Abstract:
A protection circuit that includes a first power supply system including a first power supply and a first ground, a second power supply system including a second power supply and a second ground, the second power supply system being connected to the first power supply system via a signal line through which signal transfer is performed between a circuit in the first power supply system and a circuit in the second power supply system, and a control circuit that, when coupled to an electro-static discharge (ESD) stress being applied to the first power supply system controls a switch, the switch being provided between the signal line and the first power supply.

Description:
RELATED APPLICATIONS 
     The present application is a Continuation Application of U.S. patent application Ser. No. 11/656,447 which was filed on Jan. 23, 2007 now U.S. Pat. No. 7,869,174. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a semiconductor device. More specifically, the invention relates to a device suitable for being applied to a semiconductor integrated circuit device including a plurality of power supply systems. 
     BACKGROUND OF THE INVENTION 
     In recent years, semiconductor devices have been adapted to afford multiple functions. Thus, there is a case where a plurality of power supply systems are arranged in one semiconductor device, and for each of the power supply systems, one or a plurality of circuits are arranged. 
     In a semiconductor device where an analog circuit and a digital circuit are mixed, there is a case where respective powers are supplied independently to the digital circuit and the analog circuit (in other words, a power supply system is divided), in order to prevent noise in the digital circuit from being transferred to the analog circuit. 
     In a transmitting/receiving portion in which a transfer of an input/output signal is performed through a signal line between circuits driven by different power supply systems, respectively, it is necessary to prevent a breakdown when an ESD (Electro-Static Discharge) stress is applied. 
       FIG. 25  is a diagram showing a configuration of a conventional semiconductor device in which, a plurality of different power supply systems are provided and the transfer of an input/output signal is performed through a signal line between circuits driven by the power supply systems, respectively (refer to Patent Document 1). 
     Referring to  FIG. 25 , a first circuit connected to a first power supply system comprises an analog section including an analog circuit ( 21 ), an output circuit ( 23 ), and an input protection circuit ( 25 ), for example. A second circuit connected to a second power supply system comprises a digital section including a digital circuit ( 22 ) and an input circuit ( 24 ). Both of the analog and digital sections are connected through a signal line (S 11 ). To the analog section, power is supplied from a high potential side power supply terminal (Vdd 1 ) and a low potential side power supply terminal (Vss 1 ). To the digital section, power is supplied from a high potential side power supply terminal (Vdd 2 ) and a low potential side power supply terminal (Vss 2 ). The low potential side power supply terminal (Vss 1 ) and the low potential side power supply terminal (Vss 2 ) are connected via a protection element (HK 1 ). 
     When the low-potential side power supply terminal (Vss 2 ) is grounded and the ESD stress is applied from the high potential side power supply terminal (Vdd 1 ) in this device, for example, a potential on a signal line (S 11 ) increases through a PMOS transistor constituting the output circuit ( 23 ) because the PMOS transistor is in an unstable state. Since a source of an NMOS transistor in the input circuit ( 24 ) is grounded, a potential difference Vgs is generated between the source of the NMOS transistor and a gate of the NMOS transistor. 
     Since the gate-to-source voltage Vgs is the potential difference that is generated by the ESD stress application, this voltage may exceed a breakdown voltage for a gate oxide film of the NMOS transistor in the input circuit ( 24 ). Accordingly, this voltage may cause breakdown of the gate oxide film of the NMOS transistor in the input circuit ( 24 ). 
     The above description was directed to an operation when the low potential side power supply terminal (Vss 2 ) is grounded, and the ESD stress is applied from the high potential side power supply terminal (Vdd 1 ). When the high potential side power supply terminal (Vdd 2 ) is grounded, and the ESD stress is applied from the high potential side power supply terminal (Vdd 1 ) as well, a similar operation may cause the breakdown of the gate oxide film of the PMOS transistor in the input circuit ( 24 ). 
     As a measure for reducing such a damage, there is a method of inserting a protection element such as an NMOS transistor (HK 3 ), which prevents breakdowns of gate oxide films of NMOS and PMOS transistors that constitute an input circuit ( 54 ), as shown in  FIG. 26  (refer to Patent Document 1). 
     The NMOS transistor (HK 3 ) is in an OFF state during a normal operation, and does not affect transmission of a signal between an output circuit ( 53 ) and the input circuit ( 54 ). 
     When the low potential side power supply terminal (Vss 2 ) is grounded and the ESD stress is applied from the high potential side power supply terminal (Vdd 1 ) in this device, for example, a potential on the signal line (S 11 ) increases through the PMOS transistor that constitutes the output circuit ( 53 ). When the potential exceeds a predetermined potential difference or more, the NMOS transistor (HK 3 ) is turned on, and the signal line (S 11 ) thereby has substantially the same potential as the low potential side power supply terminal (Vss 2 ). Thus, the breakdown of the gate oxide film caused by an excessive increase in a gate potential of the NMOS transistor in the input circuit ( 54 ) can be prevented. 
     [Patent Document 1]
     JP Patent Kokai Publication No. JP-A-9-172146 (FIGS. 23 and 24, and the like)   

     SUMMARY OF THE DISCLOSURE 
     In a configuration shown in  FIG. 26 , as a current that flows through the NMOS transistor (HK 3 ), a current that exceeds discharging capability of the NMOS transistor (HK 3 ) may flow into the NMOS transistor (HK 3 ), depending on a circuit condition such as a gate width of the PMOS transistor in the output circuit ( 53 ), which may cause a damage to the NMOS transistor (HK 3 ) itself. For this reason, in order to obtain a stable ESD withstand voltage, it is necessary to arrange the NMOS transistor (HK 3 ) with a gate width that does not cause the breakdown against the current flown from the PMOS transistor in the output circuit into the NMOS transistor (HK 3 ). 
     The current flown from the PMOS transistor of the output circuit ( 53 ) changes according to the size of the PMOS transistor. In leading-edge LSIs, finer dimension technologies, high-speed operation, and lower voltage operation have been achieved. A parasitic capacitance element of the NMOS transistor (HK 3 ) affects response of the high-speed operation. For this reason, it is difficult to indiscriminately increase the size of the NMOS transistor (HK 3 ) according to the size of the PMOS transistor in the output circuit ( 53 ). 
     The above described problem is solved by the invention schematically configured as follows. 
     A semiconductor integrated circuit device according to one aspect of the present invention, comprises: a plurality of power supply systems; a signal line connecting a circuit in one power supply system and a circuit in the other power supply system; and a circuit that restrains a current flowing from the circuit in said one power supply system into said signal line when an abnormal voltage is applied to said one power supply system. 
     The semiconductor integrated circuit according to the present invention, comprises a circuit that restrains the current flowing from one transistor in the one power supply system into other transistor in the other power system, the one transistor outputting a signal to the signal line, the other transistor receiving the signal through the signal line. The semiconductor integrated circuit device according to the present invention may include a circuit that restrains the current flowing into the other transistor in the other power supply system when the abnormal voltage is applied to the other power supply system, for the other transistor in the other power supply system. 
     A semiconductor integrated circuit device according to another aspect of the present invention includes: 
     an output circuit with power thereof supplied from one power supply system; 
     an input circuit with power thereof supplied from other power supply system different from the one power supply system, signal transfer being performed between the output circuit and the input circuit through a signal line; and 
     a circuit that restrains a current flowing into the signal line when ESD (Electro-Static discharge) stress is applied. 
     The semiconductor integrated circuit device in the present invention includes: 
     a transistor with a current thereof being adjustably controlled according to a signal supplied to a control terminal thereof, said transistor being disposed at least one of between said output circuit and a high potential side power supply terminal in said one power supply system and between said output circuit and a low potential side power supply terminal in said one power supply system; and 
     a control circuit that sets said transistor in an ON state at a time of a normal operation, and that changes a signal level at the control terminal of said transistor to limit the current that flows into said signal line, when the ESD stress is applied. 
     The semiconductor integrated circuit device in the present invention includes: 
     a transistor with a current thereof being adjustably controlled according to a signal supplied to a control terminal thereof, the transistor being disposed at least one of between the input circuit and a high potential side power supply terminal in the other power supply system and between the input circuit and a low potential side power supply terminal in the other power supply system; and 
     a control circuit that sets the transistor in an ON state at a time of a normal operation and changes a signal level at the control terminal of the transistor when the ESD stress is applied. 
     In the present invention, at least two cascode connected transistors are arranged at least one of between the signal line and the high potential side power supply terminal and between the signal line and the low potential side power supply terminal. 
     In the present invention, the control circuit includes a series circuit comprising a capacitance element and a resistance, disposed between the high potential side power supply terminal and the low potential side power supply terminal, and a connecting point between the capacitance element and the resistance is connected to the control terminal of the transistor. Alternatively, in the present invention, the control circuit includes a series circuit constituted from a diode and a resistance, disposed between the high potential side power supply terminal and the low potential side power supply terminal, and a connecting point between the diode and the resistance is connected to the control terminal of the transistor. 
     In the present invention, one of diffusion layers of the transistor and a tap that gives a potential to a well with the diffusion layer formed therein may be arranged in contact with each other, the transistor being connected at least one of between the output circuit (or the input circuit) and the high potential side power supply terminal and between the output circuit (or the input circuit) and the low potential side power supply terminal, the tap being of a conductive type opposite to a conductive type of the diffusion layers of the transistor. 
     In the present invention, the output circuit may include: a first transistor between the signal line and a high potential side power supply terminal in the one power supply system and a second transistor between the signal line and a low potential side power supply terminal in the one power supply system, an output of the output circuit being connected to the signal line; and 
     a control circuit that generates a signal to be supplied to control terminals of the first and second transistors so that the first and second transistors are complementarily on/off controlled in response to an input signal received by the output circuit for being output to the signal line at a time of a normal operation, and adjustably controls a level at a control terminal of at least one of the first transistor and the second transistor at a time of the application of the ESD stress to the one power supply system, thereby limiting a current flowing from the output circuit to the signal line by the application of the ESD stress. 
     In the present invention, the control circuit may include: 
     a series circuit comprising a capacitance element and a resistance, disposed between the high potential side power supply terminal in the one power supply system and the low potential side power supply terminal in the one power supply system; and 
     a logic circuit that generates a signal based on a potential at a connecting point between the capacitance element and the resistance and the input signal, the logic circuit generating the signal that set the first transistor in an ON state and sets the second transistor in an OFF state when the potential at the connecting point is of a level in which the application of the ESD stress is not detected and when the input signal is of a first value, and generating the signal that sets the first transistor in an OFF state and sets the second transistor in an ON state when the potential at the connecting point is of a level in which the application of the ESD stress is not detected and when the input signal is of a second value, the logic circuit outputting to the control terminals of the first and second transistors the signal that sets at least one of the first transistor and the second transistor in an OFF state when the potential at the connecting point of a level in which the application of the ESD stress is detected. 
     The meritorious effects of the present invention are summarized as follows. 
     According to the present invention, when the ESD stress is applied, the current that flows into the transistor which receives the input signal is restrained through the transistor that outputs the signal. The number of elements which protect a gate oxide film of the transistor that receives the input signal, against the electro-static discharge, can be thereby reduced, or such an element can be reduced in size. 
     Further, according to the present invention, a parasitic capacitance element of the protection element is reduced, and an improvement in responsiveness of a faster operation can be thereby expected. 
     Still other features and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description in conjunction with the accompanying drawings wherein embodiments of the invention are shown and described, simply by way of illustration of the mode contemplated of carrying out this invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawing and description are to be regarded as illustrative in nature, and not as restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a configuration of a circuit according to a first embodiment of the present invention; 
         FIG. 2  is a diagram showing an example of a configuration of a control circuit in the first embodiment of the present invention; 
         FIG. 3  is a diagram showing another example of the configuration of the control circuit in the first embodiment of the present invention; 
         FIG. 4  is a diagram showing an example of other configuration in the first embodiment of the present invention; 
         FIG. 5  is a diagram showing a configuration of a circuit according to a second embodiment of the present invention; 
         FIG. 6  is a diagram showing an example of a configuration of a control circuit in the second embodiment of the present invention; 
         FIG. 7  is a diagram showing other configuration of the control circuit in the second embodiment of the present invention; 
         FIG. 8  is a diagram showing a configuration of a circuit according to a third embodiment of the present invention; 
         FIG. 9  is a diagram showing an example of a configuration of a control circuit in the third embodiment of the present invention; 
         FIG. 10  is a diagram showing other configuration of the control circuit in the third embodiment of the present invention; 
         FIG. 11  is a diagram showing a configuration of a circuit according to a fourth embodiment of the present invention; 
         FIG. 12  is a diagram showing an example of a configuration of a control circuit in the fourth embodiment of the present invention; 
         FIG. 13  is a diagram showing other configuration of the control circuit in the fourth embodiment of the present invention; 
         FIG. 14  is a diagram showing a configuration of a circuit according to a fifth embodiment of the present invention; 
         FIG. 15  is a diagram showing an example of a configuration of a control circuit in the fifth embodiment of the present invention; 
         FIG. 16  is a diagram showing other configuration of the control circuit in the fifth embodiment of the present invention; 
         FIG. 17  is a diagram showing a configuration of a circuit according to a sixth embodiment of the present invention; 
         FIG. 18  is a diagram showing an example of a configuration of a control circuit in the sixth embodiment of the present invention; 
         FIG. 19  is a diagram showing other configuration of the control circuit in the sixth embodiment of the present invention; 
         FIGS. 20A ,  20 B,  20 C and  20 D include diagrams showing layout configurations in the embodiment of the present invention; 
         FIGS. 21A ,  21 B,  21 C and  21 D include diagrams showing layout configurations in the embodiment of the present invention; 
         FIG. 22  is a diagram showing a configuration of a circuit according to a seventh embodiment of the present invention; 
         FIG. 23  is a diagram showing a configuration of a circuit according to an eighth embodiment of the present invention; 
         FIG. 24  is a diagram showing a configuration of a circuit according to a ninth embodiment of the present invention; 
         FIG. 25  is a diagram showing a configuration disclosed in Patent Document 1; and 
         FIG. 26  is a diagram showing a configuration disclosed in Patent Document 1. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     A description of the invention will be given with reference to appended drawings. 
     First Embodiment 
       FIG. 1  is a diagram showing a circuit configuration according to a first embodiment of the present invention. Referring to  FIG. 1 , a first power supply system is provided from a power supply terminal VDD 1  and a ground terminal GND 1 , while a second power supply system is provided from a power supply terminal VDD 2  and a ground terminal GND 2 . 
     The first power supply system includes an ESD protection element E 1 , an output inverter circuit I 1 , and a PMOS transistor TP 1  connected between the output inverter circuit I 1  and the power supply terminal VDD 1 . The second power supply system includes an ESD protection element E 2  and an input inverter circuit I 2 . An output of the output inverter circuit I 1  in the first power supply system and an input of the input inverter circuit I 2  in the second power supply system are connected through a signal line S 1  that transfers a signal. Between the ground terminal GND 1  in the first power supply system and the ground terminal GND 2  in the second power supply system, an ESD protection element E 3  is connected. 
     Next, an operation in the first embodiment of the present invention will be described. The output inverter circuit I 1  in the first power supply system is composed by an NMOS transistor N 1  and a PMOS transistor P 1 , and the PMOS transistor TP 1  is connected between the PMOS transistor P 1  and the power supply terminal VDD 1 . 
     Referring to  FIG. 1 , when ESD is applied to the power supply terminal VDD 1  with the ground terminal GND 2  as a reference point, for example, a current flown into the PMOS transistor P 1  of the output inverter circuit I 1  can be restrained by arranging and controlling the PMOS transistor TP 1 . As a result, a current that flows into the signal line S 1  through the PMOS transistor P 1  can be restrained, thereby allowing limitation of a potential difference Vgs between a gate of an NMOS transistor N 2  of the input inverter circuit I 2  and a source of the NMOS transistor N 2  within a voltage that might cause breakdown of a gate oxide film of the NMOS transistor N 2 . With this arrangement, breakdown of the gate of the NMOS transistor N 2  can be prevented, and a stable ESD withstand voltage can be thereby obtained. 
     Electric charge applied to the power supply terminal VDD 1  by the ESD stress application is discharged to the ground terminal GND 2  through the ESD protection elements E 1  and E 3 . 
       FIG. 2  is a diagram showing a configuration of a circuit in which a circuit C 1  that controls a gate of the PMOS transistor TP 1  is provided in the first embodiment of the present invention described with reference to  FIG. 1 . The configuration except for the control circuit C 1  that controls the gate of the PMOS transistor TP 1  is the same as in  FIG. 1 . Below, a description of components that are the same as those in  FIG. 1  will be omitted as necessary, and the description will be mainly directed to a difference. 
     Referring to  FIG. 2 , in the control circuit C 1 , a resistance element R 1  is connected between the gate of the PMOS transistor TP 1  and the ground terminal GND 1 , a capacitance element (capacitor) Q 1  is connected between the gate of the PMOS transistor TP 1  and the power supply terminal VDD 1 , and the resistance element R 1  and the capacitance element Q 1  are connected. 
     Since the gate of the PMOS transistor TP 1  is connected to the ground terminal GND 1  through the resistance element R 1  of the control circuit C 1 , the PMOS transistor TP 1  is set an ON state (conductive state) when a normal operation is performed, and does not affect an operation of the circuit. 
     When the ESD stress is applied to the power supply terminal VDD 1 , with the ground terminal GND 2  as a reference point, electric charge is applied to the capacitance element Q 1  between the power supply terminal VDD 1  and the gate of the PMOS transistor TP 1 . 
     By coupling of the electric charge applied to the capacitance element Q 1 , a gate potential of the PMOS transistor TP 1  increases to become the same potential as a source potential of the PMOS transistor TP 1 . The PMOS transistor TP 1  thereby becomes the OFF state, and a current that flows into the PMOS transistor P 1  can be thereby restrained. As a result, a current that flows into the signal line S 1  through the PMOS transistor P 1  can be limited. The breakdown of the gate oxide film of the NMOS transistor N 2  can be thereby prevented, as described with reference to  FIG. 1 . 
       FIG. 3  is a diagram showing a configuration of a circuit in which a control circuit C 2  of other configuration that controls the gate of the PMOS transistor TP 1  is provided in a semiconductor device in the first embodiment of the present invention described with reference to  FIG. 1 . The configuration except for the control circuit C 2  is the same as in  FIG. 1 . Below, descriptions of components that are the same as those in  FIG. 1  will be omitted as necessary, and a description will be mainly directed a difference. 
     Referring to  FIG. 3 , in the control circuit C 2 , a resistance element R 1  is connected between the gate of the PMOS transistor TP 1  and the ground terminal GND 1 , a diode element D 1  is connected between the gate of the PMOS transistor TP 1  and the power supply terminal VDD 1 , and the resistance element R 1  and the diode element D 1  are connected. 
     In the present embodiment, the PMOS transistor TP 1  becomes the ON state (conductive state) when a normal operation is performed, and does not affect an operation of the circuit, as in an example shown in  FIG. 2 . 
     Referring to  FIG. 3 , when the ESD stress is applied to the power supply terminal VDD 1  with the ground terminal GND 1  as a reference point, the diode element D 1  is turned on due to avalanche breakdown of the diode element D 1 . Then, as in  FIG. 2 , the PMOS transistor TP 1  becomes the OFF state, and a current that flows into the PMOS transistor P 1  can be restrained. As a result, a current that flows into the signal line S 1  through the PMOS transistor P 1  can be limited. The breakdown of the gate oxide film of the NMOS transistor N 2  can be thereby prevented, as described with reference to  FIG. 1 . 
     In the present embodiment, an example is shown where the ESD protection element E 3  is used for the connection between the ground terminal GND 1  of the first power supply system and the ground terminal GND 2  of the second power system. A resistance element may be used for the connection, or the connection may be short-circuited. 
     In the present embodiment, the control circuit C 1  in  FIG. 2  and the control circuit C 2  in  FIG. 3  are not limited to configurations that control one PMOS transistor TP 1 . The control circuit C 1  (or C 2 ) may control a plurality of output circuits, as shown in  FIG. 4 , for example. In an example shown in  FIG. 4 , an output of the control circuit C 1  (at a connecting point between the capacitance element Q 1  and the resistance element R 1 ) is connected to the output inverter circuit I 1  and an output inverter circuit I 11 , and is connected in common to the gate of the PMOS transistor TP 1  and a gate of a PMOS transistor TP 11  connected between the power supplies. 
     The diode element D 1  of the control circuit C 2  in  FIG. 3  may be of course an arbitrary element that has a PN junction such as an NMOS transistor, a PMOS transistor, or a bipolar transistor. 
     Next, the present embodiment will be described with reference to layout diagrams in  FIGS. 20A-20D . The PMOS transistors P 1  and TP 1  are formed according to each layout plan view in  FIGS. 20A-20D , for example. In  FIG. 20A , a source diffusion layer of the PMOS transistor P 1  of the output inverter circuit I 1  (or a PMOS transistor P 2  of the input inverter circuit I 2 ) is separated from a drain diffusion layer of the PMOS transistor TP 1  (a PMOS transistor TP 2 ), and the source diffusion layer and the drain diffusion layer are connected via contacts and a first metal interconnect layer or the like. In  FIG. 20B , the source diffusion layer of the PMOS transistor P 1  of the output inverter circuit I 1  (or the PMOS transistor P 2  of the input inverter circuit I 2 ) and the drain diffusion layer of the PMOS transistor TP 1  (TP 2 ) are common. 
     As shown in  FIGS. 20A and 20B , an N-type diffusion layer (Tap) that assumes an N well potential is often arranged separated from a P-type diffusion layer where the PMOS transistors P 1  and TP 1  are formed. As described before, since the gate of the PMOS transistor TP 1  is controlled to turn the PMOS transistor TP 1  off when the ESD stress is applied. On this occasion, the PMOS transistor TP 1  may operate simultaneously with the ESD protection element (indicated by reference numeral E 1  in  FIG. 1 ). 
     Then, in order to prevent the operation of the PMOS transistor TP 1  simultaneously with the ESD protection element (indicated by the reference numeral E 1  in  FIG. 1 ), it is effective to bring a P-type diffusion layer of the PMOS transistor TP 1  into contact with the N-type diffusion layer (Tap) that assumes the N-well potential. 
     When the ESD stress is applied, an ESD surge flows into the drain diffusion layer through an N well resistance due to the avalanche breakdown at the PN junction between an N well and the drain diffusion layer of the PMOS transistor. 
     Due to a voltage drop caused by the N well resistance, a parasitic bipolar transistor of the PMOS transistor operates, and an ESD surge current flows between a source of the parasitic bipolar transistor and a drain of the parasitic bipolar transistor. However, by bringing the P-type diffusion layer of the PMOS transistor TP 1  into contact with the N-type diffusion layer (Tap) that assumes the N well potential, the N well resistance decreases, and the voltage drop is thereby reduced. Accordingly, the parasitic bipolar transistor of the PMOS transistor TP 1  will not operate. As a result, the PMOS transistor keeps the OFF state, making it easy to limit the current that flows into the PMOS transistor P 1 . 
     As described above, the PMOS transistor TP 1  is provided between the PMOS transistor P 1  in the output inverter circuit I 1  and the power supply terminal VDD 1 , and the gate of the PMOS transistor TP 1  is controlled, in the first embodiment of the present invention. The current that flows into the PMOS transistor P 1  can be thereby limited, and the current that flows into the signal line S 1  from the PMOS transistor P 1  can be restrained. For this reason, the potential difference Vgs between the gate of the NMOS transistor N 2  and the source of the NMOS transistor N 2  in the input inverter circuit I 2  can be limited within a voltage that might cause the breakdown of the gate oxide film of the NMOS transistor N 2 . For this reason, according to the first embodiment, the number of protection elements (HK 3 ) that prevent the breakdown of the gate oxide film, shown in  FIG. 26  can be reduced, or such a protection element (HK 3 ) can be reduced in size. 
     Second Embodiment 
       FIG. 5  is a diagram showing a configuration of a circuit according to a second embodiment of the present invention. Referring to  FIG. 5 , same reference numerals are assigned to components that are the same as those in  FIG. 1 . Below, descriptions of the same components will be omitted as necessary, and a description will be directed to a difference. 
     Referring to  FIG. 5 , the second embodiment of the present invention includes an NMOS transistor TN 1  connected between the output inverter circuit I 1  and the ground terminal GND 1 , in place of the PMOS transistor TP 1  in  FIG. 1 . 
     The output inverter circuit I 1  in the first power supply system is composed by the NMOS transistor N 1  and the PMOS transistor P 1 , and the NMOS transistor TN 1  is connected between the NMOS transistor N 1  and the ground terminal GND 1 . 
     Referring to  FIG. 5 , when the ESD stress is applied to the ground terminal GND 1  with the power supply terminal VDD 2  as a reference point, for example, a current flown into the NMOS transistor N 1  can be restrained by arranging and controlling the NMOS transistor TN 1 . As a result, a current that flows into the signal line S 1  through the NMOS transistor N 1  can be restrained, thereby allowing limitation of a potential difference Vgs between a gate of the PMOS transistor P 2  and a source of the PMOS transistor P 2  within a voltage that causes breakdown of a gate oxide film of the PMOS transistor P 2 . Breakdown of the gate of the PMOS transistor P 2  is thereby prevented, and a stable ESD withstand voltage can be thereby obtained. An electric charge injected into the ground terminal GND 1  by the ESD stress application is discharged to the power supply terminal VDD 2  through the ESD protection elements E 3  and E 2 . 
       FIG. 6  is a diagram showing a configuration of a circuit in which a circuit C 3  that controls a gate of the NMOS transistor TN 1  is provided in a semiconductor device in the second embodiment of the present invention. Referring to  FIG. 6 , same reference numerals are assigned to components that are the same as those in  FIG. 5 . Below, descriptions of the same components will be omitted as necessary, and a description will be mainly directed to a difference. 
     Referring to  FIG. 6 , the circuit C 3  that controls a gate potential of the NMOS transistor TN 1  connected between the output inverter circuit I 1  and the ground terminal GND 1  is included. 
     In the control circuit C 3 , a resistance element R 2  is connected between the gate of the NMOS transistor TN 1  and the power supply terminal VDD 1 , a capacitance element Q 2  is connected between the gate of the TMOS transistor TN 1  and the ground terminal GND 1 , and the resistance element R 2  and the capacitance element Q 2  are connected. The gate of the NMOS transistor TN 1  is connected to the power supply VDD 1  through the resistance element R 2  of the control circuit C 3 . For this reason, the NMOS transistor TN 1  becomes the ON state (conductive state) when a normal operation is performed, and does not affect an operation of the circuit. 
     Referring to  FIG. 6 , when the ESD stress is applied to the ground terminal GND 1  with the power supply terminal VDD 2  as a reference point, electric charge is charged into the capacitance element Q 2  between the ground terminal GND 1  and the gate of the NMOS transistor TN 1 . By coupling of the electric charge applied to the capacitance element Q 2 , the gate potential of the NMOS transistor TN 1  increases to become the same potential as a source potential of the NMOS transistor TN 1 . The NMOS transistor TN 1  thereby becomes the OFF state, and a current that flows into the NMOS transistor N 1  can be thereby restrained. As a result, a current that flows into the signal line S 1  through the NMOS transistor N 1  can be limited. The breakdown of the gate oxide film of the PMOS transistor P 2  can be thereby prevented, as described with reference to  FIG. 5 . 
       FIG. 7  is a diagram showing a configuration of a circuit in which a control circuit C 4  that controls the gate of the NMOS transistor TN  1  is provided in the second embodiment of the present invention. 
     Referring to  FIG. 7 , the control circuit C 4  includes an inverter circuit constituted from an NMOS transistor N 3  and a PMOS transistor P 3 , and the gate of the NMOS transistor TN 1  is connected to an output node (coupled drains of the NMOS transistor N 3  and the PMOS transistor P 3 ) of the inverter circuit. The resistance element R 1  is connected between an input node of this inverter circuit (coupled gates of the NMOS transistor N 3  and the PMOS transistor P 3 ) and the ground terminal GND 1 , and the capacitance element Q 1  is connected between the input node and the power supply terminal VDD 1 . As in  FIG. 6 , the NMOS transistor TN 1  becomes the ON state (conductive state) when a normal operation is performed, and does not affect an operation of the circuit. 
     Referring to  FIG. 7 , when the ESD stress is applied to the power supply terminal VDD 1  with the ground terminal GND 2  as a reference point, electric charge is charged into the capacitance element Q 1  through the ESD protection element E 1 . Then, due to an operation similar to that in  FIG. 2 , the PMOS transistor P 3  becomes the OFF state, and the NMOS transistor TN 1  becomes the OFF state due to an output signal of the inverter circuit constituted from the PMOS transistor P 3  and the NMOS transistor N 3 . 
     Accordingly, a current that flows into the NMOS transistor N 1  can be restricted. As a result, a current that flows into the signal line S 1  through the NMOS transistor N 1  can be limited. The breakdown of the gate oxide film of the PMOS transistor P 2  can be prevented, as described with reference to  FIG. 5 . 
     Next, the embodiment of the present invention will be described with reference to  FIGS. 21A-21D . The NMOS transistors N 1  and TN 1  are formed according to each layout plan view in  FIGS. 21A-21D , for example. In  FIG. 21A , a source diffusion layer of the NMOS transistor N 1  of the output inverter circuit I 1  (or the NMOS transistor N 2  of the input inverter circuit I 2 ) is separated from a drain diffusion layer of the NMOS transistor TN 1  (an NMOS transistor TN 2 ), and the source diffusion layer and the drain diffusion layer are mutually connected via contacts and a first interconnect layer or the like. In  FIG. 21B , the source diffusion layer of the NMOS transistor N 1  of the output inverter circuit I 1  (or the NMOS transistor N 2  of the input inverter circuit I 2 ) and the drain diffusion layer of the NMOS transistor TN 1  (TN 2 ) are common. 
     As shown in  FIGS. 21A and 21B , a P-type diffusion layer (Tap) that assumes a P well potential is often arranged separated from an N-type diffusion layer where the NMOS transistors N 1  and TN 1  are formed. As described before, since the gate of the NMOS transistor TN 1  is controlled to turn the NMOS transistor TN 1  off when the ESD stress is applied. On this occasion, the NMOS transistor TN 1  may operate simultaneously with the ESD protection element (indicated by reference numeral E 1  in  FIG. 5 ). 
     Then, in order to prevent the NMOS transistor TN 1  from operating simultaneously with the ESD protection element (indicated by reference numeral E 1  in  FIG. 5 ), it is effective to bring the N-type diffusion layer of the NMOS transistor TN 1  into contact with the P-type diffusion layer (Tap) that assumes the P-well potential. 
     When the ESD stress is applied, an ESD surge flows into the drain diffusion layer through a P well resistance due to the avalanche breakdown at the PN junction between a P well and the drain diffusion layer of the NMOS transistor. 
     Due to a voltage drop caused by the P well resistance, a parasitic bipolar transistor of the NMOS transistor operates, and an ESD surge current flows between a source of the parasitic bipolar transistor and a drain of the parasitic bipolar transistor. However, by bringing the N-type diffusion layer of the NMOS transistor TN 1  into contact with the P-type diffusion layer (Tap) that assumes the P well potential, the P well resistance decreases, and the voltage drop is thereby reduced. Accordingly, the parasitic bipolar transistor of the NMOS transistor will not operate. As a result, the NMOS transistor TN 1  keeps the OFF state, making it easy to limit the current that flows into the NMOS transistor N 1 . 
     As described above, in the second embodiment of the present invention, the NMOS transistor TN 1  is provided between the NMOS transistor N 1  in the output inverter circuit I 1  and the ground terminal GND 1 , and the gate of the NMOS transistor TN 1  is controlled. The current that flows into the NMOS transistor N 1  can be thereby limited, and the current that flows into the signal line S 1  from the NMOS transistor N 1  can be restrained. For this reason, a potential difference Vgs between the gate of the PMOS transistor P 2  and the source of the PMOS transistor P 2  in the input inverter circuit I 2  can be limited within the voltage that might cause the breakdown of the gate oxide film of the PMOS transistor P 2 . As a result, the number of the protection elements can be reduced, or the protection element can be reduced in size. 
     In the second embodiment of the present invention, the transistor that limits an ESD current is provided between the output inverter circuit and the ground terminal GND 1 . Thus, when the ESD stress is applied to the ground terminal GND 1  with the power supply terminal VDD 2  as the reference point, the potential difference Vgs on the gate oxide film of the PMOS transistor P 2  of the input inverter circuit I 2  can be reduced to a voltage that might cause the breakdown of the gate oxide film, or less. Thus, the breakdown of the gate of the PMOS transistor P 2  can be prevented. 
     Third Embodiment 
       FIG. 8  is a diagram showing a configuration of a circuit according to a third embodiment of the present invention. This embodiment is configured to combine the first embodiment shown in  FIG. 1  with the second embodiment shown in  FIG. 5 . Referring to  FIG. 8 , same reference numerals are assigned to components that are the same as those in  FIGS. 1 and 5 , and descriptions of the same components will be omitted. This embodiment includes the PMOS transistor TP 1  between the output inverter circuit I 1  and the power supply terminal VDD 1  and the NMOS transistor TN 1  between the output inverter circuit I 1  and the ground GND 1 . 
     Referring to  FIG. 8 , the output inverter circuit I 1  in the first power supply system is composed by the NMOS transistor N 1  and the PMOS transistor P 1 . Between the PMOS transistor P 1  and the power supply terminal VDD 1 , the PMOS transistor TP 1  is connected. Between the NMOS transistor N 1  and the ground terminal GND 1 , the NMOS transistor TN 1  is connected. 
     Referring to  FIG. 8 , when the ESD stress is applied to the power supply terminal VDD 1  with the ground terminal GND 2  as a reference point, for example, the same effect as that in  FIG. 1  is obtained. When the ESD stress is applied to the ground terminal GND 1  with the power supply terminal VDD 2  as a reference point as well, the same effect as that in  FIG. 5  is obtained. 
       FIG. 9  is a diagram showing a configuration of a circuit in which a circuit C 5  that controls the gates of the PMOS transistor TP 1  and the NMOS transistor TN 1 , respectively, is provided. Referring to  FIG. 9 , the control circuit C 5  includes the control circuit C 1  in  FIG. 2 , (which controls the gate of the PMOS transistor TP 1 ), and the control circuit C 3  in  FIG. 6 , (which controls the gate of the NMOS transistor TN 1 ). 
     Referring to  FIG. 9 , the PMOS transistor TP 1  is connected between the PMOS transistor P 1  of the output inverter circuit I 1  and the power supply terminal VDD 1 , and the NMOS transistor TN 1  is connected between the NMOS transistor N 1  of the output inverter circuit I 1  and the ground terminal GND 1 . To the gates of the NMOS transistor TN 1  and the PMOS transistor TP 1 , the circuit C 5  that controls potentials of the gates of the NMOS transistor TN 1  and the PMOS transistor TP 1  is connected. 
     The control circuit C 5  comprises a capacitance element Q 1  connected between the gate of the PMOS transistor TP 1  and the power supply terminal VDD 1 , a resistance element R 1  connected between the capacitance element Q 1  and the ground terminal GND 1 , a resistance element R 2  connected between the gate of the NMOS transistor TN 1  and the power supply terminal VDD 1 , and a capacitance element Q 2  connected between the ground terminal GND 1  and the resistance element R 2 . As in  FIGS. 2 and 6 , in the present embodiment as well, the PMOS transistor TP 1  becomes the ON state (conduction stage) at a time of a normal operation, and does not affect an operation of the circuit. 
     Referring to  FIG. 9 , when the ESD stress is applied to the power supply terminal VDD 1  with the ground terminal GND 2  as a reference point, for example, the same effect as that in  FIG. 2  is obtained. When the ESD stress is applied to the ground terminal GND 1  with the power supply terminal VDD 2  as a reference point as well, the same effect as that in  FIG. 6  is obtained. 
       FIG. 10  is a diagram showing a configuration of a circuit in which a circuit C 4  that controls the gates of the PMOS transistor TP 1  and the NMOS transistor TN 1 , respectively, is provided in the third embodiment of the present invention. A configuration of the control circuit C 4  is the same as in  FIG. 7 . 
     Referring to  FIG. 10 , the third embodiment of the present invention is a configuration that combines  FIG. 2  with the  FIG. 7 . As in  FIGS. 2 and 7 , the NMOS transistor TN 1  and the PMOS transistor TP 1  become the ON states (conductive states), respectively, at a time of a normal operation, and do not affect an operation of the circuit. 
     Referring to  FIG. 10 , when the ESD stress is applied to the power supply terminal VDD 1  with the ground terminal GND 2  as a reference point, for example, or the ESD stress is applied to the ground terminal GND 1  with the power supply terminal VDD 2  as a reference point, the same effect as that in  FIG. 9  is obtained. 
     As described above, in the third embodiment of the present invention, the potential difference Vgs applied between the gate of the PMOS transistor P 2  and the source of the PMOS transistor P 2  in the input inverter circuit I 2  or the potential difference Vgs applied between the gate of the NMOS transistor N 2  and the source of the NMOS transistor N 2  can be limited within the voltage that might cause the breakdown of the gate oxide film of the PMOS transistor P 2  or the NMOS transistor N 2 . The breakdown of the gate of the PMOS transistor P 2  of the input inverter circuit I 2  or the breakdown of the gate oxide film of the NMOS transistor N 2  of the input inverter circuit I 2  can be prevented. As a result, the number of the protection elements can be reduced, or the protection element can be reduced in size. 
     In the first embodiment, a protection circuit when the ESD stress is applied between the terminal VDD 1  and the terminal GND 2  is shown. In the second embodiment, a protection circuit when the ESD stress is applied between the terminal GND 1  and the terminal VDD 2  is shown. According to this third embodiment, even if the ESD stress is applied both between the terminal VDD 1  and the terminal GND 2  and between the terminal VDD 2  and the terminal GND 1 , the breakdown of the gates of the PMOS transistor P 2  and the NMOS transistor N 2  that constitute the input inverter circuit I 2  can be prevented. Thus, the third embodiment provides more excellent protection capability as compared with the first and second embodiments. 
     Fourth Embodiment 
       FIG. 11  is a diagram showing a configuration of a circuit of a semiconductor device according to a fourth embodiment of the present invention. Referring to  FIG. 11 , same reference numerals are assigned to components that are the same as those in  FIG. 1 , and descriptions of the same components will be omitted. Referring to  FIG. 11 , a PMOS transistor TP 2  connected between the PMOS transistor P 2  of the input inverter circuit I 2  and the power supply terminal VDD 2  is arranged. 
     Next, an operation of the fourth embodiment of the present invention will be described. The input inverter circuit I 2  in the second power supply system is composed by the NMOS transistor N 2  and the PMOS transistor P 2 . Between the PMOS transistor P 2  and the power supply terminal VDD 2 , the PMOS transistor TP 2  is connected. 
     Referring to  FIG. 11 , when the ESD stress is applied to the power supply terminal VDD 2  with the ground terminal GND 1  as a reference point, for example, a current that flows into the PMOS transistor P 2  can be restrained by arranging and controlling the PMOS transistor TP 2 . As a result, an increase in a source potential of the PMOS transistor P 2  is restrained, and a potential difference Vgs applied between the gate of the PMOS transistor P 2  of the input inverter circuit I 2  and the source of the PMOS transistor P 2  can be limited within the voltage that might cause the breakdown of the gate oxide film of the PMOS transistor P 2 . The breakdown of the gate of the PMOS transistor P 2  can be thereby prevented, and a stable ESD withstand voltage can be obtained. Meanwhile, electric charge applied to the power supply terminal VDD 2  by the ESD stress application is discharged to the ground terminal GND 1  through the ESD protection elements E 2  and E 3 . 
       FIG. 12  is a diagram showing a configuration of a circuit in which a circuit C 6  that controls a gate of the PMOS transistor TP 2  is provided in the fourth embodiment of the present invention. Referring to  FIG. 12 , same reference numerals are assigned to components that are the same as those in  FIG. 11 . Below, descriptions of the same components will be omitted as necessary, and a description will mainly directed to a difference. The control circuit C 6  has the same configuration as the control circuit C 1  in  FIG. 2 . 
     In the control circuit C 6 , a resistance element R 3  is connected between the gate of the PMOS transistor TP 2  and the ground terminal GND 2 , and a capacitance element Q 3  is connected between the gate of the PMOS transistor TP 2  and the power supply terminal VDD 2 . When a normal operation is performed, the PMOS transistor TP 2  becomes the ON state (conductive state) like the transistor in  FIG. 2 , and does not affect an operation of the circuit. 
     Referring to  FIG. 12 , when the ESD stress is applied to the power supply terminal VDD 2  with the ground terminal GND 1  as a reference point, the PMOS transistor TP 2  becomes the OFF state according to an operation similar to that in  FIG. 2 . As a result, as described with reference to  FIG. 11 , the breakdown of the gate oxide film of the PMOS transistor P 2  can be prevented. 
       FIG. 13  is a diagram showing a configuration of a circuit where a control circuit C 7 , which is other configuration that controls the gate of the PMOS transistor TP 2 , is provided in the fourth embodiment of the present invention. The control circuit C 7  has the same configuration as the control circuit C 2  in  FIG. 3 . 
     Referring to  FIG. 13 , in the control circuit C 7 , the resistance element R 3  is connected between the gate of the PMOS transistor TP 2  and the ground terminal GND 2 , and a diode element D 3  is connected between the gate of the PMOS transistor TP 2  and the power supply terminal VDD 2 . The resistance element R 3  and the diode element D 3  are connected. When a normal operation is performed, the PMOS transistor TP 2  becomes the ON state (conductive state) as in  FIG. 12 , and does not affect an operation of the circuit. 
     Referring to  FIG. 13 , when the ESD stress is applied to the power supply terminal VDD 2  with the ground terminal GND 1  as a reference point in the same manner as described before, the PMOS transistor TP 2  becomes OFF state according to an operation similar to that in  FIG. 3 . As a result, as described with reference to  FIG. 11 , the breakdown of the gate oxide film of the PMOS transistor P 2  can be prevented, as described with reference to  FIG. 11 . 
     As described above, in the fourth embodiment of the present invention, the PMOS transistor TP 2  is provided between the PMOS transistor P 2  of the input inverter circuit I 2  and the power supply terminal VDD 2 . By controlling the gate of the PMOS transistor TP 2 , a current that flows into the PMOS transistor P 2  can be limited, and an increase in the source potential of the PMOS transistor P 2  can be restrained. The potential difference Vgs applied between the gate of the PMOS transistor P 2  and the source of the PMOS transistor P 2  in the input inverter circuit I 2  can be therefore limited within the voltage that might cause the breakdown of the gate oxide film of the PMOS transistor P 2 . As a result, the number of the protection elements that prevent the breakdown of the gate oxide film can be reduced, or the protection element can be reduced in size. 
     In the second embodiment of the present invention, the NMOS transistor that limits the ESD current is provided between the output inverter circuit and the ground terminal GND 1 , thereby preventing the breakdown of the gate of the PMOS transistor P 2 . In the fourth embodiment of the present invention, the PMOS transistor that limits the ESD current is provided between the input inverter circuit and the power supply terminal VDD 2 . The current that flows into the PMOS transistor P 2  can be thereby limited, and the potential difference Vgs applied to the gate oxide film of the PMOS transistor P 2  of the input inverter circuit I 2  can be reduced to a voltage that might cause the breakdown of the gate oxide film, or less. For this reason, when the ESD stress is applied to the power supply terminal VDD 2  with the ground terminal GND 1  as the reference point, for example, the breakdown of the gate of the PMOS transistor P 2  can be prevented. 
     Fifth Embodiment 
       FIG. 14  is a diagram showing a configuration of a circuit of a semiconductor device according to a fifth embodiment of the present invention. Referring to  FIG. 14 , same reference numerals are assigned to components that are the same as those in  FIG. 1 . Below, descriptions of the same components will be omitted as necessary, and a description will be mainly directed to a difference. 
     While there is provided the PMOS transistor TP 1  connected between the PMOS transistor P 1  of the output inverter circuit I 1  and the power supply terminal VDD 1  in  FIG. 1 , the NMOS transistor TN 2  connected between the NMOS transistor N 2  of the input inverter circuit I 2  and the ground terminal GND 2  is provided in the configuration in  FIG. 14 . 
     That is, referring to  FIG. 14 , the input inverter circuit I 2  in the second power supply system is composed by the NMOS transistor N 2  and a PMOS transistor P 2 , and the NMOS transistor TN 2  is connected between the NMOS transistor N 2  and the ground terminal GND 2 . 
     Referring to  FIG. 14 , when the ESD stress is applied to the ground terminal GND 2  with the power supply terminal VDD 1  as a reference point, for example, a current that flows into the NMOS transistor N 2  can be restrained by arranging and controlling the NMOS transistor TN 2 . As a result, an increase in a source potential of the NMOS transistor N 2  is restrained, and the potential difference Vgs applied between the gate of the NMOS transistor N 2  of the input inverter circuit I 2  and the source of the NMOS transistor N 2  can be limited within the voltage that might cause the breakdown of the gate oxide film of the NMOS transistor N 2 . The breakdown of the gate of the NMOS transistor N 2  can be thereby prevented, and a stable ESD withstand voltage can be obtained. Electric charge applied to the ground terminal GND 2  by the ESD stress application is discharged to the power supply terminal VDD 1  through the ESD protection elements E 3  and E 1 . 
       FIG. 15  is a diagram showing a configuration of a circuit in which a circuit C 8  that controls the gate of the NMOS transistor TN 2  is provided in a semiconductor device in the fifth embodiment of the present invention. Referring to  FIG. 15 , same reference numerals are assigned to components that are the same as those in  FIG. 14 . Below, descriptions of the same components will be omitted as necessary, and a description will mainly directed to a difference. The control circuit C 8  has the same configuration as the control circuit C 3  in  FIG. 6 . 
     Referring to  FIG. 15 , in the control circuit C 8 , a resistance element R 4  is connected between the gate of the TMOS transistor TN 2  and the power supply terminal VDD 2 , and a capacitance element Q 4  is connected between the gate of the TMOS transistor TN 2  and the ground terminal GND 2 . When a normal operation is performed, the TMOS transistor TN 2  becomes the ON state (conductive state) as in  FIG. 6 , and does not affect an operation of the circuit. 
     Referring to  FIG. 15 , when the ESD stress is applied to the ground terminal GND 2  with the power supply terminal VDD 1  as a reference point, the NMOS transistor TN 2  becomes the OFF state according to an operation similar to that in  FIG. 6 . As a result, as described with reference to  FIG. 14 , the breakdown of the gate oxide film of the NMOS transistor N 2  can be prevented. 
       FIG. 16  is a diagram showing a configuration of a circuit in which a control circuit C 9 , which has other configuration that controls the gate of the NMOS transistor TN 2 , is provided in a semiconductor device in the fifth embodiment of the present invention. Since the control circuit C 9  has the same configuration as the control circuit C 4  in  FIG. 7 , a description of the control circuit C 9  will be omitted. 
     Referring to  FIG. 16 , when a normal operation is performed, the NMOS transistor TN 2  becomes the ON state (conductive state) as in  FIG. 15  and does not affect an operation of the circuit. 
     Referring to  FIG. 16 , when the ESD stress is applied to the ground terminal GND 2  with the power supply terminal VDD 1  as a reference point, the NMOS transistor TN 2  becomes the OFF state due to an operation similar to that in  FIG. 7 . As a result, as described with reference to  FIG. 14 , the breakdown of the gate of the NMOS transistor N 2  can be prevented. 
     As described above, in the fifth embodiment of the present invention, the NMOS transistor TN 2  is provided between the NMOS transistor N 2  of the input inverter circuit I 2  and the ground terminal GND 2 , and the gate of the NMOS transistor TN 2  is controlled. Thus, the current that flows into the NMOS transistor N 2  can be limited, and the increase in the source potential of the NMOS transistor N 2  can be thereby restrained. The potential difference Vgs applied between the gate of the NMOS transistor N 2  in the input inverter circuit I 2  and the source of the NMOS transistor N 2  can be limited within the voltage that might cause the breakdown of the gate oxide film of the NMOS transistor N 2 . As a result, the number of the protection elements that prevent the breakdown of the gate oxide film can be reduced, or the protection element can be reduced in size. 
     In the first embodiment of the present invention, the PMOS transistor that limits the ESD current is provided between the output inverter circuit and the power supply terminal VDD 1 , thereby preventing the breakdown of the gate of the NMOS transistor N 2 . In the fifth embodiment of the present invention, the NMOS transistor that limits the ESD current is provided between the input inverter circuit and the ground terminal GND 2 . The current that flows into the NMOS transistor N 2  can be thereby limited, and the potential difference Vgs applied to the gate oxide film of the NMOS transistor N 2  of the input inverter circuit I 2  can be reduced to a voltage that might cause the breakdown of the gate oxide film, or less. For this reason, when the ESD stress is applied to the ground terminal GND 2  with the power supply terminal VDD 1  as the reference point, the breakdown of the gate of the NMOS transistor N 2  can be prevented. 
     Sixth Embodiment 
       FIG. 17  is a diagram showing a configuration of a circuit of a semiconductor device according to a sixth embodiment of the present invention.  FIG. 17  is configured to combine the fourth embodiment shown in  FIG. 11  with the fifth embodiment shown in  FIG. 14 . Same reference numerals are assigned to components that are the same as those in  FIGS. 11 and 14 . Below, descriptions of the same components will be omitted, and a description will be directed to a difference. 
     Referring to  FIG. 17 , the input inverter circuit I 2  in the second power supply system is composed by the NMOS transistor N 2  and the PMOS transistor P 2 . The PMOS transistor TP 2  is connected between the PMOS transistor P 2  and the power supply terminal VDD 2 , and the NMOS transistor TN 2  is connected between the NMOS transistor N 2  and the ground terminal GND 2 . 
     Referring to  FIG. 17 , when the ESD stress is applied to the power supply terminal VDD 2  with the ground terminal GND 1  as a reference point, for example, the same effect as that in  FIG. 11  is obtained. When the ESD stress is applied to the ground terminal GND 2  with the power supply terminal VDD 1  as a reference point as well, the same effect as that in  FIG. 14  is obtained. 
       FIG. 18  is a diagram showing a configuration of a circuit in which a circuit C 10  that controls the gate of the PMOS transistor TP 2  and the gate of the NMOS transistor TN 2  is provided in a semiconductor device in the sixth embodiment of the present invention. The control circuit C 10  combines the control circuit C 6  shown in  FIG. 12  and the control circuit C 8  shown in  FIG. 15 . Referring to  FIG. 18 , same reference numerals are assigned to components that are the same as those in  FIGS. 12 and 15 . Below, descriptions of the same components will be omitted as necessary, and a description will mainly directed to a difference. 
     Referring to  FIG. 18 , the PMOS transistor TP 2  is connected between the PMOS transistor P 2  of the output inverter circuit I 2  and the power supply terminal VDD 2 , and the NMOS transistor TN 2  is connected between the NMS transistor N 2  of the output inverter circuit I 2  and the ground terminal GND 2 . To the gates of the NMOS transistor TN 2  and the PMOS transistor TP 2 , the circuit C 10  that controls potentials of the gates of the NMOS transistor TN 2  and the PMOS transistor TP 2  is connected. 
     The control circuit C 10  comprises a capacitance element Q 3  connected between the gate of the PMOS transistor TP 2  and the power supply terminal VDD 2 , a resistance element R 3  Connected between the capacitance element Q 3  and the ground terminal GND 2 , a resistance element R 4  connected between the gate of the NMOS transistor TN 2  and the power supply terminal VDD 2 , and a capacitance element Q 4  connected between the resistance element R 4  and the ground terminal GND 2 . 
     The control circuit C 10  operates as in  FIGS. 12 and 15 . When a normal operation is performed, the PMOS transistor TP 2  and the NMOS transistor TN 2  become the ON states (conductive states), respectively, and do not affect an operation of the circuit. 
     Referring to  FIG. 18 , when the ESD stress is applied to the power supply terminal VDD 2  with the ground terminal GND 1  as a reference point, the same effect as that in  FIG. 12  is obtained. When the ESD stress is applied to the ground terminal GND 2  with the power supply terminal VDD 1  as a reference point as well, the same effect as that in  FIG. 15  is obtained. 
       FIG. 19  is a diagram showing a configuration of a circuit in which the circuit C 9 , which is other configuration that controls the gate of the PMOS transistor TP 2  and the gate of the NMOS transistor TN 2 , is provided in a semiconductor device in the sixth embodiment of the present invention. The configuration of the control circuit C 9  is the same as that in  FIG. 16 . 
       FIG. 19  combines  FIGS. 12 and 16 . As in  FIGS. 12 and 16 , the NMOS transistor TN 2  and the PMOS transistor TP 2  become the ON states (conductive states), respectively, when a normal operation is performed, and do not affect an operation of the circuit. 
     Referring to  FIG. 19 , when the ESD stress is applied to the power supply terminal VDD 2  with the ground terminal GND 1  as a reference point, or when the ESD stress is applied to the ground terminal GND 2  with the power supply terminal VDD 1  as a reference point, for example, the same effect as that in  FIG. 18  is obtained. 
     As described above, in the sixth embodiment of the present invention, due to the same effect as those in the fourth and fifth embodiments, the breakdown of the gate oxide film of the PMOS transistor P 2  or the NMOS transistor N 2  of the input inverter circuit I 2  can be prevented. As a result, the number of the protection elements can be reduced, or the protection element can be reduced in size. 
     According to the sixth embodiment of the present invention, the transistors are provided between the input inverter circuit and the power supply terminal and between the input inverter circuit and the ground terminal, respectively. In both cases where the ESD stress is applied from the terminal VDD 2  to the terminal GND 1  and the ESD stress is applied from the terminal GND 2  to the terminal VDD 1 , the breakdown of the gates of the PMOS transistor P 2  and the NMOS transistor N 2  that constitute the input inverter circuit I 2  can be thereby prevented. The sixth embodiment provides more excellent protection capability than the fourth and fifth embodiments of the present invention. 
     Seventh Embodiment 
       FIG. 22  is a diagram showing a configuration of a circuit of a semiconductor device according to a seventh embodiment of the present invention. Referring to  FIG. 22 , same reference numerals are assigned to components that are the same as those in  FIG. 1 . Descriptions of the same components will be omitted, and a description will be mainly directed to a difference.  FIG. 22  shows the circuit configuration in which a circuit C 11  that controls a gate of the PMOS transistor P 1  in the output inverter circuit I 1  is provided. 
     Referring to  FIG. 22 , in the control circuit C 11 , a capacitance element Q 1  and a resistance element R 1  are connected in series between the power supply terminal VDD 1  and the ground terminal GND 1 . Between the PMOS transistor P 1  and a node between the capacitance element Q 1  and the resistance element R 1 , an inverter circuit B 2  and a NAND circuit A 1  are connected in series. An inverter element B 1  is connected to a gate of the NMOS transistor N 1 . 
     The output inverter circuit I 1  in the first power supply system is composed by the NMOS transistor N 1  and the PMOS transistor P 1 . 
     Since the gate of the PMOS transistor P 1  is connected to the ground terminal GND 1  through the NAND circuit A 1 , inverter circuit B 2 , and resistance element R 1 , the gate of the PMOS transistor P 1  is controlled by a value of an input signal (input signal to be output from the output circuit) to the NAND circuit A 1 . 
     The gate of the NMOS transistor N 1  is connected to the input signal to the NAND circuit A 1  through the inverter circuit B 1 . Accordingly, an output signal of the output inverter circuit I 1  is controlled by the input signal to the NAND circuit A 1 , and does not affect an operation of the circuit. 
     Referring to  FIG. 22 , when the ESD stress is applied to the power supply terminal VDD 1  with the ground terminal GND 2  as a reference point, for example, electric charge is charged into the capacitance element Q 1 . Then, due to coupling of the electric charge applied to the capacitance element Q 1 , a gate potential of a PMOS transistor (not shown) of the inverter circuit B 2  becomes the same as a source potential of the PMOS transistor (not shown) of the inverter circuit B 2 . An output of the inverter circuit B 2  therefore goes Low. Since an output of the NAND circuit A 1  that receives the Low-level output of the inverter circuit B 2  goes High, the gate of the PMOS transistor P 1  becomes the OFF state. For this reason, a current that flows into the signal line S 1  through the PMOS transistor P 1  can be limited. The breakdown of the gate oxide film of the NMOS transistor N 2  of the input inverter circuit I 2  can be thereby prevented. 
     As described above, in the seventh embodiment of the present invention, a gate potential of the PMOS transistor P 1  of the output inverter circuit I 1  is controlled. The current that flows into the signal line S 1  from the PMOS transistor P 1  can be thereby restrained, and the potential difference Vgs applied between the gate of the NMOS transistor N 2  of the input inverter circuit I 2  and a source of the NMOS transistor N 2  can be thereby limited within the voltage that might cause the breakdown of the gate oxide film of the NMOS transistor N 2 . As a result, the number of the protection elements ca be reduced, or the protection element can be reduced in size. 
     In the seventh embodiment of the present invention, by controlling the PMOS transistor P 1  of the output inverter circuit I 1  itself, driving capability can be more enhanced as compared with the first embodiment of the present invention. 
     Eighth Embodiment 
       FIG. 23  is a diagram showing a configuration of a circuit according to an eighth embodiment of the present invention. Referring to  FIG. 23 , same reference numerals are assigned to components that are the same as those in  FIG. 1 . Below, descriptions of the same components will be omitted as necessary, and a description will be mainly directed to a difference. 
     Referring to  FIG. 23 , the present embodiment includes a circuit C 12  that controls the gate of the NMOS transistor N 1  of the output inverter circuit I 1 . 
     In the control circuit C 12 , a capacitance element Q 2  and a resistance element R 2  are connected in series between the power supply terminal VDD 1  and the ground terminal GND 1 . Between the NMOS transistor N 1  and a node between the capacitance element Q 2  and the resistance element R 2 , an inverter circuit B 2  and a NOR circuit A 2  are connected in series. An inverter element B 1  is connected to the gate of the PMOS transistor P 1 . 
     The output inverter circuit I 1  in the first power supply system is composed by the NMOS transistor N 1  and the PMOS transistor P 1 . Since the gate of the NMOS transistor N 1  is connected to the power supply terminal VDD 1  through the NOR circuit A 2 , inverter circuit B 2 , and resistance element R 2 , the gate of the NMOS transistor N 1  is controlled by an input signal to the NOR circuit A 2  when a normal operation is performed. 
     The gate of the PMOS transistor P 1  is connected to the input signal to the NOR circuit A 2  through the inverter circuit B 1 . Accordingly, an output signal of the output inverter circuit I 1  is controlled by the input signal to the NOR circuit A 2 , and does not affect an operation of the circuit. 
     Referring to  FIG. 23 , when the ESD stress is applied to the ground terminal GND 1  with the power supply terminal VDD 2  as a reference point, for example, electric charge is charged into the capacitance element Q 2 . Then, due to coupling of the electric charge charged in the capacitance element Q 2 , a gate potential of an NMOS transistor of the inverter circuit B 2  becomes the same as a source potential of the NMOS transistor of the inverter circuit B 2 . An output of the inverter circuit B 2  therefore goes High. 
     Since an output of the NOR circuit A 2  therefore goes Low, the gate of the NMOS transistor N 1  becomes the OFF state. A current that flows into the signal line S 1  through the NMOS transistor N 1  can be limited. The breakdown of the gate oxide film of the PMOS transistor P 2  can be thereby prevented. 
     As described above, in the eighth embodiment of the present invention, the gate of the NMOS transistor N 1  of the output inverter circuit I 1  is controlled. The current that flows into the signal line S 1  from the NMOS transistor N 1  can be thereby restrained, and the potential difference Vgs applied between the gate of the PMOS transistor P 2  of the input inverter circuit I 2  and the source of the PMOS transistor P 2  can be thereby limited within the voltage that might cause the breakdown of the gate oxide film of the PMOS transistor P 2 . As a result, the number of the protection elements can be reduced, or the protection element can be reduced in size. 
     In the eighth embodiment of the present invention, by controlling the NMOS transistor N 1  of the output inverter circuit I 1  itself, driving capability can be more enhanced as compared with the second embodiment of the present invention. 
     Ninth Embodiment 
       FIG. 24  is a diagram showing a configuration of a circuit according to a ninth embodiment of the present invention. Referring to  FIG. 24 , same reference numerals are assigned to components that are the same as those in  FIG. 1 . Below, descriptions of the same components will be omitted as necessary, and a description will be mainly directed to a difference below. 
     Referring to  FIG. 24 , the ninth embodiment includes a circuit C 13  that controls the gates of the NMOS transistor N 1  and the PMOS transistor P 1  of the output inverter circuit I 1 . 
     In the control circuit C 13 , a capacitance element Q 1  and a resistance element R 1  are connected in series between the power supply terminal VDD 1  and the ground terminal GND 1 . Between the PMOS transistor P 1  and a node between the capacitance element Q 1  and the resistance element R 1 , an inverter circuit B 2  and a NAND circuit A 1  are connected in series. A NOR circuit A 2  is connected to the gate of the NMOS transistor N 1 . Ones of input nodes of the NAND circuit A 1  and the NOR circuit A 2  are connected through the inverter circuit B 2 , and the others of the input nodes are connected in series. 
     The output inverter circuit I 1  in the first power supply system is composed by the NMOS transistor N 1  and the PMOS transistor P 1 . A circuit formed by combination of the control circuit C 13  with the output inverter circuit I 1  becomes a three-state output circuit. 
     When a normal operation is performed, the gate of the PMOS transistor P 1  is controlled by an input signal to the NAND circuit A 1  as in the seventh embodiment. 
     Since the gate of the NMOS transistor N 1  is connected to the ground terminal GND 1  through the NOR circuit and the resistance element R 1 , the gate of the NMOS transistor N 1  is controlled by an input signal to the NOR circuit A 2 . Accordingly, the NMOS transistor N 1  does not affect an operation of the circuit. 
     Referring to  FIG. 24 , when the ESD stress is applied to the power supply terminal VDD 1  with the ground terminal GND 2  as a reference point, for example, the same effect as that in the seventh embodiment is obtained. Further, when the ESD stress is applied to the ground terminal GND 1  with the power supply terminal VDD 2  as a reference point, the same effect as that in the eighth embodiment is obtained. 
     As described above, in the ninth embodiment of the present invention, by controlling the gates of the PMOS transistor P 1  and the NMOS transistor N 1  of the output inverter circuit I 1 , a current that flows from the PMOS transistor P 1  or the NMOS transistor N 1  into the signal line S 1  can be restrained. The potential difference Vgs applied to the gate of the NMOS transistor N 2  of the input inverter circuit I 2  and the source of the NMOS transistor N 2  can be limited within the voltage that might cause the breakdown of the gate oxide film of the NMOS transistor N 2 . 
     Further, the potential difference Vgs applied between the gate of the PMOS transistor P 2  and the source of the PMOS transistor P 2  can be limited within the voltage that might cause the breakdown of the gate oxide film of the PMOS transistor P 2 . As a result, the number of the protection elements can be reduced, or the protection element can be reduced in size. 
     In the ninth embodiment of the present invention, by controlling the PMOS transistor P 1  and the NMOS transistor N 1  of the output inverter circuit I 1  themselves, the ninth embodiment provides more excellent protection capability as compared with the seventh and eighth embodiments of the present invention. In addition, driving capability can be more enhanced as compared with the third embodiment of the present invention. 
     According to the present invention, when a plurality of different power supply systems are provided and an output signal of a circuit of one power supply system is received by a circuit of the other power supply systems in the form of an input signal, an arrangement that prevents a gate breakdown of the input circuit for receiving the input signal is adopted. The number of the protection elements provided in a conventional device or the like or downsizing of the protection element can be performed. Accompanying this advantage, a parasitic capacitance element of the protection element is reduced, so that an improvement in response at a high speed operation is expected. Naturally, the present invention can also be applied to a device constituted from a plurality of chips which have powers supplied from different power supply systems respectively, in addition to a configuration in which an LSI includes a plurality of power supply systems. 
     The above description was given in connection with the embodiments described above. The present invention is not limited to the configurations of the embodiments described above, and of course includes various variations and modifications that could be made by those skilled in the art within the scope of the present invention. 
     It should be noted that other objects, features and aspects of the present invention will become apparent in the entire disclosure and that modifications may be done without departing the gist and scope of the present invention as disclosed herein and claimed as appended herewith. 
     Also it should be noted that any combination of the disclosed and/or claimed elements, matters and/or items may fall under the modifications aforementioned.