input-output terminals  111  and  112 . The gate electrode  1160  is connected to the output terminal of the level shift circuit  114 . 
     In the PMOS transistor  113 , a signal with the same potential as the ground potential, or a signal with the same potential as the back gate of the PMOS transistor  113 , is input from the level shift circuit  114  to the gate electrode. According to the input signal, the PMOS transistor  113  is switched on-off to allow or cut off current flow between the input-output terminal  111  and the input-output terminal  112 . Specifically, the PMOS transistor  113  enters the off state when a signal with the same potential as the back gate is input, and the PMOS transistor  113  enters the on state when a signal with the same potential as the ground potential is input. 
     In the above configuration, even when the potential of the gate electrode of the PMOS transistor  113  is the ground potential (in the present specification, 0V), if the difference in potential between the source or the drain (that is, either of the P+ diffusion layer  1132  and the P+ diffusion layer  1133 ) of the PMOS transistor  113  and the gate electrode is not more than a threshold voltage of the PMOS transistor  113 , the PMOS transistor  113  will not enter the on state. Thus, the potential that the switch circuit  1  can transmit is limited to not less than a value obtained by adding the threshold voltage of the PMOS transistor  113  to the ground potential, and the switch circuit  1  cannot transmit a negative potential. 
     (2) Diodes  115  and  116   
     As shown in  FIG. 2 , the diode  115  is configured from an N-well region  1110  formed on the P-type substrate  1100 , a P-well region  1120  and an N+ diffusion layer- 1111  formed in the N-well region  1110 , and a P+ diffusion layer  1121  and an N+ diffusion layer  1122  formed in the P-well region  1120 . Both the N+ diffusion layer  1111  and the P+ diffusion layer  1121  are connected to the input-output terminal  111 , and the N-well region  1110  and the P-well region  1120  have the same potential as the input-output terminal  111 . In the diode  115 , the N+ diffusion layer  1111  and the P+ diffusion layer  1121  are connected to the anode terminal, and the N+ diffusion layer  1122  is connected to the cathode terminal. 
     The anode terminal of the diode  115  is connected to the input-output terminal  111 , and the cathode terminal is connected to the back gate of the PMOS transistor  113  via the N+ diffusion layer  1131 . When the potential of the input-output terminal  111  is higher than the potential of the back gate of the PMOS transistor  113 , the PMOS transistor  113  is conductive, so the potential of the input-output terminal  111  is transmitted to the back gate of the PMOS transistor  113 . When the potential of the back gate of the PMOS transistor  113  is higher than the potential of the input-output terminal  111 , the PMOS transistor  113  is not conductive, so the transmission of potential between the input-output terminal  111  and the back gate of the PMOS transistor  113  is cut off. 
     As shown in  FIG. 2 , the diode  116  is configured from an N-well region  1140  formed on the P-type substrate  1100 , a P-well region  1150  and an N+ diffusion layer  1141  formed in the N-well region  1140 , and a P+ diffusion layer  1151  and an N+ diffusion layer  1152  formed in the P-well region  1150 . Both the N+ diffusion layer  1141  and the P+ diffusion layer  1151  are connected to the input-output terminal  112 , and the N-well region  1140  and the P-well region  1150  have the same potential as the input-output terminal  112 . 
     The N+ diffusion layer  1141  and the P+ diffusion layer  1151  are connected to the anode terminal of the diode  116 , and the N+ diffusion layer  1152  is connected to the cathode terminal. 
     The anode terminal of the diode  116  is connected to the input-output terminal  112 , and the cathode terminal is connected to the back gate of the PMOS transistor  113  via the N+ diffusion layer  1134 . When the potential of the input-output terminal  112  is higher than the potential of the back gate of the PMOS transistor  113 , the diode  116  is conductive, so the potential of the input-output terminal  112  is transmitted to the back gate of the PMOS transistor  113 . When the potential of the back gate of the PMOS transistor  113  is higher than the potential of the input-output terminal  112 , the diode  116  is not conductive, so the transmission of potential from the input-output terminal  112  to the back gate of the PMOS transistor  113  is cut off. 
     (3) Level Shift Circuit  114   
     The positive high voltage power terminal of the level shift circuit  114  is connected to the back gate of the PMOS transistor  113 , and the output terminal is connected to the gate electrode of the PMOS transistor  113 . 
     The level shift circuit  114  is able to operate normally by receiving supply of an operating potential of not less than an operating threshold value, and obtains the operating potential from the back gate of the PMOS transistor  113  via the positive high voltage power terminal. Here, the operating threshold value is the potential of the power source. 
     The level shift circuit  114  receives an H-level or L-level control signal from an external circuit. In the present specification, H-level is the power source potential, and L-level is the ground potential. 
     When the received control signal NCNT is an H-level signal, the potential of the back gate of the PMOS transistor  113  is output instead of the power source potential. When the control signal NCNT is an L-level signal, the ground potential is output as-is. In the present specification, the potentials input to the electrodes and terminals are relative potentials based on the ground potential, so in the following description 0V is used as the ground potential. 
     1.3 Operation of Switch Circuit  1   
     Following is a specific description of the operation of the switch circuit  1 . Here, the potential of the input-output terminal  111  is 10V, and the potential of the input-output terminal  112  is 3V. 
     When potential is applied to the input-output terminals  111  and  112  respectively, electric current flows in the forward bias direction to the diode  115  and the parasitic diode  117 , so that the potential of the back gate of the PMOS transistor  113  becomes 10V. 
     When the control signal NCNT is an H-level signal, the level shift circuit  114  outputs 10V, which is the potential of the back gate of the PMOS transistor  113 , to the gate electrode of the PMOS transistor  113 . At this time, because the potential difference between the gate electrode and the source (the P+ diffusion layer  1132 ) of the PMOS transistor  113  is zero, the PMOS transistor  113  enters the off state, so that current is cut off between the input-output terminals  111  and  112 . 
     When the control signal NCNT is an L-level signal, the level shift circuit  114  outputs the ground potential (0V) to the gate electrode of the PMOS transistor  113 . At this time, because the potential difference between the gate electrode and the source (the P+ diffusion layer  1132 ) of the PMOS transistor  113  is 10V, the PMOS transistor  113  enters the on state, so that current flows between the input-output terminals  111  and  112 . 
     When the size relationship of the potentials supplied to the input-output terminals  111  and  112  is reversed, the potential of the input-output terminal  112  is transmitted to the back gate of the PMOS transistor  113  via the diode  116  and the parasitic diode  118 , so that the P+ diffusion layer  1133  functions as the source. 
     1.4 Effects 
     As described above, with the switch circuit  1  of the present invention, potential is transmitted from the input-output terminal  111  to the back gate of the PMOS transistor  113  by the diode  115 , which is formed on the same substrate as the PMOS transistor  113 , and the parasitic diode  117 , which is between the N-well region  1130  and the P+ diffusion layer  1131  of the PMOS transistor  113 . That is, a forward bias voltage is applied to the PN junction used to configure the parasitic diode  117 , and forward bias current flows according to the applied voltage. That is, minority carrier injection occurs at the PN junction face of the N-well region  1130  and the P+ diffusion layer  1132 . 
     As an example of a parasitic element present on the substrate on which the switch circuit  1  is formed, a parasitic thyristor is conceivable that is configured from the P+ diffusion layer  1132 , the N-well region  1130 , the P-type substrate  1100  and an N-type diffusion layer other than the N-well region  1130  formed on the P-type substrate  1100 . This parasitic thyristor is equivalent to a circuit in which a bipolar first parasitic transistor, configured from the P+ diffusion layer  1132 , the N-well region  1130 , and the P-type substrate  1100 , is connected to a bipolar second parasitic transistor, configured from the N-well region  1130 , the P-type substrate  1100 , and an N-type diffusion layer other than the N-well region  1130  formed on the P-type substrate  1100 . 
     When minority carrier injection occurs at the PN junction face of the P+ diffusion layer  1132  and the N-well region  1130 , a base current flows to the first parasitic transistor. At this time, if the potential of the N-type diffusion layer other than the N-well region  1130  is sufficiently low, a collector current flows to the first parasitic transistor so that the parasitic transistor enters the on state, that is, there is a risk that latch-up will occur. 
     At this time, the number of minority carriers grows as the current that flows through the PN junction forming the parasitic diode  117  in the forward bias direction increases, increasing the risk that latch-up will occur. 
     Here, the effects of the switch circuit  1  of the present invention will be more specifically described assuming that in the initial state, the potential of all of the input-output terminals  111  and  112  and the back gate of the PMOS transistor  113  is the ground potential (0V), and assuming that the potential of the input-output terminal  111  is 10V and the potential of the input-output terminal  112  is 3V. 
     When supplying a potential of 10V from an external circuit to the input-output terminal  111 , a finite time Ta (Ta&gt;0) is necessary for the potential of the input-output terminal  111  to reach 10V. This can be expressed as Ea (Ta)=10V when the potential of the input-output terminal  111  for an elapsed time t since starting to supply potential to the input-output terminal  111  is expressed as Ea(t). Also, the potential of the back gate of the PMOS transistor  113  for an elapsed time (t) since starting to supply potential to the input-output terminal  111  is expressed as E(t). E(t) increases according to the total amount of the charge supplied to the back gate, and when Ea(t)=E(t)=10V, electric current between the input-output terminal  111  and the back gate is stopped. The total amount of the charge supplied to the back gate is proportional to a value obtained by integrating the current between the input-output terminal  111  and the back gate with the time t. 
     When the elapsed time since starting to supply potential to the input-output terminal  111  is t1 (0=t1=Ta), the forward bias voltage applied to the parasitic diode  117  is Ea(t1)-E(t1), so the forward bias voltage applied to the parasitic diode  117  decreases as the potential of the back gate increases, or in other words, decreases as the amount of the charge supplied to the back gate during the passage of time t1 increases. The forward bias current that flows to the parasitic diode  117  decreases as the applied forward bias voltage decreases. 
     In the switch circuit  1  of the present invention, the parasitic diode  117  and the diode  115  are connected in parallel, and a charge is supplied by both to the back gate, and so the speed with which the charge is supplied is comparatively faster than when the diode  115  is not present. Accordingly, because the potential E(t1) of the back gate at the point in time of elapsed time t1 is higher than when the diode  115  is not present, the forward bias voltage Ea(t1)-E(t1) applied to the parasitic diode is reduced. Thus, the forward bias current that flows into the parasitic diode  117  is also reduced, and therefore it is possible to suppress the occurrence of latch-up in the parasitic thyristor. 
     Conversely, also when the potential of the input-output terminal  112  is higher than the potential of the input-output terminal  111 , because the diode  116  is present, it is possible to achieve suppression of latch-up of the parasitic thyristor, which includes the PN junction face between the P+ diffusion layer  1133  and the N-well region  1130  of the PMOS transistor  113 . 
     By, in this manner, respectively providing the diodes  115  and  116  in parallel with two parasitic thyristors of the PMOS transistor  113 , in the switch circuit  1  of the present invention, it is possible to suppress the occurrence of latch-up in both the transmission of potential from the input-output terminal  111  to the input-output terminal  112 , and the transmission of potential from the input-output terminal  112  to the input-output terminal  111 . 
     Moreover, in the switch circuit  1 , the higher potential among the potentials of the input-output terminals  111  and  112  is transmitted to the PMOS transistor  113 , and the level shift circuit  114  acquires an operating potential from the back gate of the PMOS transistor  113  via the positive high voltage power terminal. Accordingly, due to the presence of the level shift circuit  114 , the switch circuit  1  can allow current to flow or be cut off between the input-output terminal  111  and the input-output terminal  112  without requiring a special external power source. 
     1.5 Properties of Diodes  115  and  116   
     In the above description, the diodes  115  and  116 , as shown in  FIG. 2 , were described as PN junction diodes formed by the PN junction of an N+ diffusion layer and a P-well region, but any desired diode may be used. However, the diodes used here are intended to mitigate the forward bias current that flows to the parasitic diodes  117  and  118 , and thus actively allow a large current to flow. Therefore, it is desirable that the diodes themselves have a configuration that does not cause latch-up. Thus, it is necessary to satisfy one of the following two conditions. 
     Condition 1: latch-up is not caused even if minority carrier injection occurs when forward bias current flows to the PN junction face included in a diode. 
     Condition 2: minority carrier injection does not occur even if forward direction bias current is allowed to flow 
     The diodes  115  and  116  shown in  FIG. 2  satisfy above Condition 1.  FIGS. 5 to 9  shows examples in which transistors are used as the diodes, and these transistors function as diodes that satisfy above Condition 2. A Schottky barrier diode is an example of a diode that satisfies Condition 2. Schottky barrier diodes are well known technology, and therefore are not described here. 
     Following is a description of the configuration and properties of diodes and transistors satisfying Conditions 1 and 2, with reference to the accompanying drawings. 
     (1) Diode Satisfying Condition 1 
     Following is a description of a diode that satisfies above Condition 1. 
     (1-1)  FIG. 3A  shows the cross-sectional structure of the diode  115  described in above Embodiment 1, and  FIGS. 3B and 3C  show an example of a parasitic thyristor present on the substrate on which the diode  115  is formed. 
     The diode  115  shown in  FIG. 3A , as previously described, is configured from the N-well region  1110  formed on the P-type substrate  1100 , the P-well region  1120  and the N+ diffusion layer  1111  formed in the N-well region  1110 , and the P+ diffusion layer  1121  and the N+ diffusion layer  1122  formed in the P-well region  1120 . The N+ diffusion layer  1111  and the P+ diffusion layer  1121  are connected, so the N-well region  1110  and the P-well region  1120  have the same potential. The P-type substrate  1100  is connected to a ground potential (0V). 
     As shown in  FIG. 3B , the diode  115  includes a parasitic thyristor  121 , which is configured from the P-type substrate  1100 , the N-well region  1110 , the P-well region  1120 , and the N+ diffusion layer  1122 . The parasitic thyristor  121  is equivalent to a circuit in which the parasitic transistors  122  and  123  are connected. The parasitic transistor  122  is a PNP bipolar transistor in which the P-type substrate  1100  is used as an emitter, the N-well region  1110  is used as a base, and the P-well region  1120  is used as a collector. The parasitic transistor  123  is an NPN bipolar transistor in which the N+ diffusion layer  1122  is used as an emitter, the P-well region  1120  is used as a base, and the N-well region  1110  is used as a collector. 
     When voltage has been applied in the forward direction between the anode terminal and the cathode terminal of the diode  115 , forward bias voltage is applied to the PN junction between the N+ diffusion layer  1122  and the P-well region  1120 , so minority carrier injection occurs. However, as shown in  FIG. 3A , both the N-well region  1110  and the P-well region  1120  have the same potential as the anode terminal. The P-type substrate  1100  has ground potential (0V), and at this time, is in a state with the bias always reversed between the base and the emitter. That is, base current does not flow to the parasitic transistor  122 , so the parasitic transistor  122 , and therefore also the parasitic thyristor  121 , do not operate. Accordingly, even if minority carrier injection occurs at the PN junction face, latch-up does not occur in the diode  115 . 
     As shown in  FIG. 3C , a parasitic thyristor  126  may be present on the substrate on which the diode  115  is configured, the parasitic thyristor  126  being configured from the P-well region  1120 , the N-well region  1110 , the P-type substrate  1100 , and an N-type diffusion layer other than the N-well region  1110  on the P-well substrate  1100 . 
     The parasitic thyristor  126  is equivalent to a circuit in which the parasitic transistors  127  and  128  are connected. The parasitic transistor  127  is a PNP bipolar transistor configured using the P-well region  1120  as an emitter, using the N-well region  1110  as a base, and using the P-type substrate  1100  is used as a collector. The parasitic transistor  128  is an NPN bipolar transistor configured using the N-well region  1110  as a collector, using the P-type substrate  1100  as a base, and using an N-type diffusion layer other than the N-well region  1110  on the P-type substrate  1100  as an emitter. 
     In this configuration, the N-well region  1110  and the P-well region  1120  have the same potential. That is, there is no difference in potential between the base and the emitter of the parasitic transistor  127 , so base current does not flow between the base and the emitter, and thus the parasitic transistor  127 , and therefore also the parasitic thyristor  126 , do not operate. Accordingly, latch-up is not caused in the diode  115  due being affected by other regions on the P-type substrate  1100  on which the diode  115  is formed. 
     (1-2)  FIG. 4  is a cross-sectional diagram that shows the configuration of a diode  130  that operates similarly to the diode  115  described in (1-1), formed on an N-type substrate. The diode  130  is configured from a P-well region  1270  formed on an N-type substrate  1260 , a P+ diffusion layer  1271  and an N-well region  1280  formed in the P-well region  1270 , and an N+ diffusion layer  1272  and a P+ diffusion layer  1273  formed in the N-well region  1280 . The N-type substrate  1260  is connected to a power source potential. 
     When potential is applied in the forward direction between the anode terminal and the cathode terminal of the diode  130 , forward bias voltage is applied to the PN junction face between the P+ diffusion layer  1273  and the N-well region  1280 , causing minority carrier injection. However, because the N-well region  1280  and the P-well region  1270  have the same potential, latch-up due to the parasitic thyristor is not caused even if minority carrier injection occurs. 
     (2) Diode Satisfying Condition 2 
     Following is a description of a transistor that functions as a diode satisfying above Condition 2. 
     (2-1) As one transistor that functions as a diode satisfying Condition 2, an NMOS transistor is conceivable in which the back gate is connected to a ground potential, and the drain is connected with the gate electrode. In this case the NMOS transistor functions as a diode in which the terminal to which the drain and the gate electrode are connected is used as the anode terminal, and the source is used as the cathode terminal. When the difference in potential between the gate and the source exceeds a threshold voltage of the NMOS transistor, the NMOS transistor becomes conductive. 
       FIG. 5  shows a switch circuit  1   a  provided with transistors  131  and  132  having this configuration, instead of the diodes  115  and  116  of the switch circuit  1  shown in  FIG. 1 . In  FIG. 5 , the parasitic diodes  117  and  118  are omitted. 
     In this configuration, the back gate of the transistor  131  always has ground potential (0V). When the difference in potential between the input-output terminal  111  and the back gate of the PMOS transistor  113  is greater than the threshold voltage of the transistor  131 , an N-type channel layer is formed in the back gate of the transistor  131 , so that it is possible to transmit potential from the input-output terminal  111  to the PMOS transistor  113 . A PN junction face is present between the N+ diffusion layer where the source and the drain of the transistor  131  are formed and the P-well region, but at this time, in the transistor  131 , current is allowed to flow in the channel so minority carrier injection does not occur at the PN junction face. 
     The configuration and function of the transistor  132  is the same as that of the transistor  131 . 
     When the potentials applied to the input-output terminals  111  and  112  are respectively Va and Vb, and the potential of the back gate of the PMOS transistor  113  prior to applying Va and Vb to the input-output terminals  111  and  112  is Vbac, in a state of ordinary usage, either Va&gt;Vbac&gt;Vb or Vb&gt;Vbac&gt;Va is satisfied, so that the transistors  131  and  132  are not in the on state at the same time. Accordingly, the transistors  131  and  132  function as diodes in which minority carrier injection does not occur. 
     (2-2) Also, a PMOS transistor is conceivable in which the drain, the gate electrode, and the back gate are connected. In this case, the PMOS transistor functions as a diode in which the terminal to which the drain, the gate electrode, and the back gate are connected is used as the cathode terminal, and the source is used as the anode terminal. When the difference in potential between the source and the drain exceeds a threshold value, the PMOS transistor enters the on state, and current is allowed to flow from the anode terminal to the cathode terminal. 
       FIG. 6  shows a switch circuit  1   b  provided with transistors  134  and  135  according to this configuration, instead of the diodes  115  and  116  shown in  FIG. 1 . In  FIG. 6 , the parasitic diodes  117  and  118  are omitted.  FIG. 7  is a cross-sectional diagram that shows the configuration of the transistor  135 . 
     The transistor  135  is configured from a P-well region  1320  formed on an N-type substrate  1310 , a P+ diffusion layer  1321  and an N-well region  1330  formed in the P-well region  1320 , and an N+ diffusion layer  1331 , a P+ diffusion layer  1332 , a P+ diffusion layer  1333  formed in the N-well region  1330 , and a gate electrode  1340 . The N-type substrate  1310  is connected to a power source potential. 
     A parasitic thyristor is present on the substrate on which the transistor  135  is formed. Conceivable as examples are a parasitic thyristor configured from the N-well region  1330 , the P-well region  1320 , the N-type substrate  1310 , and a P-type diffusion layer other than the P-well region  1320  on the N-type substrate  1310 , and a parasitic thyristor configured from the P+ diffusion layer  1333 , the N-well region  1330 , the P-well region  1320 , and the N-type substrate  1310 . However, same as in the case of the diode  115  described in (1-1) above, because the adjacent N-well region  1330  and P-well region  1320  have the same potential, these parasitic thyristors do not operate. 
     In the transistor  135 , when the difference in potential between the gate and the source exceeds the threshold voltage of the transistor  135 , a P-type channel region is formed and thus the transistor  135  enters the on state. 
     When potential is transmitted from the anode terminal to the cathode terminal, potential is applied in the forward bias direction to the PN junction face between the N-well region  1330  and the P+ diffusion layer  1333 , but in the transistor  135 , there is substantially no occurrence of minority carrier injection, because current flows more dominantly to the channel region than the PN junction face. 
     The configuration and function of the transistor  134  is the same as the transistor  135 . 
     As also described in above (2-1), in ordinary use, the potential input to the input-output terminals  111  and  112  satisfy either Va&gt;Vbac&gt;Vb or Vb&gt;Vbac&gt;Va, and thus the transistors  134  and  135  do not enter the on state at the same time. 
     Accordingly, the transistors  134  and  135  function as diodes in which there is substantially no occurrence of minority carrier injection. 
     (2-3) Also, an NMOS transistor is conceivable in which the drain, the gate electrode, and the back gate are connected. In this case, the NMOS transistor functions as a diode in which the terminal to which the drain, the gate electrode, and the back gate are connected is used as the anode terminal, and the source is used as the cathode terminal. When the difference in potential between the drain and the source is equal to or greater than a threshold voltage, the NMOS transistor enters the on state. 
       FIG. 8  shows a switch circuit  1   c  provided with transistors  141  and  142  according to this configuration, instead of the diodes  115  and  116  shown in  FIG. 1 .  FIG. 9  shows the cross-sectional structure of the transistor  141 . 
     As shown in  FIG. 9 , the transistor  141  is configured from an N-well region  1370  formed on a P-type substrate  1360 , an N+ diffusion layer  1371  and a P-well region  1380  formed in the N-well region  1370 , and a P+ diffusion layer  1381  and N+ diffusion layers  1382  and  1383  formed in the P-well region  1380 . The P-type substrate  1360  is connected to a ground potential. 
     Same as in the case of above (1-1) and (2-2), because the adjacent P-well region  1380  and N-well region  1370  have the same potential, latch-up does not occur in parasitic thyristors of the transistor  141 . 
     In the transistor  141 , when the difference in potential between the gate and the source exceeds the threshold voltage, an N-type channel region is formed and thus, via the channel region, current flows from the anode terminal to the cathode terminal. When doing so, forward bias voltage is applied to the PN junction face between the P-well region  1380  and the N+ diffusion layer  1383 . However, same as in the case of (2-1) and (2-2) above, in the transistor  141 , there is substantially no occurrence of minority carrier injection, because current flows more dominantly via the channel region than the PN junction face. 
     The configuration and function of the transistor  142  is the same as that of the transistor  141 . Also, same as in the case of (2-2), the transistors  141  and  142  are never in the on state at the same time. 
     (4) As described above, a PN junction diode with the configuration in (1) above does not cause latch-up even if minority carrier injection occurs. 
     Also, a transistor with the configuration in (2) functions as a diode in which there is substantially no occurrence of minority carrier injection. In particular, by setting the threshold voltage of the transistor to less than the built-in potential of a parasitic diode formed by a PN junction face within the transistor, the difference between the current that flows via the PN-junction face and the current that flows through the channel region is enlarged, and thus it is possible to reduce the risk that latch-up will occur. 
     Also, in the diode in (1), and the transistors with the configuration in (2-2) and (2-3), by adopting the same potential for the adjacent P-well region and N-well region, latch-up caused by surrounding circuits on the same substrate does not occur. 
     Also, compared to a Schottky barrier diode, a transistor according to (2) above has the merit that it can be produced using widespread transistor production processes as-is, without including a process of producing a Schottky barrier junction. 
     Further, because minority carrier injection does not occur in a diode that satisfies Condition 2, it is not necessary to be concerned about latch-up caused by the diode itself. Accordingly, there is the benefit that when configuring a switch circuit adopting a plurality of these diodes, there is greater freedom in the cross-sectional structure of the switch circuit. 
     2. Embodiment 2 
     Following is a description of a switch circuit  2  according to Embodiment 2 of the present invention. 
     2.1. Overview of Switch Circuit  2   
     The switch circuit  2  allows current to flow or be cut off between two input-output terminals using an NMOS transistor  153 , and is provided with two diodes that are connected in parallel to a parasitic diode that is parasitic on a PN junction face between a back gate and the drain and the source of the NMOS transistor  153 . Forward bias current that flows into the parasitic diode between the back gate and the drain or the source is mitigated by these diodes, thus preventing latch-up due to a parasitic thyristor present on the substrate on which the switch circuit  2  is formed. 
     2.2 Configuration of Switch Circuit  2   
       FIG. 10  is circuit diagram that shows the configuration of the switch circuit  2 . In  FIG. 10 , the same reference numerals are given to the same elements of the configuration as in the switch circuit  1  in Embodiment 1. 
     As shown in  FIG. 10 , the switch circuit  2  is configured from an NMOS transistor  153 , diodes  155  and  156 , a level shift circuit  154 , and the input-output terminals  111  and  112 . Either the drain or the source of the NMOS transistor  153  is connected to the input-output terminal  111 , and the other is connected to the input-output terminal  112 . An input terminal of the level shift circuit  154  is connected to an external circuit that outputs a control signal CNT, and an output terminal is connected to a gate electrode of the NMOS transistor  153 . A negative high voltage power terminal is connected to the back gate of the NMOS transistor  153 . The anode terminal of the diode  155  is connected to the back gate of the NMOS transistor  153 , and the cathode terminal of the diode  155  is connected to the input-output terminal  111 . The anode terminal of the diode  156  is connected to the back gate of the NMOS transistor  153 , and the cathode terminal of the diode  156  is connected to the input-output terminal  112 . 
     Also, parasitic diodes  157  and  158  are parasitic diodes present in the NMOS transistor  153 . 
       FIG. 11  shows an example cross-section of a substrate on which the switch circuit  2  shown in  FIG. 10  has been formed. In  FIG. 11 , the level shift circuit  154  shown in  FIG. 10  is omitted. 
     (1) NMOS Transistor  153   
     As shown in  FIG. 11 , the NMOS transistor  153  is configured from a P-well region  1430  formed on an N-type substrate  1400 , N+ diffusion layers  1432  and  1433  formed in the P-well region  1430 , and a gate electrode  1443 . Although not shown in  FIG. 11 , an insulating oxide film is formed between the gate electrode  1443  and the P-well region  1430 . The N-type substrate  1400  is connected to a ground potential. 
     Here, a PN junction between the N+ diffusion layer  1432  and the P-well region  1430  is the parasitic diode  157 , and a PN junction between the N+ diffusion layer  1433  and the P-well region  1430  forms the parasitic diode  158 . 
     In the NMOS transistor  153 , when electric potential is transmitted bi-directionally and the potential of the input-output terminal  111  is higher than the potential of the input-output terminal  112 , the N+ diffusion layer  1432  functions as the drain, and the N+ diffusion layer  1433  functions as the source. Conversely, when the potential of the input-output terminal  112  is higher than the potential of the input-output terminal  111 , the N+ diffusion layer  1432  functions as the source, and the N+ diffusion layer  1433  functions as the drain. 
     As shown in  FIGS. 10 and 11 , the N+ diffusion layer  1432  of the NMOS transistor  153  is connected to the input-output terminal  111 , and the N+ diffusion layer  1433  is connected to the input-output terminal  112 . The NMOS transistor  153  is connected to the anode terminal of the diodes  156  and  157  via P+ diffusion layers  1431  and  1434  formed in the P-well region  1430  (the back gate). Thus, the potential of the P-well region  1430  (the back gate) is the same as the lower potential among the potentials of the input-output terminals  111  and  112 . The gate electrode  1443  is connected to the output terminal of the level shift circuit  154 . 
     In the NMOS transistor  153 , a signal with the same potential as the power source potential, or a signal with the same potential as the back gate of the NMOS transistor  153 , is input from the level shift circuit  154  to the gate electrode, and according to the potential of the input signal, the NMOS transistor  113  is switched on-off. 
     Specifically, when a signal with the same potential as the back gate of the NMOS transistor  153  is input, the NMOS transistor  153  enters the off state and the connection between the input-output terminals  111  and  112  is cut off. When a signal with the same potential as the power source is input, an N-type channel is formed in the P-well region  1430  and the NMOS transistor  153  enters the on state, so that current flows between input-output terminals  111  and  112 . 
     In the above configuration, even when the potential of the gate electrode of the NMOS transistor  153  is the power source potential, if the difference in potential between the gate electrode and either the source or the drain (the N+ diffusion layers  1432  and  1433 ) is less than a threshold voltage of the NMOS transistor  153 , the NMOS transistor  153  will not enter the on state. Accordingly, the potential that the switch circuit  2  can transmit is limited to not more than a value obtained by subtracting the threshold voltage of the NMOS transistor  153  from the power source potential. 
     (2) Diodes  155  and  156   
     As shown in  FIG. 11 , the diode  155  is configured from a P-well region  1410  formed on the N-type substrate  1400 , an N-well region  1420  and a P+ diffusion layer  1411  formed in the P-well region  1410 , and an N+ diffusion layer  1421  and a P+ diffusion layer  1422  formed in the N-well region  1420 . In the diode  155 , the P+ diffusion layer  1411  and the N+ diffusion layer  1421  are connected to the cathode terminal, and the P+ diffusion layer  1422  is connected to the anode terminal. 
     The cathode terminal of the diode  155  is connected to the input-output terminal  111 , and the P-well region  1410  and the N-well region  1420  have the same potential as the input-output terminal  111 . The anode terminal is connected to the back gate of the NMOS transistor  153  via the P+ diffusion layer  1431 . When the potential of the input-output terminal  111  is lower than the potential of the back gate of the NMOS transistor  153 , the diode  155  is conductive, so the potential of the back gate of the NMOS transistor  153  is reduced to the potential of the input-output terminal  111 . Conversely, when the potential of the input-output terminal  111  is higher than the potential of the back gate of the NMOS transistor  153 , the diode  155  is not conductive, so the transmission of the potential of the input-output terminal  111  is cut off. 
     As shown in  FIG. 11 , the diode  156  is configured from a P-well region  1440  formed on the N-type substrate  1400 , a P+ diffusion layer  1441  and an N-well region  1450  formed in the P-well region  1440 , and an N+ diffusion layer  1451  and a P+ diffusion layer  1452  formed in the N-well region  1450 . In the diode  156 , the P+ diffusion layer  1441  and the N+ diffusion layer  1451  are connected to the cathode terminal, and the P+ diffusion layer  1452  is connected to the anode terminal. 
     The cathode terminal of the diode  156  is connected to the input-output terminal  112 , and the P-well region  1440  and the N-well region  1450  have the same potential. The anode terminal is connected to the back gate of the NMOS transistor  153  via the P+ diffusion layer  1434 . The diode  156  becomes conductive when the potential of the input-output terminal  112  is lower than the potential of the back gate of the NMOS transistor  153 , so that the potential of the back gate of the NMOS transistor  153  is reduced to the potential of the input-output terminal  112 . Conversely, when the potential of the input-output terminal  112  is greater than the potential of the back gate of the NMOS transistor  153 , the diode  156  becomes non-conductive, so that transmission of the potential of the input-output terminal  112  is cut off. 
     The configuration of the diodes  155  and  156  is the same as that of the diode  130  shown in  FIG. 4 . 
     (3) Level Shift Circuit  154   
     The negative high voltage power terminal of the level shift circuit  154  is connected to the back gate of the NMOS transistor  153 , and the output terminal is connected to the gate electrode the NMOS transistor  153 . 
     The level shift circuit  154  is able to operate normally by receiving supply of an operating potential of not less than a predetermined operating threshold value, and obtains the operating potential from the back gate of the NMOS transistor  153  via the negative high voltage power terminal. Here, the operating threshold value of the level shift circuit  154  is the ground potential (0V). 
     The level shift circuit  154  receives an H-level or L-level control signal CNT from an external circuit. When the received control signal is an H-level signal, the power source potential is output, and when the control signal is an L-level signal, the potential of the back gate of the NMOS transistor  153  is output. 
     2.3 Operation of Switch Circuit  2   
     Following is a specific description of the operation of the switch circuit  2 . Here, it is assumed that the potential of the input-output terminal  111  is −10V, and the potential of the input-output terminal  112  is −3V. 
     When voltage is applied to the input-output terminals  111  and  112  respectively, current flows in the forward bias direction to the diode  155  and the parasitic diode  157 , to reduce the potential of the back gate of the NMOS transistor  153  to −10V. 
     When the control signal is an H-level signal, the level shift circuit  154  outputs the power source potential to the gate electrode of the NMOS transistor  153 . At this time the difference in potential between the gate electrode and the source (the N+ diffusion layer  1432 ) of the NMOS transistor  153  becomes the power source potential—(−10V), so that the NMOS transistor  153  enters the on state and thus current flows between the input-output terminals  111  and  112 . 
     When the control signal is an L-level signal, the level shift circuit  154  outputs −10V, which is the potential of the back gate of the NMOS transistor  153 , to the gate electrode of the NMOS transistor  153 . At this time, the difference in potential between the gate and the source of the NMOS transistor  153  becomes zero, so that the NMOS transistor  153  enters the off state and thus current is cut off between the input-output terminals  111  and  112 . 
     When the size relationship of the potentials supplied to the input-output terminals  111  and  112  is reversed, the potential of the back gate of the NMOS transistor  153  is reduced to the potential of the input-output terminal  112  via the diode  156  and the parasitic diode  158 , so that the N+ diffusion layer  1433  functions as the source. 
     2.4 Effects 
     As described above, in the switch circuit  2  of the present invention, the transmission of potential from the input-output terminal  111  to the back gate of the NMOS transistor  153  is performed by the diode  155 , which is formed on the same substrate as the NMOS transistor  153 , and the parasitic diode  157 , which is between the P-well region  1430  and the N+ diffusion layer  1432  of the NMOS transistor  153 . That is, a forward bias voltage is applied to the PN junction used to configure the parasitic diode  157 , and forward bias current flows according to the applied voltage. When the forward bias current flows, minority carrier injection occurs at the PN junction face. 
     As an example of a parasitic element present on the substrate on which the switch circuit  2  is formed, a parasitic thyristor is conceivable that is configured from the N+ diffusion layer  1432 , the P-well region  1430 , the N-type substrate  1400  and a P-type diffusion layer other than the P-well region  1430  formed on the N-type substrate  1400 . This parasitic thyristor is equivalent to a circuit in which a bipolar first parasitic transistor, configured from the N+ diffusion layer  1432 , the p-well region  1430 , and the N-type substrate  1400 , is connected to a bipolar second parasitic transistor, configured from the P-well region  1430 , the N-type substrate  1400 , and a P-type diffusion layer other than the P-well region  1430  formed on the N-type substrate  1400 . 
     When minority carrier injection occurs at the PN junction face of the N+ diffusion layer  1432  and the P-well region  1430 , a base current flows to the first parasitic transistor, and if the potential of the P-type diffusion layer other than the P-well region  1430  is sufficiently high, a collector current flows to the first parasitic transistor, and thus it is possible that latch-up will occur in the parasitic thyristor. 
     At this time, the number of minority carriers grows as the current that flows through the PN junction forming the parasitic diode  157  increases, and thus the risk that latch-up will occur increases. 
     Here, the effects of the switch circuit  2  of the present invention will be more specifically described, assuming that in the initial state, the potential of all of the input-output terminals  111  and  112  and the back gate of the NMOS transistor  153  is the ground potential (0V). 
     When supplying a potential of −10V from an external circuit to the input-output terminal  111 , a finite time Ta (Ta&gt;0) is necessary for the potential of the input-output terminal  111  to reach −10V. This can be expressed as Ea(Ta)=−10V when the potential of the input-output terminal  111  for an elapsed time t since starting to supply potential to the input-output terminal  111  is expressed as Ea(t). Also, the potential of the back gate of the NMOS transistor  153  for an elapsed time (t) since starting to supply potential to the input-output terminal  111  is expressed as E(t). E(t) decreases according to the total amount of the charge moved from the back gate, and the total amount of the charge moved from the back gate is proportional to a value obtained by integrating the current between the input-output terminal  111  and the back gate with the time t. 
     When the elapsed time since starting to supply potential to the input-output terminal  111  is t1 (0=t1=Ta), the forward bias voltage applied to the parasitic diode  157  is E(t1)−Ea(t1), so the forward bias voltage applied to the parasitic diode  157  decreases as the potential of the back gate decreases, or in other words, decreases as the amount of the charge moved from the back gate during the passage of time t1 increases. The forward bias current that flows to the parasitic diode  157  decreases as the applied forward bias voltage decreases. 
     In the switch circuit  2  of the present invention, the parasitic diode  157  and the diode  155  are connected in parallel, and movement of a charge to the back gate is performed by both, so the speed with which the charge is moved is comparatively faster than when the diode  155  is not present. Accordingly, because the potential E(t1) of the back gate at the point in time of elapsed time t1 is lower than when the diode  155  is not present, the forward bias voltage E(t1)−Ea(t1) applied to the parasitic diode  157  is reduced. Thus, the forward direction current that flows to the parasitic diode  157  is also reduced, and therefore it is possible to suppress the occurrence of latch-up in the parasitic thyristor. 
     Conversely, also when the potential of the input-output terminal  112  is lower than the potential of the input-output terminal  111 , because the diode  156  is present, it is possible to achieve suppression of latch-up of the parasitic thyristor, which includes the PN junction face between the N+ diffusion layer  1433  and the P-well region  1430 . 
     By, in this manner, respectively providing the diodes  155  and  156  in parallel with the parasitic diodes  157  and  158  of the NMOS transistor  153 , in the switch circuit  2  of the present invention, it is possible to suppress the occurrence of latch-up in both the transmission of potential from the input-output terminal  111  to the input-output terminal  112 , and the transmission of potential from the input-output terminal  112  to the input-output terminal  111 . 
     Moreover, in the switch circuit  2 , the lower potential among the potentials of the input-output terminals  111  and  112  is transmitted to the back gate of the NMOS transistor  153 , and the level shift circuit  154  acquires an operating potential from the back gate of the NMOS transistor  153  via the negative high voltage power terminal. Accordingly, the switch circuit  2  can allow current to flow or be cut off between the input-output terminal  111  and the input-output terminal  112  without requiring a special external power source to the level shift circuit  154 . 
     2.5 Modified Example of Embodiment 2 
     In the above description, as shown in  FIG. 11 , the diodes  155  and  156  were described as PN junction diodes formed from the PN junction of a P+ diffusion layer and an N-well region, but diodes configured as desired may be used. However, because the diodes used here are intended to mitigate the current that flows to the parasitic diodes  157  and  158 , they actively allow current to flow therethrough. Therefore, it is desirable that the diodes themselves have a configuration that does not cause latch-up. Thus, same as in the case of Embodiment 1, it is necessary to satisfy one of the following two conditions. 
     Condition 1: latch-up is not caused even if minority carrier injection occurs when forward bias current flows to the PN junction face included in a diode. 
     Condition 2: minority carrier injection does not occur even if forward direction bias current is allowed to flow. 
     The above diodes  155  and  156  satisfy Condition 1. A diode with the configuration shown in  FIG. 3A  described in Embodiment 1 is conceivable as a diode that satisfies Condition 1. 
     With respect to a circuit that satisfies Condition 2, an example is conceivable in which a Schottky barrier diode and a transistor are used as diodes, as described in section 1.5 of Embodiment 1. Schottky barrier diodes are well known technology, and therefore are not described here. 
     Following is a description of a transistor that functions as a diode that satisfies Condition 2. 
     (1) A PMOS transistor in which the back gate is connected to a power source potential and the drain is connected to the gate electrode is conceivable as one transistor that functions as a diode that satisfies Condition 2. This PMOS transistor functions as a diode in which the drain and the gate electrode are connected to the cathode terminal, and the source is connected to the anode terminal. The PMOS transistor enters the on state when the difference in potential between the gate and the source is equal to or greater than a threshold voltage. 
       FIG. 12  shows a switch circuit  2   a  provided with transistors  161  and  162  with this configuration instead of the diodes  155  and  156  of the switch circuit  2  in  FIG. 10 . In  FIG. 12 , the parasitic diodes  157  and  158  are omitted. 
     In this configuration, the back gate of the transistor  161  always has the power source potential. The difference in potential between the drain and the source, that is, the difference in potential between the potential of the back gate of the NMOS transistor  153  and potential applied to the input-output terminal  111  is greater than the threshold voltage of the transistor  161 , a P-type channel layer is formed in the back gate of the transistor  161 , and the transistor  161  enters the on state. Thus, the potential of the back gate of the NMOS transistor  153  can be reduced to the potential of the input-output terminal  111 . A PN junction face is present between the N+ diffusion layer where the source and the drain of the transistor  161  are formed and the P-well region, and at this time, in the transistor  161 , current flows more dominantly to the channel, and thus there is substantially no occurrence of minority carrier injection at the PN junction face. 
     (2)  FIG. 13  shows a switch circuit  2   b  in which transistors  163  and  164  are used instead of the diodes  155  and  156  in the switch circuit  2 . The transistors  163  and  164  have the same configuration as the transistor  135  shown in  FIG. 7 , and function as diodes in which the source is connected to the anode terminal, and the drain, the gate electrode and the back gate are connected to the cathode terminal. This transistor was already described with reference to  FIGS. 6 and 7 , and thus is not described again here.
 
(3)  FIG. 14  shows a switch circuit  2   c  in which transistors  166  and  167  are used instead of the diodes  155  and  156  in the switch circuit  2 . The transistors  166  and  167  have the same configuration as the transistor  141  shown in  FIG. 9 , and function as diodes in which the drain, the gate electrode and the back gate of an NMOS transistor are connected to the anode terminal, and the source of a PMOS transistor is connected to the cathode terminal. This transistor was already described with reference to  FIGS. 8 and 9 , and thus is not described again here.
 
(4) Because latch-up is not caused in the transistors and diodes configured as described above, they are adopted in the switch circuit of the present invention.
 
     3. Embodiment 3 
     Following is a description of a switch circuit  3  according to Embodiment 3 of the present invention, with reference to the accompanying drawings. 
     3.1 Overview of Switch Circuit  3   
     In the switch circuit  3 , a diode is further connected to the switch circuit  1  described in Embodiment 1. In this diode, the anode terminal is connected to a power source potential, and the cathode terminal is connected to the back gate of the PMOS transistor  113 . The potential of the back gate of the PMOS transistor  113  is, at least, not less than the power source potential, and so the level shift circuit can operate normally even when the potential input from an input-output terminal is less than an operating threshold value of the level shift circuit. 
     In the following description, a description of the same portions as in Embodiment 1 is omitted, so that mainly distinguishing portions of the present embodiment are described. Also, in  FIG. 15 , constituent elements that are the same as in Embodiment 1 have the same reference numerals. 
     3.2 Configuration of Switch Circuit  3   
     As shown in  FIG. 15 , the switch circuit  3  is configured from the input-output terminals  111  and  112 , the PMOS transistor  113 , the diodes  115  and  116 , the level shift circuit  114 , and a diode  171 . Although omitted in  FIG. 15 , same as in the switch circuit  1  shown in  FIG. 1 , the PMOS transistor  113  includes the two parasitic diodes  117  and  118 . The constituent elements other than the diode  171  and their connections to each other are the same as in the switch circuit  1  of Embodiment 1, and so a description thereof is omitted here. 
     In the diode  171 , the anode terminal is connected to a power source potential, and the cathode terminal is connected to the back gate of the PMOS transistor  113 . When the potential of the back gate of the PMOS transistor  113  is less than the power source potential, the on state is entered, and thus the power source potential is transmitted to the back gate of the PMOS transistor  113 . A diode configured as desired can be adopted as the diode  171 , but here, the diode  171  has the same configuration as the diode  115  shown in  FIG. 3A . 
     The operation of the switch circuit  3  is the same as that of the switch circuit  1  in Embodiment 1, and so a description thereof is omitted here. 
     3.3 Effects 
     With the above configuration, the potential of the back gate of the PMOS transistor  113  used to configure the switch circuit  3  is the same as the highest potential among the potential of the input-output terminal  111 , the potential of the input-output terminal  112 , and the power source potential. 
     In the level shift circuit  114  used to configure the switch circuit  3 , the positive high voltage power terminal is connected to the back gate of the PMOS transistor  113 , and acquires an operating potential from the back gate. If the acquired operating potential is equal to or greater than an operating threshold value, the switch circuit  3  operates normally. 
     In the switch circuit  3  of the present invention, even when the potential of the input-output terminals  111  and  112  is less than the operating threshold value of the level shift circuit  114 , the power source potential is supplied via the diode  171  and the back gate of the PMOS transistor  113 . Thus, the level shift circuit  114  can always control the on-off state of the PMOS transistor  113  normally. 
     3.4 Modified Example 
     A PN junction diode as shown in  FIGS. 3A to 3C  is used as the diodes  115 ,  116 , and  171  used to configure the above switch circuit  3 , but any desired diode may be used. However, as also stated in Embodiment 1, the diodes  115  and  116  are intended to mitigate the forward bias current that flows to the parasitic diode of the PMOS transistor  113 , and thus it is necessary that they actively allow a large current to flow. Also, when the potential of the input-output terminals  111  and  112  is low, potential is transmitted to the back gate of the PMOS transistor  113  only by the diode  171 . Accordingly, it is desirable to have a configuration in which latch-up is not caused, and more specifically, it is desirable that one of the two conditions stated in Embodiment 1 is satisfied. 
     In the above switch circuit  3 , a PN junction diode with the configuration shown in  FIG. 3A  was adopted as a diode that satisfies Condition 1, but a PN junction diode with the configuration shown in  FIG. 4  may be used instead. 
     Also, as described also in Embodiment 1, a transistor that functions as a diode that satisfies Condition 2 may be used. This transistor was already described in detail with reference to  FIGS. 5 to 9  in Embodiment 1, and so it described here only briefly. 
       FIGS. 16 to 18  show a switch circuit in which a transistor that functions as a diode is adopted instead of each of the diodes  115 ,  116 , and  171 . 
       FIG. 16  shows a switch circuit  3   a  provided with transistors  173 ,  174 , and  175  instead of the three diodes provided in the switch circuit  3 . The transistors  173 ,  174 , and  175  have the same configuration as the transistor  131  shown in  FIG. 5 . These are NMOS transistors in which the back gate is connected to a ground potential, that function as a diode in which the drain is connected to the cathode terminal, and the source and the gate electrode are connected to the anode terminal. 
       FIG. 17  shows a switch circuit  3   b  provided with transistors  176 ,  177 , and  178  instead of the three diodes provided in the switch circuit  3 . The transistors  176 ,  177 , and  178  have the same configuration as the transistor  135  shown in  FIG. 6 , and function as a diode in which the source of a PMOS transistor is connected to the anode terminal, and the drain, the back gate, and the gate electrode are connected to the cathode terminal. 
       FIG. 18  shows a switch circuit  3   c  provided with transistors  178 ,  179 , and  181  instead of the three diodes provided in the switch circuit  3 . The transistors  178 ,  179 , and  181 , same as the transistor  141  shown in  FIG. 8 , function as diodes in which the source of an NMOS transistor is connected to the cathode terminal, and the drain, the back gate, and the gate electrode are connected to the anode terminal. 
     4. Embodiment 4 
     Following is a description of a switch circuit  4  according to Embodiment 4 of the present invention, with reference to the accompanying drawings. 
     4.1 Overview of Switch Circuit  4   
     In the switch circuit  4 , a diode is further connected to the switch circuit  2  described in Embodiment 2. In this diode, the anode terminal is connected to the back gate of the NMOS transistor  153 , and the cathode terminal is connected to a ground potential. The potential of the back gate of the transistor  153  is certainly not less than the ground potential, so the level shift circuit can operate normally even when the potential of the input-output terminals is higher than the operating threshold value of the level shift circuit. In the following description, a description of the same portions as in Embodiment 2 is omitted, so that mainly distinguishing portions of the present embodiment are described. Also, in  FIG. 19 , constituent elements that are the same as in Embodiment 2 have the same reference numerals. 
     4.2 Configuration of Switch Circuit  4   
     As shown in  FIG. 19 , the switch circuit  4  is configured from the input-output terminals  111  and  112 , the NMOS transistor  153 , diodes  155  and  156 , the level shift circuit  154 , and a diode  191 . Although omitted in  FIG. 19 , same as in the switch circuit  2  shown in  FIG. 10 , the NMOS transistor  153  includes the parasitic diodes  157  and  158 . The constituent elements other than the diode  191  and their connections to each other are the same as in the switch circuit  2  of Embodiment 2, and so a description thereof is omitted here. 
     In the diode  191 , the anode terminal is connected to the back gate of the NMOS transistor  153 , and the cathode terminal is connected to a ground potential. When the potential of the back gate of the NMOS transistor  153  is greater than the ground potential, current is allowed to flow in the forward direction, and thus the potential of the back gate of the NMOS transistor  153  is reduced to the ground potential. A diode configured as desired can be adopted as the diode  191 , but as an example, here the diode  191  has the same configuration as the diode  155  shown in  FIG. 11 . 
     The operation of the switch circuit  4  is the same as that of the switch circuit  2  in Embodiment 2, and so a description thereof is omitted here. 
     4.3 Effects 
     With the above configuration, the potential of the back gate of the NMOS transistor  153  used to configure the switch circuit  4  is the same as the lowest potential among the potential of the input-output terminal  111 , the potential of the input-output terminal  112 , and the ground potential. 
     In the level shift circuit  154  used to configure the switch circuit  4 , acquires an operating potential from the back gate of the NMOS transistor  153  via the negative high voltage power terminal. If the acquired operating potential is equal to or less than an operating threshold value (here, the ground potential), the switch circuit  4  operates normally. 
     In the switch circuit  4  of the present invention, even when the potential of the input-output terminals  111  and  112  is greater than the operating threshold value of the level shift circuit  154 , the ground potential is supplied via the diode  191  and the back gate of the NMOS transistor  153 . Thus, the level shift circuit  154  can always control the on-off state of the NMOS transistor  153  normally. 
     4.4 Modified Example 
     In the above switch circuit  4 , the diodes  155 ,  156 , and  191  were described as PN junction diodes as shown in  FIG. 11 , but any desired diode may be used. However, as also stated in Embodiments 1 and 2, these diodes, which are connected in parallel with the parasitic diodes  157  and  158 , are intended to mitigate the forward bias current that flows to the parasitic diodes  157  and  158 , and thus it is necessary that the diodes  155 ,  156 , and  191  actively allow a large current to flow. Also, when the potential of the input-output terminals  111  and  112  is high, potential is transmitted to the back gate of the NMOS transistor  153  only by the diode  191 . Accordingly, it is desirable to have a configuration in which latch-up is not caused, and more specifically, it is desirable that one of the two conditions stated in Embodiment 1 is satisfied. 
     In the above switch circuit  4 , a PN junction diode with the configuration shown in  FIG. 11  that satisfies Condition 1, but a PN junction diode with the configuration shown in  FIG. 3A  may be used instead. 
     Also, as described also in Embodiment 2, a transistor that functions as a diode that satisfies Condition 2 may be used. This transistor was already described in detail with reference to  FIGS. 6 to 9  and  12  to  14  in Embodiments 1 and 2, and so it described here only briefly.  FIGS. 20 to 22  show a switch circuit in which a transistor that functions as a diode is adopted instead of each of the diodes  155 ,  156 , and  191 . 
       FIG. 20  shows a switch circuit  4   a  provided with transistors  196 ,  197 , and  198  instead of the three diodes provided in the switch circuit  4 . The transistors  196 ,  197 , and  198 , same as the transistor  161  described in  FIG. 12 , are PMOS transistors in which the back gate is connected to the power source potential, and function as diodes in which the source is connected to the anode terminal, and the drain and the gate electrode are connected to the cathode terminal. 
       FIG. 21  shows a switch circuit  4   b  provided with transistors  201 ,  202 , and  203  instead of the three diodes provided in the switch circuit  4 . The transistors  201 ,  202 , and  203 , same as the transistor  135  described in  FIG. 7 , function as a diode in which the source of a PMOS transistor is connected to the anode terminal, and the drain, the back gate, and the gate electrode are connected to the cathode terminal. 
       FIG. 22  shows a switch circuit  4   c  provided with transistors  206 ,  207 , and  208  instead of the three diodes provided in the switch circuit  4 . The transistors  206 ,  207 , and  208 , same as the transistor  141  described in  FIG. 8 , function as a diode in which the source of an NMOS transistor is connected to the cathode terminal, and the drain, the back gate, and the gate electrode are connected to the anode terminal. 
     5. Embodiment 5 
     Following is a description of a switch circuit  5  according to Embodiment 5 of the present invention, with reference to the accompanying drawings. 
     5.1 Overview of Switch Circuit  5   
     In the switch circuit  5 , an NMOS transistor and an inverter are further connected to the switch circuit  3  described in Embodiment 3. The NMOS transistor is connected to the PMOS transistor  113  in parallel. When the difference in potential between the input-output terminals  111  and  112  and the ground potential is less than the threshold voltage of the PMOS transistor  113 , the switch circuit  5  allows current to flow between the input-output terminals  111  and  112 , via the NMOS transistor connected in parallel to the PMOS transistor  113 . 
     In the following description, a description of the same portions as in Embodiment 3 is omitted, and mainly distinguishing portions of the present embodiment are described. Also, in  FIG. 23 , constituent elements that are the same as in Embodiment 3 have the same reference numerals. 
     5.2 Configuration of Switch Circuit  5   
     As shown in  FIG. 23 , the switch circuit  5  is configured from the input-output terminals  111  and  112 , the PMOS transistor  113 , the level shift circuit  114 , the diodes  115 ,  116 , and  171 , an NMOS transistor  211 , and an inverter  212 . 
     The input-output terminals  111  and  112 , the PMOS transistor  113 , the level shift circuit  114 , and the diodes  115 ,  116 , and  171  have the same configuration and connections to each other as described in Embodiment 3, and so a description thereof is omitted here. 
     In the NMOS transistor  211 , the back gate is connected to a ground potential, either the source or the drain is connected to the input-output terminal  111 , and the other is connected to the input-output terminal  112 . The gate electrode is connected to the output terminal of the inverter  212 , and an H-level (power source potential) or L-level (ground potential) signal is input from the inverter  212 . When the difference in potential between the gate electrode and the input-output terminal  111  is equal to or greater than the threshold voltage of the NMOS transistor  211 , or when the difference in potential between the gate electrode and the input-output terminal  112  is equal to or greater than the threshold voltage of the NMOS transistor  211 , the NMOS transistor  211  enters the on state. The range of potential that the NMOS transistor  211  can transmit is from the ground potential to a potential obtained by subtracting the threshold voltage from the power source potential. 
     The inverter  212  inverts a control signal input from an external circuit and outputs the inverted control signal to the gate electrode of the NMOS transistor. Specifically, if the control signal is an H-level signal, the inverter  212  outputs an L-level (ground potential) signal, and if the control signal is an L-level signal, the inverter  212  outputs an H-level (power source potential) signal. 
     5.3 Operation of Switch Circuit  5   
     Following is a description of the operation of the switch circuit  5 . Here, the power source potential is expressed as E, the potential of the input-output terminal  111  is expressed as Va, the potential of the input-output terminal  112  is expressed as Vb, the threshold voltage of the PMOS transistor  113  is expressed as Tp, the threshold voltage of the NMOS transistor  211  is expressed as Tn, and the operating threshold value of the level shift circuit  114  is expressed as Ts. Here, it is assumed that the power source potential E is sufficiently large compared to the threshold voltages Tp and Tn, and the operating threshold value Ts. 
     As stated in Embodiment 1, the potential that can be transmitted by the PMOS transistor  113  is limited to a potential not less than a value obtained by adding the threshold voltage of the PMOS transistor  113  to the ground potential (0V), that is, a potential that is not less than Tp. 
     When potential is applied to the input-output terminals  111  and  112  respectively, the potential of the back gate of the PMOS transistor  113  becomes equal to the largest among the potentials Va, Vb, and E. The potential of the back gate is equal to or greater than the operating threshold value Ts, so the level shift circuit  114  operates normally. 
     Here, a case is assumed in which Va&lt;Tp and Vb&lt;Tp. The potential of the back gate of the PMOS transistor  113  is E. 
     When the control signal is an H-level signal, the level shift circuit  114  outputs the potential E of the back gate of the PMOS transistor  113 . At this time, the difference in potential between the gate electrode and the back gate of the PMOS transistor  113  is 0V, so the PMOS transistor  113  enters the off state. 
     The inverter  212  inverts the control signal and outputs an L-level signal to the gate electrode of the NMOS transistor  211 . The back gate of the NMOS transistor  211  is connected to a ground potential, so the NMOS transistor  211  enters the off state. Because both the PMOS transistor  113  and the NMOS transistor  211  are in the off state, the switch circuit  5  cuts off the flow of current between the input-output terminals  111  and  112 . 
     When the control signal is an L-level signal, the level shift circuit  114  outputs the ground potential. At this time, the difference in potential between the gate electrode and the back gate is E, but the difference in potential between the input-output terminal  111  and the gate electrode is Va, and the difference in potential between the input-output terminal  112  and the gate electrode is Vb, and both are less than Tp, so the PMOS transistor  113  remains in the off state. 
     On the other hand, the inverter  212  inverts the L-level control signal and outputs an H-level (power source potential) control signal to the NMOS transistor  211 . The power source potential E is sufficiently large compared to the threshold voltages Tp and Tn, so E−Tn&gt;Tp is realized. Also, because Tp&gt;Va, E−Tn&gt;Va. Accordingly, E−Va&gt;Tn, that is, the difference is potential between the gate electrode and the input-output terminal  111  is equal to or greater than the threshold value of the NMOS transistor  211 , so the NMOS transistor  211  enters the on state. 
     The PMOS transistor  113  is in the off state, but the NMOS transistor  211  is in the on state, so current is allowed to flow between the input-output terminals  111  and  112 . 
     5.4 Effect 
     As described above, the back gate of the PMOS transistor  113  has the same potential as the highest among the potentials of the input-output terminals  111  and  112  and the power source potential. When the control signal is an L-level signal, the level shift circuit  114  outputs the ground potential, so that a difference in potential occurs between the back gate and the gate electrode of the PMOS transistor  113 , and thus the PMOS transistor  113  is expected to enter the on state. However, when both the difference in potential between the input-output terminal  111  and the gate electrode, and the difference in potential between the input-output terminal  112  and the gate electrode, are less than the threshold value of the PMOS transistor  113 , the PMOS transistor  113  remains in the off state. 
     However, the switch circuit  5  of the present embodiment is provided with the NMOS transistor  211 , and when the PMOS transistor  113  is in the off state, the NMOS transistor  211  enters the on state. Accordingly, it is possible to expand the range of potential that can be transmitted by the switch circuit  5  to not less than the ground potential. 
     5.5 Modified Example of Embodiment 5 
     PN junction diodes as shown in  FIG. 3A  are adopted as the diodes  115 ,  116 , and  171  used to configure the above switch circuit  5 . A diode as desired may be adopted instead, but same as in Embodiments 1 to 4, it is desirable to adopt a diode that satisfies one of Conditions 1 and 2 above. The diodes  115 ,  116 , and  171  used to configure the switch circuit  5  satisfy Condition 1. Instead, a PN junction diode with the configuration shown in  FIG. 4  may be used. 
     Also, as described in Embodiments 1 to 4, a transistor may be used that functions as a diode that satisfies Condition 2.  FIGS. 24 to 26  each show an example of a switch circuit provided with a transistor that functions as a diode. The transistors used to configure the switch circuits  5   a  to  5   c  shown in  FIGS. 24 to 26  are as already stated in Embodiment 3, and so a description thereof is omitted here. 
     6. Embodiment 6 
     Following is a description of a switch circuit  6  according to Embodiment 6 of the present invention, with reference to the accompanying drawings. 
     6.1 Switch Circuit  6   
     The switch circuit  6  is configured with a PMOS transistor and an inverter further connected to the switch circuit  4  described in Embodiment 4. The PMOS transistor is connected to the NMOS transistor  153  in parallel. When the difference in potential between the input-output terminals  111  and  112  and the power source potential is less than the threshold voltage of the NMOS transistor  153 , the switch circuit  6  allows current to flow between the input-output terminals  111  and  112 , via the NMOS transistor connected in parallel to the NMOS transistor  153 . 
     In the following description, a description of the same portions as in Embodiment 4 is omitted, and mainly distinguishing portions of the present embodiment are described. Also, in  FIG. 27 , constituent elements that are the same as in Embodiment 4 have the same reference numerals. 
     6.2 Configuration of Switch Circuit  6   
     As shown in  FIG. 27 , the switch circuit  6  is configured from the input-output terminals  111  and  112 , the NMOS transistor  153 , the level shift circuit  154 , the diodes  155 ,  156 , and  191 , a PMOS transistor  231 , and an inverter  232 . 
     The input-output terminals  111  and  112 , the NMOS transistor  153 , the level shift circuit  154 , and the diodes  155 ,  156 , and  191  have the same configuration and connections to each other as described in Embodiment 4. 
     In the PMOS transistor  231 , the back gate is connected to a power source potential, either the source or the gate is connected to the input-output terminal  111 , and the other is connected to the input-output terminal  112 . The gate electrode is connected to the output terminal of the inverter  232 , and an H-level (power source potential) or L-level (ground potential) signal is input from the inverter  232 . When an L-level signal is input to the gate electrode, and the difference in potential between the gate electrode and the input-output terminal  111  or  112  is equal to or greater than the threshold voltage of the PMOS transistor  231 , or when the difference in potential between the gate electrode and the input-output terminal  112  is equal to or greater than the threshold voltage of the PMOS transistor  231 , the PMOS transistor  231  enters the on state. The range of potential that the NMOS transistor  231  can transmit is from the threshold voltage to the power source potential. 
     The inverter  232  inverts a control signal input from an external circuit and outputs the inverted control signal to the gate electrode of the PMOS transistor  231 . Specifically, if the control signal is an H-level signal, the inverter  232  outputs an L-level signal, and if the control signal is an L-level signal, the inverter  232  outputs an H-level signal. 
     6.3 Operation of Switch Circuit  6   
     Following is a description of the operation of the switch circuit  6 . Here, the power source potential is expressed as E, the potential of the input-output terminal  111  is expressed as Va, the potential of the input-output terminal  112  is expressed as Vb, the threshold voltage of the NMOS transistor  153  is expressed as Tn, the threshold voltage of the PMOS transistor  231  is expressed as Tp, and the operating threshold value of the level shift circuit  154  is expressed as Ts ((0V). Here, it is assumed that the power source potential E is sufficiently large compared to the threshold voltages Tp and Tn, and the operating threshold value Ts. 
     As stated in Embodiment 2, the potential that can be transmitted by the NMOS transistor  153  is limited to not more than a potential calculated by subtracting the threshold voltage of the NMOS transistor  153  from the power source potential, that is, a potential that is not more than E−Tn. 
     When potential is applied to the input-output terminals  111  and  112  respectively, the potential of the back gate of the NMOS transistor  153  becomes equal to the lowest among the potential of the input-output terminals  111  and  112  and the ground potential. 
     At this time, the potential of the back gate of the NMOS transistor  153  is equal to or less than Ts, so the level shift circuit  154  operates normally. 
     Here, it is assumed that E−Va&lt;Tn, and E−Vb&lt;Tn. At this time, the potential of the back gate of the NMOS transistor  153  is the ground potential (0V). 
     When the control signal is an L-level signal, the level shift circuit  154  outputs the potential of the back gate of the NMOS transistor  153 . At this time, the difference in potential between the gate electrode and the back gate of the NMOS transistor  153  is 0V, so the NMOS transistor  153  enters the off state. 
     The inverter  232  inverts the control signal and outputs an H-level signal to the gate electrode of the PMOS transistor  231 . An H-level signal is input to the gate electrode, so the PMOS transistor  231  enters the off state. 
     Because both the NMOS transistor  153  and the PMOS transistor  231  are in the off state, the switch circuit  6  cuts off the flow of current between the input-output terminals  111  and  112 . 
     When the control signal is an H-level signal, the level shift circuit  154  outputs the power source potential. At this time, the difference in potential between the back gate and the gate electrode is E−0=E&gt;Tn. However, the difference in potential between the gate electrode and the input-output terminal  111  is E−Va, and the difference in potential between the gate electrode and the input-output terminal  112  is E−Vb, and both are less than Tp, so the NMOS transistor  153  remains in the off state. 
     On the other hand, the inverter  232  inverts the H-level control signal and outputs an L-level control signal to the gate electrode of the PMOS transistor  231 . At this time, the difference in potential between the input-output terminal  111  and the gate electrode of the PMOS transistor  231  becomes Va. Now E−Va&lt;Tn is realized, so E−Tn&lt;Va. Also, E is sufficiently large compared to Tp and Tn, so Tp&lt;E−Tn. Thus, Tp&lt;Va is realized, so the difference in potential between the gate electrode of the PMOS transistor  231  and the input-output terminal  111  is equal to or greater than the threshold value Tp, and thus the PMOS transistor  231  enters the on state. 
     The NMOS transistor  153  is in the off state, but the PMOS transistor  231  is in the on state, so current is allowed to flow between the input-output terminals  111  and  112 . 
     6.4 Effects 
     As described above, the back gate of the NMOS transistor  153  has the same potential as the lowest among the potentials of the input-output terminals  111  and  112  and the ground potential. When the control signal is an H-level signal, the level shift circuit  154  outputs the power source potential, so that a difference in potential occurs between the back gate and the gate electrode of the NMOS transistor  153 , but when both the difference in potential between the input-output terminal  111  and the gate electrode, and the difference in potential between the input-output terminal  112  and the gate electrode, are less than the threshold voltage of the NMOS transistor  153 , the NMOS transistor  153  remains in the off state. 
     However, the switch circuit  6  of the present embodiment is provided with the PMOS transistor  231 , and even when the NMOS transistor  153  is in the off state because the potential applied to the input-output terminals  111  and  112  is high, the PMOS transistor  231  enters the on state. Accordingly, the upper limit of the range of potential that can be transmitted by the switch circuit  6  can be reduced to the power source potential. 
     6.5 Modified Example of Embodiment 6 
     PN junction diodes as shown in  FIG. 11  are adopted as the diodes  155 ,  156 , and  191  used to configure the above switch circuit  6 . A diode as desired may be adopted instead, but same as in Embodiments 1 to 4, it is desirable to adopt a diode that satisfies one of Conditions 1 and 2 above. The diodes  155 ,  156 , and  191  satisfy Condition 1. Instead, a PN junction diode with the configuration shown in  FIG. 3A  may be used. 
     Also, as described in Embodiments 1 to 4, a transistor may be used that functions as a diode that satisfies Condition 2.  FIGS. 28 to 30  each show an example of a switch circuit provided with a transistor that functions as a diode instead of the diodes  155 ,  156 , and  191 . The transistors used to configure the switch circuits  6 A to  6 C shown in  FIGS. 28 to 30  are as already stated in Embodiment 4, and so a description thereof is omitted here. 
     7. Embodiment 7 
     Following is a description of a switch circuit  7  according to Embodiment 7 of the present invention, with reference to the accompanying drawings. 
     7.1 Overview of Switch Circuit  7   
     The switch circuit  7  is configured with the switch circuit  3  described in Embodiment 3 connected in parallel to the switch circuit  4  described in Embodiment 4, and can transmit positive and negative potential. 
     In the following description, a description of the same portions as in Embodiments 3 and 4 is omitted, and mainly distinguishing portions of the present embodiment are described. 
     7.2 Configuration of Switch Circuit  7   
     As shown in  FIG. 31 , the switch circuit  7  is configured from the input-output terminals  111  and  112 , the PMOS transistor  113 , the level shift circuit  114 , the diodes  115 ,  116 , and  171 , the NMOS transistor  153 , the level shift circuit  154 , the diodes  155 ,  156 , and  191 , and an inverter  249 . In  FIG. 31 , constituent elements that are the same as in Embodiments 3 and 4 have the same reference numerals as in  FIGS. 15 and 19 . 
     Within the switch circuit  7 , the portions configured from the input-output terminals  111  and  112 , the PMOS transistor  113 , the level shift circuit  114 , and the diodes  115 ,  116 , and  171  are the same as in the switch circuit  3  in Embodiment 3. 
     Within the switch circuit  7 , in the portion configured from the input-output terminals  111  and  112 , the NMOS transistor  153 , the level shift circuit  154 , the inverter  249 , and the diodes  155 ,  156 , and  191 , the inverter  249  is disposed between the input-output terminal  111  and the level shift circuit  154  of the switch circuit  4  in Embodiment 4. 
     The constituent elements of the switch circuit  7  other than the inverter  249  and their connections to each other were described in Embodiments 3 and 4, so here, a detailed description thereof is omitted. Same as in Embodiments 3 and 4, the PMOS transistor  113  and the NMOS transistor  153  include the parasitic diodes  117 ,  118 ,  157 , and  158  formed by an internal PN junction face, but they are omitted from  FIG. 31 . 
     The input terminal of the inverter  249  is connected to an external circuit that outputs a control signal, and the output terminal is connected to the gate electrode of the NMOS transistor  153 . The inverter  249  inverts the control signal input from the external circuit, and outputs the inverted control signal to the level shift circuit  154 . 
     7.3 Operation of the Switch Circuit  7   
     Following is a description of the operation of the switch circuit  7 . 
     Here, the power source potential is expressed as E, the potential of the input-output terminal  111  is expressed as Va, the potential of the input-output terminal  112  is expressed as Vb, the threshold voltage of the PMOS transistor  113  is expressed as Tp, the threshold voltage of the NMOS transistor  153  is expressed as Tn, and the operating threshold value of the level shift circuit  114  is expressed as Ts. Here, it is assumed that the power source potential E is sufficiently large compared to the threshold voltages Tp and Tn, and the operating threshold value Ts. 
     As stated in Embodiment 1, the range of the potential that can be transmitted by the PMOS transistor  113  is limited to not less than a value calculated by adding the threshold voltage of the PMOS transistor  113  to the ground potential (0V), that is, a potential that is not less than Tp. 
     Also, as stated in Embodiment 2, the potential that can be transmitted by the NMOS transistor  153  is limited to not more than a potential calculated by subtracting the threshold voltage of the NMOS transistor  153  from the power source potential, that is, a potential that is not more than E−Tn. 
     (1) Va=Tp or Vb=Tp 
     The range of potential that can be transmitted by the NMOS transistor  153  is equal to or less than E−Tn, so the NMOS transistor  153  is always in the off state when the potential is outside of this range. When either Va or Vb is equal to or less than E−Tn, the on-off state is switched by the signal input to the gate electrode. 
     Either Va or Vb is equal to or greater than Tp, so the PMOS transistor  113  operates normally, and thus when the control signal input to the switch circuit  7  is an H-level signal, the PMOS transistor  113  enters the on state. When the control signal is an L-level signal, the PMOS transistor  113  enters the off state. 
     The inverter  249  inverts the control signal output from the external circuit, and outputs the inverted control signal to the level shift circuit  154 , so when either Va or Vb is equal to or less than E−Tn, the PMOS transistor  113  and the NMOS transistor  153  are synchronized to switch the on-off state. 
     (2) E−Tn=Va or E−Tn=Vb 
     The range of potential that can be transmitted by the PMOS transistor  113  is not less than Tp, so the PMOS transistor  113  is always in the off state when the potential is outside of this range. When either Va or Vb is equal to or greater than Tp, the on-off state is switched by the signal input to the gate electrode. 
     Either Va or Vb is equal to or less than E−Tn, so the NMOS transistor  153  always operates normally. The inverter  249  inverts the control signal input to the switch circuit  7 , so when the control signal input from the external circuit is an H-level signal, the NMOS transistor  153  enters the off state. When the control signal input from the external circuit is an L-level signal, the NMOS transistor  153  enters the on state. 
     (3) E−Tn=Va=Tp or E−Tn=Vb=Tp 
     At this time, either Va or Vb is included in a portion shared between the range of potential that can be transmitted by the PMOS transistor  113  and the range of potential that can be transmitted by the NMOS transistor  153 , so both the PMOS transistor  113  and the NMOS transistor  153  operate normally. 
     At this time, the inverter  249  inverts the control signal input from the external circuit, so the level shift circuits  114  and  154  are synchronized to switch the on-off state of the PMOS transistor  113  and the NMOS transistor  153 . 
     7.4 Effects 
     As described above, the level shift circuits  114  and  154  used to configure the switch circuit  7  of Embodiment 7 are synchronized to switch the on-off state of the PMOS transistor  113  and the NMOS transistor  153 . When the signal input from the input-output terminals  111  and  112  are equal to or greater than the threshold voltage of the PMOS transistor  113 , the on-off state of the PMOS transistor  113  is switched according to the signal from the level shift circuit  114 . 
     Regardless of the potential input from the input-output terminals  111  and  112 , at least one of the PMOS transistor  113  and the NMOS transistor  153  operates normally. Accordingly, the switch circuit can transmit potential in a wide range, from a positive voltage to a negative voltage. 
     In the above description, the six diodes included in the switch circuit  7  are PN junction diodes as shown in  FIGS. 3 and 4 , but same as in the above Embodiments, they may be replaced with diodes that satisfy Condition 1 or 2. 
     8. Embodiment 8 
     Following is a description of a switch circuit  8  according to Embodiment 8 of the present invention, with reference to the accompanying drawings. 
     8.1 Overview of Switch Circuit  8   
     The switch circuit  8  is configured with two PMOS transistors further connected to the switch circuit  1  described in Embodiment 1. In these PMOS transistors, either the source or the drain is connected to the back gate of the PMOS transistor  113 , which functions to allow or cut off current between the input-output terminals. These PMOS transistors enter the on state as necessary, allowing current to flow in parallel with the diodes  115  and  116 , thus controlling the current that flows to a parasitic diode of the PMOS transistor  113 . 
     In the following description, a description of the same portions as in Embodiment 1 is omitted, and mainly distinguishing portions of the present embodiment are described. 
     8.2 Configuration of Switch Circuit  8   
       FIG. 32  is a circuit diagram that shows the configuration of the switch circuit  8 . In  FIG. 32 , constituent elements that are the same as in the switch circuit  1  in Embodiment 1 have the same reference numerals. 
     As shown in  FIG. 32 , the switch circuit  8  is configured from the PMOS transistor  113 , the diodes  115  and  116 , the level shift circuit  114 , the input-output terminals  111  and  112 , and PMOS transistors  301  and  302 . The PMOS transistor  113 , the diodes  115  and  116 , the level shift circuit  114 , and the input-output terminals  111  and  112  have the same configuration and connections with each other as in Embodiment 1, and so a description thereof is omitted here. Also, the parasitic diodes  117  and  118  described in Embodiment 1 are omitted here. 
     In the PMOS transistor  301 , either the source or the drain is connected to the input-output terminal  111 , and the other is connected to the back gate of the PMOS transistor  113 . Moreover, the gate electrode is connected to the input-output terminal  112 , and the back gate is connected to the back gate of the PMOS transistor  113 . 
     In the PMOS transistor  302 , either the source or the drain is connected to the input-output terminal  112 , and the other is connected to the back gate of the PMOS transistor  113 . Moreover, the gate electrode is connected to the input-output terminal  111 , and the back gate is connected to the back gate of the PMOS transistor  113 . 
     8.3 Operation of Switch Circuit  8   
     The operation of the switch circuit  8  is described divided into the following three cases. For the convenience of description, the potential applied to the input-output terminal  111  is expressed as Va, the potential applied to the input-output terminal  112  is expressed as Vb, and the potential of the back gate of the PMOS transistor  113  is expressed as Vbac. 
     (1) Va&gt;Vb, Vbac&lt;Va, and Vbac&lt;Vb 
     It is assumed that at a point in time before Va and Vb are applied to the input-output terminals  111  and  112  respectively, the potential of the back gate of the PMOS transistor  113  satisfies Vbac&lt;Va and Vbac&lt;Vb. At this time, Va−Vb is not less than the threshold value of the PMOS transistor  301 . 
     When Va and Vb are applied to the input-output terminals  111  and  112  respectively, the potential of the gate electrode of the PMOS transistor  301  becomes Vb. The PMOS transistor  301  enters the on state. In parallel with the diode  115  and the parasitic diode of the PMOS transistor  113 , the PMOS transistor  301  allows current to flow to the back gate of the PMOS transistor  113  from the input-output terminal  111 , and current stops when the potential Vbac of the back gate of the PMOS transistor  113  becomes equal to Va. 
     At this time, the potential of the gate electrode of the PMOS transistor  302  is Va. The potential of the back gate of the PMOS transistor  113 , that is, the potential of the back gate of the PMOS transistor  302 , is always equal to or less than Va, so the PMOS transistor  302  remains in the off state. 
     (2) Va&lt;Vb, Vbac&lt;Va, and Vbac&lt;Vb 
     It is assumed that at a point in time before Va and Vb are applied to the input-output terminals  111  and  112  respectively, the potential of the back gate of the PMOS transistor  113  is less than Va and Vb. At this time, Vb−Va is not less than the threshold value of the PMOS transistor  301 . 
     In this case, opposite to the case in (1) above, the PMOS transistor  302  enters the on state, and allows current to flow to the back gate of the PMOS transistor  113  in parallel with the diode  116  and the parasitic diode of the PMOS transistor  113 , and the PMOS transistor  301  remains in the off state. 
     (3) Va&lt;Vbac and Vb&lt;Vbac 
     A case is assumed in which, at a stage before Va and Vb are applied to the input-output terminals  111  and  112  respectively, the potential Vbac of the back gate of the PMOS transistor  113  is higher than Va and Vb. 
     When Va and Vb are applied to the input-output terminals  111  and  112  respectively, the potential of the gate electrode of the PMOS transistor  301  becomes Vb. The potential of the back gate of the PMOS transistor  301  is Vbac, same as the potential of the back gate of the PMOS transistor  113 , and Vb&lt;Vbac, so the PMOS transistor  301  enters the on state. Because Va&lt;Vbac, current flows from the back gate of the PMOS transistor  113  to the input-output terminal  111 . 
     On the other hand, the potential of the gate electrode of the PMOS transistor  302  becomes Va. The potential of the back gate of the PMOS transistor  302  is Vbac, same as the potential of the back gate of the PMOS transistor  113 , and Va&lt;Vbac, so the PMOS transistor  302  enters the on state. Because Vb&lt;Vbac, current flows from the back gate of the PMOS transistor  113  to the input-output terminal  112 . 
     The PMOS transistors  301  and  302  both enter the on state, allowing current to flow from the back gate of the PMOS transistor  113  to the input-output terminals  111  and  112 , and this is accompanied by a reduction in the potential Vbac of the back gate of the PMOS transistor  113 . 
     If Va&gt;Vb, when Vbac=Va, in the PMOS transistor  302 , the difference in potential between the back gate and the gate electrode becomes 0V, so the PMOS transistor  302  enters the off state. Also, when Vbac=Va, the PMOS transistor  301  remains in the on state, but current stops because the difference in potential between the source and the drain is 0V. 
     If Va&lt;Vb, when Vbac=Vb, the difference in potential between the back gate and the gate electrode of the PMOS transistor  301  becomes 0V, so the PMOS transistor  301  enters the off state. Also, when Vbac=Vb, the PMOS transistor  302  remains in the on state, but current stops because the difference in potential between the source and the drain is 0V. 
     8.4 Effects 
     As described above, in the switch circuit  8 , when the potential of the input-output terminal  111  is greater than that of the input-output terminal  112 , the PMOS transistor  301  enters the on state to allow current to flow from the input-output terminal  111  to the back gate of the PMOS transistor  113 . Accordingly, it is possible to control the forward bias current that flows to the parasitic diode  117  (not shown in  FIG. 32 , but shown in  FIG. 1 ) of the PMOS transistor  113 . On the other hand, the PMOS transistor  302  enters the off state, so current does not flow directly from the input-output terminal  111  to the input-output terminal  112  via the PMOS transistors  301  and  302 . 
     Conversely, when the potential of the input-output terminal  112  is greater than that of the input-output terminal  111 , the PMOS transistor  301  enters the off state, and the PMOS transistor  302  enters the on state to allow current to flow from the input-output terminal  112  to the back gate of the PMOS transistor  113 . Accordingly, it is possible to control the forward bias current that flows to the parasitic diode  118  (not shown in  FIG. 32 , but shown in  FIG. 1 ) of the PMOS transistor  113 . 
     Further, when, after applying a high potential to the input-output terminal  111  or  112 , a low potential is then applied, the PMOS transistors  301  and  302  both enter the on state, and reduce the potential of the back gate of the PMOS transistor  113 . Thus, it is possible to prevent a reduction in current capacity due to a substrate bias effect of the PMOS transistor  113 . 
     8.5 Modified Example of Embodiment 8 
     Same as in the case of Embodiment 1, a diode that satisfies above condition 1 or 2 may be used instead of the diodes  115  and  116  used to configure the switch circuit  8 . 
       FIGS. 33 to 35  show a switch circuit provided with transistors that function as diodes that satisfy Condition 2 instead of the diodes  115  and  116 . These transistors included in the switch circuit were previously described in Embodiment 1, and so a description thereof is omitted here. 
     9. Embodiment 9 
     Following is a description of a switch circuit  9  according to Embodiment 9 of the present invention, with reference to the accompanying drawings. 
     9.1 Overview of Switch Circuit  9   
     The switch circuit  9  is configured with two NMOS transistors further connected to the switch circuit  2  described in Embodiment 2. In these NMOS transistors, either the source or the drain is connected to the back gate of the NMOS transistor  153 , which functions to allow or cut off current between the input-output terminals. These NMOS transistors enter the on state as necessary, allowing current to flow in parallel with the diodes  155  and  156 , thus controlling the current that flows into a parasitic diode of the NMOS transistor  153 . 
     In the following description, a description of the same portions as in Embodiment 2 is omitted, and mainly distinguishing portions of the present embodiment are described. 
     9.2 Configuration of Switch Circuit  9   
       FIG. 36  is a circuit diagram that shows the configuration of the switch circuit  9 . In  FIG. 36 , constituent elements that are the same as in the switch circuit  2  in Embodiment 2 have the same reference numerals. 
     As shown in  FIG. 36 , the switch circuit  9  is configured from the NMOS transistor  153 , the diodes  155  and  156 , the level shift circuit  154 , the input-output terminals  111  and  112 , and NMOS transistors  321  and  322 . The NMOS transistor  153 , the diodes  155  and  156 , the level shift circuit  154 , and the input-output terminals  111  and  112  have the same configuration and connections with each other as in Embodiment 2, and so a description thereof is omitted here. 
     In the NMOS transistor  321 , either the source or the drain is connected to the input-output terminal  111 , and the other is connected to the back gate of the NMOS transistor  153 . Moreover, the gate electrode is connected to the input-output terminal  112 , and the back gate is connected to the back gate of the NMOS transistor  153 . 
     In the NMOS transistor  322 , either the source or the drain is connected to the input-output terminal  112 , and the other is connected to the back gate of the NMOS transistor  153 . Moreover, the gate electrode is connected to the input-output terminal  111 , and the back gate is connected to the back gate of the NMOS transistor  153 . 
     9.3 Operation of Switch Circuit  9   
     The operation of the switch circuit  9  is described divided into the following three cases. For the convenience of description, the potential applied to the input-output terminal  111  is expressed as Va, the potential applied to the input-output terminal  112  is expressed as Vb, and the potential of the back gate of the NMOS transistor  153  is expressed as Vbac. 
     (1) Va&lt;Vb, Vbac&gt;Va, and Vbac&gt;Vb 
     It is assumed that Va&lt;Vb, and at a point in time before Va and Vb are applied to the input-output terminals  111  and  112  respectively, the potential Vbac of the back gate of the NMOS transistor  153  satisfies Vbac&gt;Va and Vbac&gt;Vb. At this time, Vb−Va is not less than the threshold value of the NMOS transistor  321 . 
     When Va and Vb are applied to the input-output terminals  111  and  112  respectively, the potential of the gate electrode of the NMOS transistor  321  becomes Vb, and the NMOS transistor  321  enters the on state. In parallel with the diode  155  and the parasitic diode of the NMOS transistor  153 , the NMOS transistor  321  allows current to flow the back gate of the NMOS transistor  153  from the input-output terminal  111 . Current stops when the potential of the back gate of the NMOS transistor  153  becomes equal to Va. 
     On the other hand, the potential of the gate electrode of the NMOS transistor  322  is Va. The potential of the back gate of the NMOS transistor  153 , that is, the potential of the back gate of the NMOS transistor  322 , is always equal to or greater than Va, so the NMOS transistor  322  remains in the off state. 
     (2) Va&gt;Vb, Vbac&gt;Va, and Vbac&gt;Vb 
     A case is assumed in which Va&gt;Vb, and at a point in time before Va and Vb are applied to the input-output terminals  111  and  112  respectively, the potential Vbac of the back gate of the NMOS transistor  153  is greater than Va and Vb. Va−Vb is assumed to be greater than the threshold value of the NMOS transistors  321  and  322 . 
     In this case, opposite to the case in (1) above, the NMOS transistor  322  enters the on state, and allows current to flow from the back gate of the NMOS transistor  153  to the input-output terminal  112 , in parallel with the diode  156  and the parasitic diode of the NMOS transistor  153 , so the NMOS transistor  321  remains in the off state. 
     (3) Va&gt;Vbac and Vb&gt;Vbac 
     A case is assumed in which, at a stage before Va and Vb are applied to the input-output terminals  111  and  112  respectively, the potential Vbac of the back gate of the NMOS transistor  153  is lower than Va and Vb. 
     When Va and Vb are applied to the input-output terminals  111  and  112  respectively, the potential of the gate electrode of the NMOS transistor  321  becomes Vb. At this time, the potential Vbac of the back gate of the NMOS transistor  321  is less than Vb, so the NMOS transistor  321  enters the on state, and allows current to flow from the input-output terminal  111  to the back gate of the NMOS transistor  153 . 
     On the other hand, the potential of the gate electrode of the NMOS transistor  322  becomes Va, and the potential Vbac of the back gate is greater than Va, so the NMOS transistor  322  enters the on state, and allows current to flow from the input-output terminal  112  to the back gate of the NMOS transistor  153 . 
     The NMOS transistors  321  and  322  both enter the on state, allowing current to flow from the input-output terminals  111  and  112  to the back gate of the NMOS transistor  153 , and this is accompanied by an increase in the potential Vbac of the back gate of the NMOS transistor  153 . 
     If Va&lt;Vb, when Vbac=Va, in the NMOS transistor  322 , the difference in potential between the back gate and the gate electrode becomes 0V, so the NMOS transistor  322  enters the off state. Also, when Vbac=Va, the NMOS transistor  321  remains in the on state, but current stops because the difference in potential between the source and the drain is 0V. 
     If Va&gt;Vb, when Vbac=Vb, the potential of the gate electrode and the back gate of the NMOS transistor  321  becomes the same, so the NMOS transistor  321  enters the off state. Also, when Vbac=Vb, in the NMOS transistor  322 , the difference in potential between the source and the drain is 0V, so current stops. 
     9.4 Effects 
     As described above, in the switch circuit  9 , when the potential of the input-output terminal  111  is less than that of the input-output terminal  112 , the NMOS transistor  321  enters the on state, allowing current to flow from the back gate of the NMOS transistor  153  to the input-output terminal  111 . Accordingly, it is possible to control the forward bias current allowed to flow to the parasitic diode  157  (shown in  FIG. 2 ) of the NMOS transistor  153 . On the other hand, the NMOS transistor  322  enters the off state, so current is not allowed to flow directly from the input-output terminal  111  to the input-output terminal  112  via the NMOS transistors  321  and  322 . 
     Conversely, when the potential of the input-output terminal  112  is less than that of the input-output terminal  111 , the NMOS transistor  321  enters the off state, and the NMOS transistor  322  enters the on state, allowing current to flow from the back gate of the NMOS transistor  153  to the input-output terminal  112 . Accordingly, it is possible to control the forward bias current allowed to flow to the parasitic diode  158  (shown in  FIG. 2 ) of the NMOS transistor  153 . 
     Further, when, after applying a low potential to the input-output terminal  111  or  112 , a high potential is then applied, the NMOS transistors  321  and  322  both enter the on state, and reduce the potential of the back gate of the NMOS transistor  153 . Thus, it is possible to prevent a reduction in current capacity due to a substrate bias effect of the PMOS transistor  153 . 
     9.5 Modified Example of Embodiment 9 
     Same as in the case of Embodiment 2, a diode that satisfies above Condition 1 or 2 may be used instead of the diodes  155  and  156  used to configure the switch circuit  9 . 
       FIGS. 37 to 39  show a switch circuit provided with transistors that function as diodes that satisfy Condition 2 instead of the diodes  155  and  156 . These transistors used to configure the switch circuit were previously described in Embodiment 1, and so a description thereof is omitted here. 
     10. Other Modified Examples 
     Above, the present invention was described with reference to Embodiments 1 to 9, but the present invention is not limited to these embodiments. The present invention also includes the examples described below. 
     (1) In Embodiment 1, one diode  115  is connected in parallel with the parasitic diode  117 , but the present invention is not limited to this configuration. A plurality of diodes may be connected in parallel with the parasitic diode  117 . The parasitic diode  118  may likewise be connected in parallel with a plurality of diodes, not only the diode  116 . 
     The same is true with respect to Embodiments 2 to 7; a plurality of diodes may be connected in parallel with a parasitic diode. 
     (2) Also, any of the PN junction diodes described with reference to  FIG. 3  or  4 , a Schottky barrier diode, and the transistors described with reference to  FIGS. 5 to 9 , or a combination of these, may be used for the plurality of diodes connected in parallel in Modified Example (1). 
     (3) In Embodiment 1, an element that causes a voltage drop may be connected to the parasitic diode  117  in series. As examples, a resistor or a MOS transistor are conceivable. By connecting such an element, it is possible to reduce the voltage applied to the parasitic diode  117 . This is also true with respect to the parasitic diode  118 . 
     Also, the same is true with respect to Embodiments 2 to 9; a resistor or the like may be connected to a parasitic diode in series. 
     (4) Also, in the present invention, the above Embodiments 1 to 9 and modified examples may be variously combined. 
     Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.