Abstract:
A diode element circuit uses a junction between the base and collector of a vertical type PNP transistor as a diode, and is further designed that a reverse bias voltage is applied between base and emitter of a parasitic PNP transistor in the vertical type PNP transistor, thereby, a diode having a small leakage current and a high break down voltage is realized without necessitating an additional manufacturing process.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a diode element circuit and a switch circuit using the same and, more specifically, relates a diode element circuit formed in an IC form having a high break down voltage and a small leakage current which is suitably used for the switch circuit in an IC tester.  
           [0003]    2. Background Art  
           [0004]    As typical conventional diodes which are formed in a monolithic IC circuit and perform rectification, a schottky barrier diode which utilizes a schottky junction between a metal and a semiconductor and a PN junction diode which utilizes a PN junction between a P type semiconductor and an N type semiconductor are enumerated. Among those, the schottky barrier diode has an advantage of a high break down voltage and a disadvantage of a large leakage current in a reverse direction.  
           [0005]    The PN junction diodes include one using a PN junction between base and emitter of a transistor and one using a PN junction between base and collector of a transistor. A PN junction diode using a PN junction between base and emitter of a transistor which is generally used has an advantage of a small leakage current, but has a disadvantage of a low break down voltage. On the other hand, a PN junction diode using a PN junction between base and collector of a transistor shows a high break down voltage, but has a disadvantage of causing a leakage current to a substrate during being used in forward direction operation because of an influence of parasitic transistors around the junction.  
           [0006]    Accordingly, in order to achieve at the same time both a small leakage current and a high break down voltage, a variety of measures have been proposed which devise material and structure of the diodes. As examples thereof, with regard to the schottky barrier diodes, JP-A-9-199733 (1997) is enumerated and with regard to the PN junction diodes JP-A-7-66433 (1995) is enumerated.  
           [0007]    However, these measures bring about problems of requiring a special additional manufacturing process and difficulty of using a conventional manufacturing method and manufacturing device as they are. Further, since these measures complicate the manufacturing process, which causes problems of reducing the yield and increasing the manufacturing cost.  
         SUMMARY OF THE INVENTION  
         [0008]    An object of the present invention is to resolve the above conventional problems and to provide a diode element circuit which requires no additional manufacturing process and realizes a small leakage current and a high break down voltage.  
           [0009]    Another object of the present invention is to provide a switch circuit which achieves at the same time both a small leakage current and a high break down voltage and is, in particular, suitable for use in an IC tester.  
           [0010]    A diode element circuit and a switch circuit using the same according to the present invention which achieve the above objects are characterized, in that in a diode element circuit formed in an IC which includes an anode electrode and a cathode electrode and uses a diode of a PN junction between base and collector of a PNP transistor, the PNP transistor is a vertical type transistor formed in a well region, and a voltage drop element which is connected between the collector of the transistor and the anode electrode is included, wherein the base of the transistor is connected to the cathode electrode and the well region is connected to the anode electrode.  
           [0011]    As indicated in the above structure, the PN junction between base and collector of the PNP transistor which shows a small leakage current in the reverse direction is used, and in order to reduce a leakage current to a substrate due to parasitic transistors in the PNP transistor the voltage drop element is inserted between the anode electrode and the collector thereof as a bias circuit. Thereby, the collector side is placed in a lower potential than the well region to induce a potential difference therebetween. With this measure, the emitter potential of a PNP parasitic transistor is reduced which is constituted by the collector region of the PNP transistors serving as an emitter and the well region thereof serving as a base.  
           [0012]    As a result, since a reverse bias is applied between the base and emitter of the parasitic transistor, when a forward current is flown and even when a current between the base and collector of the PNP transistor increases and a voltage drop is generated in the well region, a current value where the parasitic transistor is turned ON can be increased. Accordingly, if the diode is operated under a condition below such large current value, the leakage current from the collector region to the substrate is reduced to substantially zero.  
           [0013]    As a result, the diode element circuit according to the present invention can realize a diode having a small leakage current and a high break down voltage in a monolithic IC circuit without adding a manufacturing process. Thereby, the switch circuit constituted by the diode element circuit likely enjoys the above advantages. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a circuit diagram showing one embodiment of a diode element circuit with a voltage drop means and a PNP transistor according to the present invention;  
         [0015]    [0015]FIG. 2 is an explanatory view of a cross sectional structure of a vertical type PNP transistor used in a general monolithic IC circuit;  
         [0016]    [0016]FIG. 3 is an explanatory view of an embodiment of a switch circuit according to the present invention;  
         [0017]    [0017]FIG. 4 is an explanatory view of another embodiment of a switch circuit according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    Hereinbelow, an embodiment according to the present invention will be explained with reference to FIGS. 1 and 2.  
         [0019]    [0019]FIG. 1 is a circuit diagram showing one embodiment of a diode element circuit with a voltage drop means and a PNP transistor according to the present invention; and FIG. 2 is a cross sectional structural view of a vertical type PNP transistor used in a general monolithic IC circuit.  
         [0020]    As shown in FIG. 1, a diode element circuit  3  is constituted by a PNP transistor  1  serving as a rectification element and a voltage drop means  2 . An emitter terminal  101  and a cathode terminal  103  of the PNP transistor  1  are connected each other and are used as a cathode electrode  5  for the diode element circuit  3 . Further, a collector terminal  102  of the PNP transistor  1  is connected to one terminal of the voltage drop means  2  and a well terminal  104  of the PNP transistor  1  is connected to the other terminal of the voltage drop means  2 , and further, the well terminal  104  serves as an anode electrode  4  for the diode element circuit  3 .  
         [0021]    With this structure, a well region  6  which is connected to the well terminal  104  and is shown by a dotted line frame is set at a higher potential than a collector region (see FIG. 2) under a forward bias condition.  
         [0022]    As illustrated in FIG. 2, the vertical type PNP transistor  1  as used in a general monolithic IC circuit is structured by alternatively sandwiching a P type semiconductor and an N type semiconductor, there exist parasitic transistor (NPN transistor)  11  and another parasitic transistor (PNP transistor)  12  other than the main body of the PNP transistor  10 .  
         [0023]    Among these parasitic transistors, when the parasitic transistor  11  is rendered conductive, a current flows from the well terminal  104  to the base terminal  103 . As a result, when constituting a diode by making use of the junction between the base and collector of the PNP transistor  1 , a current from the well terminal  104  is added to a current from the collector terminal  102  serving as the anode side for the current flowing into the base terminal serving as the cathode terminal. For this reason, when constituting a diode by making use of the junction between the base and collector of the PNP transistor  1 , a positive electrode (anode terminal) is constituted by connecting the well terminal  104  and the collector terminal  102 .  
         [0024]    Further, when the resistance of the well is large, a voltage drop is caused inside the well due to the current flowing through the well, therefore, if the well terminal  104  and the collector terminal  102  are simply connected as explained above, the well potential lowers below the collector potential near the parasitic transistor  12  to render the parasitic transistor  12  conductive.  
         [0025]    As a result, a current flows from the base terminal  103  serving as the cathode side to a substrate  105  to reduce the forward direction current and further, because of substrate potential rise an erroneous operation is caused in the surrounding circuits. Therefore, practically such circuit can not be used. For this reason, the potential of an emitter  121  (which corresponds to the collector region  7  of the PNP transistor main body  10 ) of the parasitic transistor  12  in the PNP transistor  1  is lowered by the voltage drop means  2  below that of the base  123  of the parasitic transistor  12 , thereby, an application of a forward bias voltage (usually more than 0.7 V) between the base and emitter of the parasitic transistor  12  is prevented, which will be explained later in detail. Because the conduction of the parasitic transistor  12  is prevented in this way, the leakage current to the substrate which is caused during use of the diode in its forward direction can be reduced.  
         [0026]    Now, an operation of the diode element circuit  3  will be explained with reference to FIGS. 1 and 2.  
         [0027]    Since in a general PNP transistor, the collector resistance is high, the potential of the cathode electrode  5  is higher than that of the anode electrode  4  in the above structure and under a condition where a reverse direction voltage is applied, a substantial part of the applied voltage is applied to the junction between the base and collector of the PNP transistor  1 .  
         [0028]    Accordingly, the characteristic of the diode element circuit  3  at the time when a reverse direction voltage is applied is determined by the characteristic of the junction between the base and collector thereof (terminals  103  and  102 ), in that shows a large reverse break down voltage as well as a small leakage current in reverse direction.  
         [0029]    On the other hand, under a condition where a forward direction voltage is applied in which the potential at the anode electrode  4  is higher than the potential at the cathode electrode  5 , the potential at the collector terminal  102  of the PNP transistor  1  is lowered below the potential of the well terminal  104  through the voltage drop means  2  by the voltage drop therein, therefore, a reverse bias voltage is applied between the base and emitter (terminals  121  and  123 ) of the parasitic transistor  12 . Thus, the conduction of the parasitic transistor  12  is prevented and the leakage current from the anode electrode  4  to the substrate  105  is reduced.  
         [0030]    At this moment, a reverse bias voltage corresponding to the voltage drop in the voltage drop means  2  is applied between the well region  6  and the collector region  7 .  
         [0031]    Further, when a current is flown from the anode electrode  4  to the cathode electrode  5 , in that a current is flown in forward direction, a voltage drop is caused by the well resistance in a passage from the well terminal  104  of the PNP transistor  1  to the base terminal  123  of the parasitic transistor  12 . With the voltage caused the parasitic transistor  12  is usually rendered conductive to cause a leakage current to the substrate  105 , however, because of the provision of the voltage drop means  2 , the parasitic transistor  12  is reversely biased and the limit current where the parasitic transistor  12  is rendered conductive can be increased.  
         [0032]    Further, as the voltage drop means  2  other than the resistor such as a schottky barrier diode and PN junction diode formed by the junction between the base and emitter of an NPN and PNP transistor can be used. Still further, a plurality of these elements can be used therefor while connecting in combination.  
         [0033]    [0033]FIG. 3 is a circuit diagram showing an embodiment of a switch circuit, in that an analogue switch for switching an input and output in an IC tester, in which the diode element circuit  3  according to the present invention is used. A switch circuit  30  for switching an input and output for an IC tester is constituted by four diode element circuits  32 ˜ 35  each corresponding to the diode element circuit  3 , current sources  301  and  311  and switches  302  and  312  as shown in FIG. 3.  
         [0034]    The four diode element circuits  32 ˜ 35  are respectively formed by a transistor  32   a  and a diode  32   b  serving as a voltage drop means, a transistor  33   a  and a diode  33   b  serving as a voltage drop means, a transistor  34   a  and a diode  34   b  serving as a voltage drop means and a transistor  35   a  and a diode  35   b  serving as a voltage drop means, and constitute a diode bridge circuit  31  as shown in FIG. 3. When the entirety of these constitutes an analogue switch, an input terminal  321  for the analogue switch is connected to the junction between the cathode electrode of the diode element circuit  34  and the anode electrode of the diode element circuit  35 , and an output terminal  322  for the analogue switch is connected to the junction between the cathode electrode of the diode element circuit  32  and the anode electrode of the diode element circuit  33 . As the remaining two terminals for the analogue switch into which a biasing current is flown, the junction between the anode electrode of the diode element circuit  32  and the anode electrode of the diode element circuit  34  constitutes an upstream side terminal, and the junction between the cathode electrode of the diode element circuit  33  and the cathode electrode of the diode element circuit  35  constitutes a downstream side terminal. The upstream side terminal is connected via the switch  302  to the current source  301  for current discharge, and the downstream side terminal is connected via the switch  312  to the current source  311  for current sink.  
         [0035]    The current sources  301  and  311  are for flowing a bias current to the diode bridge circuit  31 , and the current source  301  is connected to a power source line Vcc and causes to flow a current received from the line to the diode bridge circuit.  
         [0036]    The current source  311  is connected to a power source line V EE  at negative side and causes to sink the current flowing out from the diode bridge circuit  31  into the line.  
         [0037]    Herein, the diodes  32   b˜   35   b  are diodes which are formed at the same time in the same well region  6  and which can be formed as vertical type transistors as shown in FIG. 2, as other type transistors or as separate diodes formed separately in the well region  6 . Now, an operation of the switch circuit  30  will be explained. Under a condition when the switches  302  and  312  are connected, by means of the upper current source  301  and the lower current source  311  a bias current is flown into the diode bridge circuit  31 , therefore, the bridge circuit  31  is placed in an electrically balanced condition and the voltage at the input terminal  321  appears at the output terminal  322 . Further, under a condition when the switches  302  and  312  are interrupted, since no bias current flows into the diode bridge circuit  31 , the respective diodes are placed in an OFF condition, thus the output terminal  322  gives a high resistance. As will be seen from the above, the switch circuit  30  functions as an analogue switch which can perform switching of high/low impedance between the input terminal  321  and the output terminal  322  through connection/interruption of the switches  302  and  321 .  
         [0038]    Further, when the output terminal  322  is connected to an arbitrary device to be inspected (DUT), the switch circuit  30  can be utilized as a load current supply circuit (a current load circuit) for an IC tester.  
         [0039]    More specifically, under an ON condition of the switches  302  and  321 , when the voltage of the output terminal  322  connected to the output terminal of DUT, is lower than the voltage of the input terminal  321 , a current flows out from the current source  301  to the output terminal of DUT, and further, when the voltage of the output terminal  322  is higher than the voltage of the input terminal  321 , the current source  311  performs an operation of sinking a flow out current from the output terminal of DUT via the output terminal  322 . Further, in this instance, the current sources  301  and  311  can be formed as a constant current source.  
         [0040]    Herein, the respective diode element circuits  32 ˜ 35  are constituted likely as the diode element circuit  3  and are provided with characteristics of high break down and a low leakage current. Therefore, a high voltage can be applied between the input and output terminals of the switch circuit  30 , the switch circuit is suitable for a switch circuit for an IC tester and further, the switch circuit shows a characteristic of a small leakage current at the time of switch interruption.  
         [0041]    [0041]FIG. 4 is a circuit diagram of another embodiment of a switch circuit for switching input and output in an IC tester in which a diode element circuit  3  according to the present invention is used.  
         [0042]    A switch circuit  40  as shown in FIG. 4 is an example in which the switches  302  and  312  are respectively formed by change-over switches  303  and  313 . Further, the current sources  301  and  311  are constituted by variable current sources  301   a  and  311   a , so that the current values thereof can be set separately at predetermined constant current values through external control signal CONT, thereby, the current load condition to DUT can be varied. Other structure in FIG. 4 are the same as those in FIG. 3.  
         [0043]    The change-over switches  303  and  313  are for changing over the current passages of the two variable current sources  301   a  and  311   a  between the side of the diode bridge  31  and the side of short circuiting (ground GN). The respective single pole sides in the respective single pole double throw type change-over switch are connected to the respective variable current sources  301   a  and  311   a  and each one of the double throws is connected to each one terminal of the diode bridge  31  and the other of the double throws are connected to the ground.  
         [0044]    When both passages are changed over toward the diode bridge  31 , like the switch circuit as shown in FIG. 3, a bias current is flown into the diode bridge by the upper and lower current sources and a voltage at the input terminal  321  appears at the output terminal  322 . Further, when the both current passages are changed over toward the short circuiting sides, no bias current flows through the diode bridge, therefore, the respective diodes are rendered into OFF condition to give a high resistance at the output terminal  322 . The other functions and advantages than the above in the switch circuit  40  are the same as those in the switch circuit  30 .