Abstract:
An electrostatic protection circuit includes a first impurity region, a second impurity region, a first electrode, a third impurity region, a fourth impurity region, a second electrode, a fifth impurity region, a sixth impurity region, a third electrode, a gate insulating film, and a fourth electrode.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present invention contains subject matter related to Japanese Patent Applications JP 2007-264702, and JP 2008-059502 all filed with the Japan Patent Office on Oct. 10, 2007, and Mar. 10, 2008, respectively the entire contents of which being incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electrostatic protection circuit that diverts a surge voltage applied to a signal line away from the circuit that should be protected. 
     2. Description of Related Art 
     In general, a semiconductor integrated circuit (IC) is susceptible to a surge voltage arising due to electrostatic discharge (ESD), and is easily broken by the surge voltage. The surge voltage frequently arises when a human (user), who can store therein static electricity of about 2000 V, treats an IC without taking measures against static electricity. 
     Typically, in order to protect an IC from the surge voltage, an electrostatic protection circuit that diverts the surge voltage away from the circuit that should be protected is provided in the IC. For example, a signal line and a ground potential line of the IC are connected to each other via a diode. In this case, the diode is turned on when the surge voltage is applied to the signal line, and thus the surge voltage can be diverted into the ground potential line. Alternatively, it is also possible to provide a field effect transistor (FET) between the signal line and the ground potential line instead of the diode and control the FET in the gate-controlled drain avalanche breakdown mode, to thereby divert the surge voltage into the ground potential line. 
     Furthermore, it is also possible to divert the surge voltage away from the circuit that should be protected by using e.g. metal-oxide-semiconductor (MOS) transistors.  FIG. 10  is a diagram showing one example of the circuit arrangement of the electrostatic protection circuit employing MOS transistors. An electrostatic protection circuit  100  shown in  FIG. 10  includes an n-type MOS transistor  110  and a p-type MOS transistor  120 . The n-type MOS transistor  110  has a gate, source, drain, and p-type semiconductor substrate. The gate, source, and p-type semiconductor substrate of the n-type MOS transistor  110  are connected to a ground line L 3 , and the drain of the n-type MOS transistor  110  is connected to a signal line L 1 . The p-type MOS transistor  120  has a gate, source, drain, and n-type semiconductor substrate. The gate, source, and n-type semiconductor substrate of the p-type MOS transistor  120  are connected to a power supply line L 2 , and the drain of the p-type MOS transistor  120  is connected to the signal line L 1 . Due to this arrangement, the electrostatic protection circuit  100  does not operate when a signal voltage is applied to the signal line. On the other hand, when a surge voltage is applied to the signal line, the p-type MOS transistor  120  is turned on, or the breakdown of the n-type MOS transistor  110  is caused, depending on the magnitude of the surge voltage. This operation makes it possible to divert the surge voltage away from the circuit that should be protected (refer to Japanese Patent Laid-open No. 2003-133434). 
     SUMMARY OF THE INVENTION 
     A MOS transistor for high-breakdown-voltage driving is often used for the above-described electrostatic protection circuit  100 . For this MOS transistor for high-breakdown-voltage driving, the breakdown voltage Vb (see  FIG. 11 ) thereof is set high so that the MOS transistor can withstand high voltage. Therefore, the following problem will occur when the MOS transistor for high-breakdown-voltage driving is used for the electrostatic protection circuit  100 . Specifically, when a signal voltage is applied to the signal line, the temperature surpasses the allowable temperature at the moment of snap-back (see the area surrounded by the dashed line in  FIG. 11 ) because the heat generation amount is large although the current is small, and thus the MOS transistor itself in the electrostatic protection circuit  100  is broken. 
     There is a need for the present invention to provide an electrostatic protection circuit that is prevented from being broken due to a surge voltage and a semiconductor device including the electrostatic protection circuit. 
     According to an embodiment of the present invention, there is provided a first electrostatic protection circuit including the following components (A) to (K). According to another embodiment of the present invention, there is provided a semiconductor device that has over a semiconductor substrate the first electrostatic protection circuit including the following components (A) to (K).
         (A) a first impurity region configured to contain an impurity of a first conductivity type,   (B) a second impurity region configured to be formed on the surface of the first impurity region and contain an impurity of the first conductivity type with concentration higher than the concentration of the impurity of the first conductivity type in the first impurity region,   (C) a first electrode configured to be formed on the surface of the second impurity region and be electrically connected to a signal line,   (D) a third impurity region configured to be formed on the surface of the first impurity region and contain an impurity of a second conductivity type different from the first conductivity type,   (E) a fourth impurity region configured to be formed on the surface of the third impurity region and contain an impurity of the second conductivity type with concentration higher than the concentration of the impurity of the second conductivity type in the third impurity region,   (F) a second electrode configured to be formed on the surface of the fourth impurity region and be electrically connected to the signal line,   (G) a fifth impurity region configured to be formed in an area adjacent to the third impurity region, of a surface area of the first impurity region, and contain an impurity of the second conductivity type,   (H) a sixth impurity region configured to be formed on the surface of the fifth impurity region and contain an impurity of the first conductivity type,   (I) a third electrode configured to be formed on the surface of the sixth impurity region and be electrically connected to a reference potential line,   (J) a gate insulating film configured to be formed at least on the part of the surface of the first impurity region, between the third impurity region and the fifth impurity region, and   (K) a fourth electrode configured to be formed on the surface of the gate insulating film and be electrically connected to the reference potential line when a surge voltage is applied to the signal line.       

     In the first electrostatic protection circuit and the semiconductor device according to the embodiments of the present invention, a bipolar transistor is formed by the first impurity region, the fifth impurity region, and the sixth impurity region, and a MOS transistor is formed by the first impurity region, the third impurity region, the fifth impurity region, the gate insulating film, and the fourth electrode. The fifth impurity region serves as both the base of the bipolar transistor and the drain or source of the MOS transistor, and therefore it can be said that the base of the bipolar transistor and the drain or source of the MOS transistor are electrically connected to each other. Due to this feature, in the case in which a surge voltage is applied to the signal line so as to be transmitted to the first impurity region and the third impurity region and thus the voltage of the first impurity region and the third impurity region becomes the surge voltage, when the third electrode and the fourth electrode are electrically connected to the reference potential line, a channel is formed in the partial portion of the first impurity region directly beneath the fourth electrode, so that the surge voltage of the third impurity region is transmitted to the fifth impurity region via the channel. When the surge voltage is thus transmitted to the fifth impurity region, the junction between the fifth impurity region and the sixth impurity region electrically connected to the reference potential line is forward-biased. Furthermore, because the voltage of the first impurity region is the surge voltage, the bipolar transistor starts its bipolar operation, so that the surge voltage is discharged from the first impurity region to the sixth impurity region via the fifth impurity region. 
     According to yet another embodiment of the present invention, there is provided a second electrostatic protection circuit including a bipolar transistor and a MOS transistor. The bipolar transistor has a base, a collector electrically connected to a signal line, and an emitter electrically connected to a reference potential line. The MOS transistor has a gate, a source, and a drain. The gate is electrically connected to the reference potential line when a surge voltage is applied to the signal line. One of the source and the drain is electrically connected to the signal line, and the other thereof is electrically connected to the base. 
     In the second electrostatic protection circuit according to this embodiment of the present invention, the base of the bipolar transistor and the source or drain of the MOS transistor are electrically connected to each other. Due to this feature, in the case in which a surge voltage is applied to the signal line so as to be transmitted to the collector and the source or drain electrically connected to the signal line and thus the voltage of the collector and the source becomes the surge voltage, when the emitter is electrically connected to the reference potential line, a channel is formed in the MOS transistor, so that the surge voltage of the source or drain electrically connected to the signal line is transmitted to the base via the channel. When the surge voltage is thus transmitted to the base, the junction between the base and the emitter electrically connected to the reference potential line is forward-biased. Furthermore, because the voltage of the collector is the surge voltage, the bipolar transistor starts its bipolar operation, so that the surge voltage is discharged from the collector to the emitter via the base. 
     In the first electrostatic protection circuit and the semiconductor device according to the embodiments of the present invention, the fifth impurity region is so designed as to serve as both the base of the bipolar transistor and the drain or source of the MOS transistor. Thus, the trigger of the bipolar operation at the time of the electrostatic protection can be controlled based on the threshold voltage of the MOS transistor. This can start the electrostatic protection operation with low voltage, which can prevent breakdown of the electrostatic protection circuit itself due to a surge voltage. 
     In the second electrostatic protection circuit according to the embodiment of the present invention, the base of the bipolar transistor and the drain or source of the MOS transistor are electrically connected to each other. Thus, the trigger of the bipolar operation at the time of the electrostatic protection can be controlled based on the threshold voltage of the MOS transistor. This can start the electrostatic protection operation with low voltage, which can prevent breakdown of the electrostatic protection circuit itself due to a surge voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional structural diagram of an electrostatic protection circuit according to a first embodiment of the present invention; 
         FIG. 2  is a circuit arrangement diagram of a control circuit of  FIG. 1 ; 
         FIG. 3  is an equivalent circuit diagram of a bipolar transistor and MOS transistors of  FIG. 1 ; 
         FIG. 4  is a circuit arrangement diagram for explaining operation when a surge voltage is applied to the electrostatic protection circuit of  FIG. 1 ; 
         FIG. 5  is a circuit arrangement diagram for explaining operation when a signal voltage is applied to the electrostatic protection circuit of  FIG. 1 ; 
         FIG. 6  is a characteristic diagram showing one example of the current-voltage characteristic of the electrostatic protection circuit of  FIG. 1 ; 
         FIG. 7  is a sectional structural diagram of an electrostatic protection circuit according to a second embodiment of the present invention; 
         FIG. 8  is a circuit arrangement diagram of one modification example of the electrostatic protection circuit of  FIG. 1  or  FIG. 7 ; 
         FIG. 9  is a circuit arrangement diagram of another modification example of the electrostatic protection circuit of  FIG. 1  or  FIG. 7 ; 
         FIG. 10  is a circuit arrangement diagram of a related-art electrostatic protection circuit; and 
         FIG. 11  is a characteristic diagram showing one example of the current-voltage characteristic of the related-art electrostatic protection circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described in detail below with reference to the drawings. 
     First Embodiment 
       FIG. 1  is a diagram showing the sectional structure and the connection relationship of an electrostatic protection circuit  1  according to a first embodiment of the present invention. The electrostatic protection circuit  1  of the present embodiment is formed over a silicon substrate together with an integrated circuit in a semiconductor device, and is provided and connected between a signal line L 1  electrically connected to the integrated circuit and a ground line L 3  (reference potential line). 
     As shown in  FIG. 1 , this electrostatic protection circuit  1  includes, over a substrate  10 , one bipolar transistor  20 , two MOS transistors  30 , and a control circuit  40 . 
     The semiconductor substrate  10  is e.g. a silicon substrate containing a p-type impurity. 
     The bipolar transistor  20  has a collector region  21  formed to have a large depth around the surface of the semiconductor substrate  10 , a base region  22  formed on a part of the surface of the collector region  21 , and an emitter region  23  formed on a part of the surface of the base region  22 . 
     The collector region  21  contains e.g. an impurity of a conductivity type (n-type) different from that of the semiconductor substrate  10 . The base region  22  contains e.g. an impurity of the same conductivity type (p-type) as that of the semiconductor substrate  10 . The emitter region  23  contains e.g. an impurity of a conductivity type (n-type) different from that of the semiconductor substrate  10 , with impurity concentration higher than that of the collector region  21 . 
     At two places on the surface of the collector region  21 , first collector potential extraction regions  24  are formed. The first collector potential extraction region  24  contains an impurity of the same conductivity type as that of the collector region  21 , with impurity concentration higher than that of the collector region  21 , and is electrically connected to the collector region  21 . Furthermore, second collector potential extraction regions  25  are formed on the surfaces of the respective first collector potential extraction regions  24 . The second collector potential extraction region  25  contains an impurity of the same conductivity type as that of the first collector potential extraction region  24 , with impurity concentration higher than that of the first collector potential extraction region  24 , and is electrically connected to the first collector potential extraction region  24 . Over the surfaces of the respective second collector potential extraction regions  25 , collector electrodes  27  are formed with the intermediary of vias  26  therebetween. The via  26  and the collector electrode  27  are composed of e.g. a metal such as aluminum (Al) and are electrically connected to the second collector potential extraction region  25 . Therefore, the collector electrode  27  is electrically connected to the collector region  21  via the via  26 , the second collector potential extraction region  25 , and the first collector potential extraction region  24 . The collector electrode  27  is electrically connected also to the signal line L 1 . 
     Over the surface of the emitter region  23 , an emitter electrode  28  is formed with the intermediary of the via  26  therebetween. The emitter electrode  28  is composed of e.g. a metal such as aluminum (Al) and is electrically connected to the emitter region  23  via the via  26 . In addition, the emitter electrode  28  is always electrically connected to the ground line L 3 . 
     Two MOS transistors  30  are formed in areas adjacent to the bipolar transistor  20 , of the surface area of the collector region  21 . Each MOS transistor  30  has a source region  31  and a drain region formed on the surface of the collector region  21 , a gate insulating film  32  formed at least on the part of the surface of the collector region  21 , between the source region  31  and the drain region, and a gate electrode  33  formed on the gate insulating film  32 .  FIG. 1  shows an example in which the gate insulating film  32  is formed over a part of the surface of the source region  31 , a part of the surface of the drain region, and the part of the surface of the collector region  21 , between the source region  31  and the drain region. 
     The source region  31  contains e.g. an impurity of the same conductivity type (p-type) as that of the semiconductor substrate  10 . The drain region contains e.g. an impurity of the same conductivity type (p-type) as that of the semiconductor substrate  10 . The drain region is electrically connected to the base region  22  of the bipolar transistor  20 , or is formed monolithically with (or is used also as) the base region  22  of the bipolar transistor  20 . This drain region (the base region  22 ) is surrounded by regions of the different conductivity type (the collector region  21  and the emitter region  23 ) and an insulating layer  52  to be described later, and is not electrically connected to other regions but electrically floating. The gate insulating film  32  is composed of e.g. silicon oxide (SiO 2 ). The gate electrode  33  has e.g. a two-layer structure obtained by sequentially stacking, from the gate insulating film side, a poly-silicon layer containing an impurity of the same conductivity type (p-type) as that of the semiconductor substrate  10  and a silicide layer. 
     Source potential extraction regions  34  are formed on partial portions of the surfaces of the respective source regions  31 . The source potential extraction region  34  contains an impurity of the same conductivity type as that of the source region  31 , with impurity concentration higher than that of the source region  31 , and is electrically connected to the source region  31 . Over the surfaces of the respective source potential extraction regions  34 , source electrodes  35  are formed with the intermediary of the vias  26  therebetween. The source electrode  35  is composed of e.g. a metal such as aluminum (Al) and is electrically connected to the source potential extraction region  34 . Therefore, the source electrode  35  is electrically connected to the source region  31  via the via  26  and the source potential extraction region  34 . The source electrode  35  is electrically connected also to the signal line L 1 . 
     Between the source potential extraction region  34  and the second collector potential extraction region  25 , an element isolation layer  50  that isolates these regions from each other is provided. Between the element including one bipolar transistor  20  and two MOS transistors  30  and another element formed over the semiconductor substrate  10 , an element isolation layer  51  that isolates these elements from each other is provided. Moreover, the insulating layer  52  is formed on the part of the surface of the semiconductor substrate  10  on which the via  26  is not formed (specifically, on the partial portions exposed at the surface of the semiconductor substrate  10 , of the collector region  21 , the base region  22 , the emitter region  23 , the second collector potential extraction regions  25 , the source regions  31 , and the source potential extraction regions  34 ). 
     The element isolation layer  50  has e.g. a shallow trench isolation (STI) structure or a local-oxidation-of-silicon (LOCOS) structure, and the upper surface thereof is at a position slightly higher than that of the upper surface of the semiconductor substrate  10 . The element isolation layer  51  has a lower isolation layer  51 A and an upper isolation layer  51 B. The lower isolation layer  51 A contains e.g. an impurity of a conductivity type different from that of the collector region  21 . The upper isolation layer  51 B has e.g. an STI structure or a LOCOS structure, and the upper surface thereof is at a position slightly higher than that of the upper surface of the semiconductor substrate  10 . The insulating layer  52  is composed of e.g. silicon oxide (SiO 2 ). 
     The control circuit  40  serves to electrically connect the gate electrode  33  and the ground line L 3  to each other when a surge voltage is applied to the signal line L 1 , and electrically connect the gate electrode  33  and the ground line L 1  to each other when a signal voltage is applied to the signal line L 1 . As shown in  FIG. 2 , this control circuit  40  includes e.g. two p-type MOS transistors Tr 1  and Tr 2 , two n-type MOS transistors Tr 3  and Tr 4 , a resistance element R, and a capacitance element C. 
     Each of the p-type MOS transistors Tr 1  and Tr 2  has a gate, source, drain, and n-type well (not shown) formed over the semiconductor substrate. Each of the n-type MOS transistors Tr 3  and Tr 4  has a gate, source, drain, and p-type well (not shown) formed over the semiconductor substrate. 
     For the p-type MOS transistor Tr 1 , the source and the n-type well are connected to the collector electrode  27  and the source electrode  35 , the gate is connected to the gate of the n-type MOS transistor Tr 3 , and the drain is connected to the drain of the n-type MOS transistor Tr 3 . For the n-type MOS transistor Tr 3 , the source and the p-type well are connected to the emitter electrode  28 , the gate is connected to the gate of the p-type MOS transistor Tr 1  as described above, and the drain is connected to the drain of the p-type MOS transistor Tr 1  as described above. A connecting node P 1  between the gate of the p-type MOS transistor Tr 1  and the gate of the n-type MOS transistor Tr 3  is connected to a connecting node P 0  of series connection between the resistance element R and the capacitance element C. 
     For the p-type MOS transistor Tr 2 , the source and the n-type well are connected to the collector electrode  27  and the source electrode  35 , the gate is connected to the gate of the n-type MOS transistor Tr 4 , and the drain is connected to the drain of the n-type MOS transistor Tr 4 . For the n-type MOS transistor Tr 4 , the source and the p-type well are connected to the emitter electrode  28 , the gate is connected to the gate of the p-type MOS transistor Tr 2  as described above, and the drain is connected to the drain of the p-type MOS transistor Tr 2  as described above. A connecting node P 3  between the gate of the p-type MOS transistor Tr 2  and the gate of the n-type MOS transistor Tr 4  is connected to a connecting node P 2  between the drain of the p-type MOS transistor Tr 1  and the drain of the n-type MOS transistor Tr 3 . A connecting node P 4  between the drain of the p-type MOS transistor Tr 2  and the drain of the n-type MOS transistor Tr 4  is connected to the gate electrode  33 . 
     One end of the resistance element R is connected to the collector electrode  27  and the source electrode  35 , and the other end of the resistance element R is connected to the connecting node P 0 . One end of the capacitance element C is connected to the connecting node P 0 , and the other end of the capacitance element C is connected to the emitter electrode  28 . 
     In the electrostatic protection circuit  1  of the present embodiment, one bipolar transistor  20  and two MOS transistors  30  shown in  FIG. 1  can be represented by e.g. an equivalent circuit shown in  FIG. 3 . In this equivalent circuit, numeral  30 A denotes a bipolar transistor composed of the source region  31  of the MOS transistor  30 , the partial portion of the collector region  21  directly beneath the gate electrode  33  (so-called channel body), and the drain region (the base region  22 ). 
     As is apparent also from this equivalent circuit, in the present embodiment, the base region  22  of the bipolar transistor  20  and the drain region of the MOS transistor  30  are electrically connected to each other, and the drain region (the base region  22 ) is electrically floating. 
     Due to this structure, when a surge voltage V 1  is applied to the signal line L 1  as shown in  FIG. 4 , the surge voltage V 1  is transmitted to the collector region  21  and the source region  31 , so that the voltage of the collector region  21  and the source region  31  becomes the surge voltage V 1 . At this time, in the control circuit  40 , the surge voltage V 1 , which rises up rapidly, is input before charging of the capacitance element C. Therefore, the gate potential of the p-type MOS transistor Tr 1  is at Low, and thus the MOS transistor Tr 1  is in the on-state. On the other hand, the n-type MOS transistor Tr 3  is in the off-state, and therefore the output of the n-type MOS transistor Tr 3  is at High. Thus, the p-type MOS transistor Tr 2  is in the off-state, and the n-type MOS transistor Tr 4  is in the on-state. Therefore, the output of the n-type MOS transistor Tr 4  is at Low. As a result, the gate electrode  33  of the MOS transistor  30  is electrically connected to the ground line L 3  via the n-type MOS transistor Tr 4 . Furthermore, because the emitter electrode  28  is also electrically connected to the ground line L 3 , a channel is formed in the partial portion of the collector region  21  directly beneath the gate electrode  33  (channel body), so that the surge voltage V 1  of the source region  31  is transmitted to the base region  22  via the channel. When the surge voltage V 1  is thus transmitted to the base region  22 , the junction between the base region  22  and the emitter region  23  electrically connected to the ground line L 3  is forward-biased. In addition, because the voltage of the collector region  21  is the surge voltage V 1 , the bipolar transistor  20  starts its bipolar operation, so that the surge voltage V 1  is discharged from the collector region  21  to the ground line L 3  via the base region  22 , the emitter region  23 , and the emitter electrode  28 . Consequently, the surge voltage V 1  does not transmit in the signal line L 1  but is diverted into the ground line L 3  via the electrostatic protection circuit  1 . 
     On the other hand, when a signal voltage V 0  is applied to the signal line L 1  as shown in  FIG. 5 , the capacitance element C is charged in the control circuit  40 . Therefore, the gate potential of the p-type MOS transistor Tr 1  is at High, and thus the MOS transistor Tr 1  is in the off-state. On the other hand, the n-type MOS transistor Tr 3  is in the on-state, and therefore the output of the n-type MOS transistor Tr 3  is at Low. Thus, the p-type MOS transistor Tr 2  is in the on-state, and the n-type MOS transistor Tr 4  is in the off-state. Therefore, the output of the n-type MOS transistor Tr 4  is at High. As a result, the gate electrode  33  of the MOS transistor  30  is not electrically connected to the ground line L 3  but electrically floating. Thus, the electrostatic protection circuit  1  does not operate, but the signal voltage V 0  transmits in the signal line L 1 , so that the integrated circuit (not shown) connected to the signal line L 1  operates. 
     In this manner, in the present embodiment, the base region  22  is so designed as to serve as both the base of the bipolar transistor  20  and the drain of the MOS transistor  30 . Thus, the trigger of the bipolar operation at the time of the electrostatic protection can be controlled based on the threshold voltage of the MOS transistor  30 . Due to this feature, the electrostatic protection operation can be started even when the voltage Vd between the signal line L 1  and the ground line L 3  is low (e.g. 0.3 V) as shown in  FIG. 6 , which allows prevention of the breakdown of the electrostatic protection circuit  1  itself due to the surge voltage V 1 . 
     Furthermore, the internal impedance at the time of the electrostatic protection operation is very low. Therefore, even when static electricity of high voltage is applied, the voltage Vd can be suppressed to as low as about 10 V, and thus low power consumption can be realized. This allows suppression of the heat generation of the electrostatic protection circuit  1 , which greatly enhances the electrostatic protection resistance. Moreover, as shown in  FIG. 6 , the resistance can be maintained for large current of up to about 6.5 A. Thus, even when a high voltage of about 10400 V is applied in the human body model or a high voltage of about 520 V is applied in the machine model, the resistance can be maintained, and hence the electrostatic protection resistance is extremely excellent. 
     Second Embodiment 
       FIG. 7  is a diagram showing the sectional structure and the connection relationship of an electrostatic protection circuit  2  according to a second embodiment of the present invention. Similarly to the electrostatic protection circuit  1  of the above-described embodiment, the electrostatic protection circuit  2  of the present embodiment is formed over a silicon substrate together with an integrated circuit, and is provided and connected between the signal line L 1  electrically connected to the integrated circuit and the ground line L 3  (reference potential line). 
     As shown in  FIG. 7 , this electrostatic protection circuit  2  is different from the electrostatic protection circuit  1  of the above-described embodiment, mainly in that the base region  22  and the source region  31  of the above-described embodiment are provided with a pillar structure  60  in the electrostatic protection circuit  2 . Furthermore, the electrostatic protection circuit  2  does not include the second collector potential extraction region  25  on the surface of the first collector potential extraction region  24  but includes a source potential extraction region  29  adjacent to the first collector potential extraction region  24 . Also in this feature, the electrostatic protection circuit  2  is different from the electrostatic protection circuit  1  of the above-described embodiment, which includes the second collector potential extraction region  25  on the surface of the first collector potential extraction region  24  and does not include the source potential extraction region  29 . In the following, the differences from the above-described embodiment will be mainly described, and the description of the common points of these embodiments is omitted according to need. 
     As shown in  FIG. 7 , this electrostatic protection circuit  2  includes two bipolar transistors  20 , two MOS transistors  30 , and three pillar structures  60 . 
     Two bipolar transistors  20  are formed between two MOS transistors  30 . The drain region of one of the MOS transistors  30  is electrically connected to the base region  22  of one of the bipolar transistors  20 , or is formed monolithically with (or is used also as) this base region  22 . The drain region of the other of the MOS transistors  30  is electrically connected to the base region  22  of the other of the bipolar transistors  20 , or is formed monolithically with (or is used also as) this base region  22 . 
     Of three pillar structures  60 , one is formed between two bipolar transistors  20 , another one is formed between one of the MOS transistors  30  and the first collector potential extraction region  24  adjacent thereto, and the remaining one is formed between the other of the MOS transistors  30  and the first collector potential extraction region  24  adjacent thereto. Each pillar structure  60  has e.g. a deep trench isolation (DTI) structure and a pillar shape that ranges from the outermost surface of the semiconductor substrate  10  to the vicinity of the bottom of the collector region  21 . Furthermore, each pillar structure  60  has e.g. a multilayer structure obtained by stacking plural layers along the direction from the center of the pillar structure  60  toward the collector region  21 . This multilayer structure is composed of e.g. a pillar layer  60 A that has a pillar shape and is provided at the center of the multilayer structure, a pillar layer  60 B that covers the side surfaces and the bottom of the pillar layer  60 A, and a pillar layer  60 C that covers the side surfaces and the bottom of the pillar layer  60 B. 
     Of three pillar structures  60 , in the pillar structure  60  provided between two bipolar transistors  20 , the pillar layer  60 A is surrounded by the pillar layer  60 B and the insulating film  52  (insulating film  52 A) formed on the pillar structure  60 . Thus, the pillar layer  60 A is spatially isolated from the collector region  21 , the pillar layer  60 C, and the base regions  22  in the periphery thereof. The pillar layer  60 C is formed between the pillar layer  60 B and the collector region  21 , and is in contact with two base regions  22  adjacent to each other. 
     The pillar layer  60 A contains e.g. poly-silicon containing an impurity of the same conductivity type as that of the semiconductor substrate  10 . The pillar layer  60 B is composed of e.g. silicon oxide (SiO 2 ), and insulates, together with the insulating film  52  (insulating film  52 A) formed on the pillar structure  60 , the pillar layer  60 A from the collector region  21 , the pillar layer  60 C, and the base regions  22  in the periphery of the pillar layer  60 A. The pillar layer  60 C contains e.g. an impurity of a conductivity type different from that of the collector region  21 , and is electrically connected to two base regions  22  adjacent to each other. Due to this structure, the pillar layer  60 C has a roll of, when high voltage is applied to the collector electrode  27 , causing the collector region  21  and the pillar layer  60 C to be completely depleted and equalizing the electric field directly beneath the base region  22  to thereby increase the breakdown voltage. 
     Of three pillar structures  60 , in two pillar structures  60  provided between one of the MOS transistors  30  and the first collector potential extraction region  24  adjacent thereto and provided between the other of the MOS transistors  30  and the first collector potential extraction region  24  adjacent thereto, the pillar layer  60 A is surrounded by the pillar layer  60 B and the insulating film  52  (insulating film  52 A) formed on the pillar structure  60 . Thus, the pillar layer  60 A is spatially isolated from the collector region  21 , the pillar layer  60 C, the source region  31 , and the source potential extraction region  29  (to be described later) in the periphery thereof. The pillar layer  60 C is formed between the pillar layer  60 B and the collector region  21 , and is in contact with the source region  31  and the source potential extraction region  29  that are adjacent to each other with the intermediary of the pillar structure  60  therebetween. 
     The pillar layer  60 A contains e.g. poly-silicon containing an impurity of the same conductivity type as that of the semiconductor substrate  10 . The pillar layer  60 B is composed of e.g. silicon oxide (SiO 2 ), and insulates, together with the insulating film  52  (insulating film  52 A) formed on the pillar structure  60 , the pillar layer  60 A from the collector region  21 , the pillar layer  60 C, the source region  31 , and the source potential extraction region  29  in the periphery of the pillar layer  60 A. The pillar layer  60 C contains e.g. an impurity of a conductivity type different from that of the collector region  21 , and is electrically connected to the source region  31  and the source potential extraction region  29  that are adjacent to each other with the intermediary of the pillar structure  60  therebetween. Due to this structure, the pillar layer  60 C has a roll of, when high voltage is applied to the collector electrode  27 , causing the collector region  21  and the pillar layer  60 C to be completely depleted and equalizing the electric field directly beneath the source region  31  to thereby increase the breakdown voltage. 
     The reason why the pillar layers  60 A,  60 B, and  60 C are provided as the pillar structure  60  is that the following process is used in order to form the pillar layer  60 C, which contributes to increase in the breakdown voltage, in terms of reduction in the manufacturing cost. Specifically, initially three deep trenches (not shown) are formed in predetermined areas in the collector region  21 , and then the thin pillar layer  60 B is formed in each deep trench. Subsequently, the pillar layer  60 C is formed directly beneath the pillar layer  60 B by oblique implantation and diffusion, and the pillar layer  60 A is so formed on the pillar layer  60 B as to fill the deep trench. In this way, the pillar structure  60  can be formed. 
     If the manufacturing cost is not taken into consideration, it is also possible to use the following process for forming the pillar structure  60 , which contributes to increase in the breakdown voltage. Specifically, three deep trenches (not shown) are formed in predetermined areas in the collector region  21 , and then the deep trenches are filled by growing e.g. a semiconductor layer (pillar layer) containing an impurity of a conductivity type different from that of the collector region  21  in the respective deep trenches. 
     The source potential extraction region  29  is provided on the outermost surface of the semiconductor substrate  10  together with the first collector potential extraction region  24 . Over the surfaces of the source potential extraction region  29  and the first collector potential extraction region  24 , the collector electrode  27  is formed with the intermediary of the via  26  therebetween. The source potential extraction region  29  contains an impurity of the same conductivity type as that of the pillar layer  60 C, with impurity concentration higher than that of the pillar structure  60 C. Due to this structure, the via  26  and the collector electrode  27  are electrically connected to the first collector potential extraction region  24  and the source potential extraction region  29 . Furthermore, as described later, the source potential extraction region  29  is in contact with the pillar layer  60 C, which is in contact with the source region  31 , and thus is electrically connected to the source region  31  via the pillar layer  60 C. Consequently, the collector electrode  27  is electrically connected to the collector region  21  via the via  26  and the first collector potential extraction region  24 , and is electrically connected also to the source region  31  via the via  26 , the source potential extraction region  29 , and the pillar layer  60 C. Moreover, the collector electrode  27  is electrically connected also to the signal line L 1 . 
     In the electrostatic protection circuit  2  of the present embodiment, two bipolar transistors  20  and two MOS transistors  30  shown in  FIG. 7  can be represented by e.g. the equivalent circuit shown in  FIG. 3 , similarly to the above-described embodiment. Therefore, also in the present embodiment, the base region  22  of the bipolar transistor  20  and the drain region of the MOS transistor  30  are electrically connected to each other, and the drain region (the base region  22 ) is electrically floating. 
     Due to this structure, when the surge voltage V 1  is applied to the signal line L 1  as shown in  FIG. 4 , the surge voltage V 1  does not transmit in the signal line L 1  but is diverted into the ground line L 3  via the electrostatic protection circuit  2 , similarly to the above-described embodiment. On the other hand, when the signal voltage V 0  is applied to the signal line L 1  as shown in  FIG. 5 , the electrostatic protection circuit  2  does not operate, but the signal voltage V 0  transmits in the signal line L 1 , so that the integrated circuit (not shown) connected to the signal line L 1  operates, similarly to the above-described embodiment. 
     In this manner, in the present embodiment, the base region  22  is so designed as to serve as both the base of the bipolar transistor  20  and the drain of the MOS transistor  30 . Thus, the trigger of the bipolar operation at the time of the electrostatic protection can be controlled based on the threshold voltage of the MOS transistor  30 . Due to this feature, the electrostatic protection operation can be started even when the voltage Vd between the signal line L 1  and the ground line L 3  is low (e.g. 0.3 V) as shown in  FIG. 6 , which allows prevention of the breakdown of the electrostatic protection circuit  2  itself due to the surge voltage V 1 . 
     Furthermore, the internal impedance at the time of the electrostatic protection operation is very low. Therefore, even when static electricity of high voltage is applied, the voltage Vd can be suppressed to as low as about 10 V, and thus low power consumption can be realized. This allows suppression of the heat generation of the electrostatic protection circuit  2 , which greatly enhances the electrostatic protection resistance. Moreover, as shown in  FIG. 6 , the resistance can be maintained for large current of up to about 6.5 A. Thus, even when a high voltage of about 10400 V is applied in the human body model or a high voltage of about 520 V is applied in the machine model, the resistance can be maintained, and hence the electrostatic protection resistance is extremely excellent. 
     This is the end of the description of the electrostatic protection circuits according to two embodiments of the present invention. The present invention is not limited to the above-described embodiments, but the structures of the electrostatic protection circuits can optionally be modified as long as the same advantageous effects as those by the above-described embodiments can be achieved. 
     For example, in the above-described embodiments, the drain region of the MOS transistor  30  (the base region  22  of the bipolar transistor  20 ) is electrically floating. Alternatively, it is also possible to employ e.g. a configuration in which a base electrode (not shown) electrically connected to the base region  22  is provided on a part of the surface of the base region  22  and a high-resistance element R 1  is provided and connected between this base electrode and the ground line L 3 . Due to this structure, for example, as shown in  FIG. 8 , the drain region of the MOS transistor  30  (the base region  22  of the bipolar transistor  20 ) is electrically connected to the ground line L 3  via the high-resistance element R 1 . Thus, erroneous operation due to noise can be prevented without deteriorating the electrically-floating state. Specifically, in the structures of the above-described embodiments, when the surge voltage V 1  is applied, the surge voltage V 1  of the source region  31  is transmitted via the channel to the base region  22  in the floating state, which offers the advantageous effect. Thus, the base region  22  should be set to the electrically-floating state. However, this would possibly cause erroneous operation due to noise. In contrast, if the high-resistance element R 1  is provided like the present modification example, even in the case of the occurrence of noise, the noise can be discharged to the ground line L 3  via the high-resistance element R 1 , and thus the potential of the base region  22  can be stabilized, which allows prevention of erroneous operation due to the noise. 
     In the above-described embodiments, the emitter electrode  28  is connected directly to the ground line L 3 . Alternatively, it is also possible to employ e.g. a configuration shown in  FIG. 9  in which the p-type MOS transistor Tr 4  in the control circuit  40  is interposed between the emitter electrode  28  and the ground line L 3 . In this case, due to the control circuit  40 , the emitter electrode  28  and the gate electrode  33  are connected to the ground line L 3  via the p-type MOS transistor Tr 4  when the surge voltage V 1  is applied to the signal line L 1 , and are connected to the signal line L 1  via the p-type MOS transistor Tr 2  when the signal voltage V 0  is applied to the signal line L 1 . 
     In the above-described embodiments, the semiconductor substrate  10  is a silicon substrate containing a p-type impurity. Alternatively, it may be a silicon substrate containing an n-type impurity. In this case, when the conductivity type of another component is the p-type, this conductivity type is replaced by the n-type. When the conductivity type of another component is the n-type, this conductivity type is replaced by the p-type. 
     In the above-described embodiments, two MOS transistors  30  are provided. Alternatively, only one MOS transistor  30  may be provided, or three or more MOS transistors  30  may be provided. In the first embodiment, one bipolar transistor  20  is provided. Alternatively, two or more bipolar transistors  20  may be provided. In the second embodiment, two bipolar transistors  20  are provided. Alternatively, only one bipolar transistor  20  may be provided, or three or more bipolar transistors  20  may be provided. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.