Semiconductor device

Abstract: A semiconductor device having a SOI structure in which an ESD resistance can be enhanced is obtained. The semiconductor device comprises PMOS transistors Q21 and Q22 which are brought into a forward bias state if a positive high voltage is applied as a surge voltage to a signal terminal 30, and NMOS transistors Q11 and Q12 which are brought into the forward bias state if a negative high voltage is applied as the surge voltage to the signal terminal 30. Furthermore, if a normal operation signal is applied from the signal terminal 30, all the NMOS transistors Q11 and Q12 and the PMOS transistors Q21 and Q22 are brought into an OFF state.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a semiconductor device having a MOS transistor formed on a SOI substrate, and more particularly to an input - output protecting function thereof. Description of the Background Art In the case where a MOS device is formed on a bulk silicon substrate, a surge can be discharged into the substrate through a PN junction. In a SOI (Silicon - on - insulator) device in which the MOS device is formed on a SOI layer provided on a buried insulation layer, a path for transferring the surge does not structurally exist on the substrate because the buried insulation layer is formed. In particular, a longitudinal discharge path does not exist in a thin film SOI structure in which source and drain regions of a MOS transistor reach a buried oxide film. Consequently, it is structurally necessary to perform a transverse discharge. For this reason, the discharge is transversely performed into a power line and a grounding line through the MOS transistor and a diode. Fig. 18 is a circuit diagram showing a structure of an input - output protecting circuit having a SOI structure according to the prior art. As shown in Fig. 18, one of ends of a resistor 36 (hereinafter referred to as a "protecting resistor") for limiting a rush current is first connected to a signal terminal 30 to delay propagation of a surge voltage to an inside through the signal terminal 30, thereby preventing an excessive current from flowing. Then, a PMOS transistor Q31 and an NMOS transistor Q32 are provided in series as discharge elements for transferring electric charges between a power supply (node) 32 and a ground level (node) 33. The PMOS transistor Q31 and the NMOS transistor Q32 have gates connected to the power supply 32 and the ground level 33, and drains connected to the other end of the protecting resistor 36 in common, respectively. Accordingly, the PMOS transistor Q31 and the NMOS transistor Q32 are. usually brought into an OFF State. A similar semiconductor device is disclosed in DE 196 51 247 A1, however, the gates of the transistors are floating. In the case where the surge voltage is applied to the signal terminal 30, the electric charges are quickly discharged into the power supply 32 or the ground level 33 by avalanche breakdown of each of the MOS transistors Q31 and Q32 to protect an internal element 31. Furthermore, the other end of the protecting resistor 36 is connected to one of ends of an internal resistor 37, and the internal element 31 is connected to the other end of the internal resistor 37. Consequently, the surge voltage is propagated to the internal element 31 with difficulty. Fig. 19 shows an input - output protecting circuit formed by using diodes 38 and 39 as discharge elements in the same manner. As shown in Fig. 19, one of ends of a protecting resistor 36 is first connected to a signal terminal 30 to delay propagation of a surge voltage to an inside through the signal terminal 30, thereby preventing an excessive current from flowing. Then, the diodes 38 and 39 are provided in series as discharge elements for transferring electric charges between a power supply 32 and a ground level 33. A cathode of the diode 38 is connected to the power supply 32, and an anode of the diode 39 is connected to the ground level 33. Accordingly, the diodes 38 and 39 are connected in reverse directions between the power supply 32 and the ground level 33. If the surge voltage is applied to the signal terminal 30, the electric charges are quickly discharged into the power supply 32 or the ground level 33 by backward avalanche breakdown of the diodes 38 and 39 or a forward current, thereby protecting an internal element 31. Furthermore, an internal resistor 37 is inserted between the protecting resistor 36 and the internal element 31 to propagate the surge voltage to the internal element 31 with difficulty. Fig. 20 shows an inverter circuit acting as one of typical internal circuits. As shown in Fig. 20, a PMOS transistor Q33 and an NMOS transistor Q34 are connected in series between a power supply 32 and a ground level 33. The PMOS transistor Q33 and the NMOS transistor Q34 have gates connected to an input signal terminal 82 in common, and drains connected to an output signal terminal 83 in common. In the case where the input - output protecting circuits having the structures shown in Figs. 18 and 19 are connected to the input signal terminal 82 (an input section of the internal circuit) for the internal circuit shown in Fig. 20, they function as input protecting circuits. In the case where the same input - output protecting circuits are connected to the output signal terminal 83 (an output section of the internal circuit), they function as output protecting circuits. The function and operation of the protecting circuit are the same in the input and output sections. Therefore, the protecting circuit will be hereinafter referred to as an "input - output protecting circuit". If the protecting circuit is used as the output protecting circuit, no resistor is often added thereto. Fig. 21 is a plan view showing a planar structure of the MOS input - output protecting circuit shown in Fig. 18, and Fig. 22 is a sectional view taken along the line A - A shown in Fig. 21. As shown in Figs. 21 and 22, a thin silicon film 3 acting as a SOI layer is provided on a silicon substrate 1 with a buried oxide film 2 acting as an insulation layer interposed therebetween. The thin silicon film 3 is divided into two islands 18A and 18B by an interlayer dielectric film 11. Channel formation regions 6 and 6d into which an impurity having a concentration of about 10 17 / cm 3 (p - type in NMOS and n - type in PMOS) is implanted are provided, drain and source regions 7 and 8 into which an impurity having a concentration of about 10 20 / cm 3 (n - type in NMOS and p - type in PMOS) is implanted are provided with the channel formation region 6 interposed therebetween, and drain and source regions 7d and 8d into which an impurity having a concentration of about 10 20 / cm 3 is implanted are provided with the channel formation region 6d interposed therebetween. Furthermore, gate electrodes 5 and 5d are formed on the channel formation regions 6 and 6d and a part of each of the drain regions 7 and 7d and the source regions 8 and 8d in the thin silicon film 3 with gate oxide films 4 and 4d interposed therebetween, respectively. Accordingly, the NMOS transistor Q32 is formed by the gate oxide film 4, the gate electrode 5, the channel formation region 6, the drain region 7 and the source region 8 in the island 18A, and the PMOS transistor Q31 is formed by the gate oxide film 4d, the gate electrode 5d, the channel formation region 6d, the drain region 7d and the source region 8d in the island 18B. Furthermore, the interlayer dielectric film 11 divides the thin silicon film 3 into the islands 18A and 18B and is formed over the whole surface of the thin silicon film 3. Contact holes 12A to 12D are provided on the interlayer dielectric film 11 in a part of each of the drain regions 7 and 7d and the source regions 8 and 8d. An aluminum wiring 14 is electrically connected to the drain regions 7 and 7d through the contact holes 12A and 12B, an aluminum wiring 13 is electrically connected to the source region 8 through the contact hole 12C, and an aluminum wiring 15 is electrically connected to the source region 8d through the contact hole 12D. The aluminum wiring 13 is connected to a ground level 33, the aluminum wiring 14 is connected to an input section 30d, and the aluminum wiring 15 is connected to a power supply 32. The input section 30d means a portion to be connected to the other end of the protecting resistor 36 as shown in Fig. 19. In Fig. 21, the interlayer dielectric film 11 is omitted. As shown in Fig. 21, the protecting resistor 36 formed of a gate electrode material and a thin silicon film is provided between the signal terminal 30 and the PMOS transistor Q31 and NMOS transistor Q32 which act as discharge elements, and the internal resistor 37 formed of the gate electrode material and the thin silicon film is provided between the discharge elements Q31 and Q32 and the internal element 31. The protecting resistor 36 is usually resistant to a rush current flowing into the discharge elements Q31 and Q32. Therefore, a width of the protecting resistor 36 is made greater than that of the internal resistor 37, thus reducing a current density. The gate electrode 5d of the PMOS transistor Q31 is connected to the power supply 32 by a gate potential fixing wiring 40, and the gate electrode 5 of the NMOS transistor Q32 is connected to the ground level 33 by a gate potential fixing wiring 41. The PMOS transistor Q31 and the NMOS transistor Q32 are set into an OFF state, respectively. The next operation will be described below with reference to Figs. 18, 21 and 22. A surge generated by static electricity is sent through the signal terminal 30 and is first delayed by the protecting resistor 36 to prevent a great rush current from flowing into the discharge elements Q31 and Q32. The surge propagated through the protecting resistor 36 is then discharged quickly into the power supply 32 or the ground level 33 through the discharge elements Q31 and Q32. The internal resistor 37 does not function to cause the surge to flow in a direction of the internal element but in a direction of the discharge element. The surge propagated through the protecting resistor 36 is applied to a discharge element section. If a positive surge voltage is applied, a junction breaks down between the drain region 7 and the channel formation region 6 of the NMOS transistor Q32 to emit the surge to the ground level 33. At the same time, the PMOS transistor Q31 is forward biased and the surge is emitted to the power supply 32 as well. If a negative surge voltage is applied, a junction breaks down between the drain region 7d and the channel formation region 6d of the PMOS transistor Q31 to emit the surge to the power supply 32 and the NMOS transistor Q32 is forward biased to emit the surge. During normal operation, the PMOS transistor Q31 and the NMOS transistor Q32 are set in the OFF state. Therefore, an normal operation signal applied to the signal terminal 30 is propagated to the internal element 31 through the protecting resistor 36 and the internal resistor 37. Fig. 23 is a sectional view showing a sectional structure of the diode type input - output protecting circuit shown in Fig. 19. As shown in Fig. 23, a thin silicon film 3 is provided on a silicon substrate 1 with a buried oxide film 2 acting as an insulation layer interposed therebetween. The thin silicon film 3 is divided into two islands 19A and 19B by an interlayer dielectric film 11. An anode region 9B into which a p-type impurity having a concentration of about 10 17 / cm 3 is implanted, and a cathode region 10B into which an n-type impurity having a concentration of about 10 17 / cm 3 is implanted are provided, respectively. An anode region 9A and a cathode region 10A are provided with the anode region 9B interposed therebetween. An anode region 9C and a cathode region 10C are provided with the cathode region 10B interposed therebetween. A p-type impurity having a concentration of about 10 20 / cm 3 is implanted into the anode regions 9A and 9C, and an n-type impurity having a concentration of about 10 20 /cm 3 is implanted into the cathode regions 10A and 10C. Furthermore, an electrode 5 is formed on the anode region 9B and a part of the anode region 9A and the cathode region 10A in the thin silicon film 3 with an oxide film 4 interposed therebetween, and an electrode 5d is formed on the cathode region 10B and a part of the anode region 9C and the cathode region 10C in the thin silicon film 3 with an oxide film 4d interposed therebetween. A diode 39 is formed by the anode regions 9A and 9B and the cathode region 10A. A diode 38 is formed by the anode region 9C and the cathode regions 10B and 10C. A structure of a diode section is the same as that shown in Fig. 22. The anode regions 9B and 9A and the cathode region 10A are replaced with the channel formation region 6, the source region 8 and the drain region 7, respectively. The cathode region 10B, the anode region 9C and the cathode region 10C are replaced with the channel formation region 6d, the drain region 7d and the source region 8d, respectively. While the electrodes 5 and 5d are used as masks during formation of the anode regions 9A and 9C and the cathode regions 10A and 10C, they do not function as components of the element The next operation of the input - output protecting circuit shown in Figs. 19 and 23 is the same as that of the input - output protecting circuit shown in Figs. 18 and 22 except that emission is performed with breakdown of a junction of the anode region 9B and the cathode region 10A of the diode 39 and breakdown of a junction of the anode region 9C and the cathode region 10B of the diode 38. In the SOI structure, a thermal conductivity is poor due to existence of the buried oxide film 2, a temperature rise is great even if the same power as that of a bulk element formed on a bulk substrate is put into the discharge element (the temperature rise being about three times as much as that in the bulk substrate), and secondary breakdown (thermal runaway) is caused so that destruction is easily caused. Furthermore, a current flows ununiformly and concentrates on a part of the elements. Consequently, a rapid temperature rise occurs in the element on which the current concentrates. Thus, only the same element causes the secondary breakdown. As a result, an ESD (Electrostatic discharge) resistance of a whole device is deteriorated. With the SOI structure, furthermore, quality of the thin silicon film 3 is not so good as that of the bulk substrate. Therefore, quality of the gate oxide film 4 formed on the thin silicon film 3 is easily deteriorated so that the ESD resistance is degraded. The deterioration in the quality of the gate oxide film 4 is particularly remarkable in an active region boundary portion (an isolating end) of the thin silicon film 3 having a height difference from the insulation layer 2. From US 4,989,057 a semiconductor device having an input-output protecting circuit section is known which includes a signal node connected to a signal terminal and an internal circuit connected to the signal node, which are formed on a SOI substrate. The semiconductor device comprises a power supply node and a reference potential node. The input-output protection circuit section includes first discharge elements provided between the power supply node and the signal node been forward biased when a surge voltage having a first polarity is applied to the signal terminal. Further discharge elements are provided between the signal node and the reference potential node being forward biased when the surge voltage having a second polarity is applied to the signal terminal. SUMMARY OF THE INVENTION Therefore, it is the object of the present invention to provide a semiconductor device having a SOI structure in which an ESD resistance can be enhanced. This object is solved by a semiconductor device as set forth in claim 1. Preferred developments of the invention are given in the dependent claims. According to claim 1, the input - output protecting circuit includes the first to fourth discharge elements having PN junctions, each of which is provided between the power supply node or reference potential node and the signal terminal, is reversely biased when the normal operation signal is applied to the signal terminal and is forward biased when the voltage having the first or second polarity is applied to the signal terminal. Accordingly, when the surge voltage having the first polarity is applied from the signal terminal, the first and third discharge elements in a forward bias state can cause the power supply node and the reference potential node to discharge a high surge voltage respectively. When the surge voltage having the second polarity is applied from the signal terminal, the second and fourth discharge elements in the forward bias state can cause the power supply node and the reference potential node to discharge the surge voltage respectively. As a result, there are two discharge elements for discharging any surge voltage in the forward bias state into the power supply node and the reference potential node, respectively. Therefore, an input - output protecting circuit having an excellent ESD resistance can be obtained. All the PN junctions of the first to fourth discharge elements are brought into a reverse bias state when the normal operation signal is applied to the signal terminal. Therefore, the normal operation signal does not leak to the power supply node or the grounding line through the first to fourth discharge elements, and normal operation of the internal circuit is not affected by the input - output protecting circuit. According to claim 2, the input - output protecting circuit is formed by the first and third MOS transistors of the first conductivity type and the second and fourth MOS transistors of the second conductivity type. Two transistors whose conductivity types are different from each other are used, respectively. Therefore, the input - output protecting circuit can easily be manufactured. According to claim 3, the input - output protecting circuit is formed by the first transistor of the first conductivity type, the second transistor of the second conductivity type, and the first and second diodes. Consequently, the transistors whose conductivity types are different from each other are used one by one. Therefore, the input - output protecting circuit can easily be manufactured. According to claim 5, the first internal resistor is inserted between the signal terminal and the signal node, and the second internal resistor is inserted between the signal node and the internal circuit. Consequently, if the surge voltage having the first or second polarity is applied to the signal terminal, propagation delay is caused by the first internal resistor and propagation to the internal circuit is prevented by the second internal resistor, thus protecting the internal circuit. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a circuit diagram showing a structure of an input - output protecting circuit of a semiconductor device according to an embodiment of the present invention; Figure 2 is a circuit diagram showing another structure of the input - output protecting circuit of the semiconductor device according to the embodiment of the present invention; Figure 3 is a circuit diagram showing an NMOS transistor having a gate coupling sectional structure; Figure 4 is a graph showing a discharge characteristic of the NMOS transistor in Figure 3; Figure 5 is a sectional view showing a SOI sectional structure; Figure 6 is a graph showing a discharge characteristic of an NMOS transistor which is reverse bias connected; Figure 7 is a circuit diagram showing the NMOS transistor which is reverse bias connected; Figure 8 is a plan view showing a planar structure of a MOS transistor in which thermal runaway destruction occurs; Figure 9 is a plan view showing a planar structure in which a plurality of NMOS transistors are connected in parallel between an input and a ground; Figure 10 is a circuit diagram showing a circuit structure in Figure 9; Figure 11 is a circuit diagram showing an NMOS transistor which is forward bias connected; Figure 12 is a graph showing a discharge characteristic of the NMOS transistor which is forward bias connected; Figure 13 is a circuit diagram showing a structure in which forward bias connection is performed by using a plurality of NMOS transistors; Figure 14 is a circuit diagram showing a diode which is reverse bias connected; Figure 15 is a graph showing a discharge characteristic of the diode which is reverse bias connected; Figure 16 is a circuit diagram showing a diode which is forward bias connected; Figure 17 is a graph showing a discharge characteristic of the diode which is forward bias connected; Figure 18 is a circuit diagram showing a structure of an input - output protecting circuit according to the prior art; Figure 19 is a circuit diagram showing a structure of the input - output protecting circuit according to the prior art; Figure 20 is a circuit diagram showing an example of an internal element; Figure 21 is a plan view showing a planar structure of a part of the input - output protecting circuit in Figure 18; Figure 22 is a sectional view showing a sectional structure of the part of the input - output protecting circuit in Figure 18; and Figure 23 is a sectional view showing a sectional structure of a part of the input - output protecting circuit in Figure 19. DESCRIPTION OF THE PREFERRED EMBODIMENT <Principle> Fig. 5 is a sectional view showing a SOI structure. As shown in Fig. 5, a buried oxide film 2 is- formed on a silicon substrate 1, and a thin silicon film 3 is formed on the buried oxide film 2. Thus, a structure comprising the silicon substrate 1, the buried oxide film 2 and the thin silicon film 3 is referred to as a SOI substrate. A gate oxide film 4 is selectively formed on the thin silicon film 3. A gate electrode 5 is formed on the gate oxide film 4. The thin silicon film 3 provided under the gate oxide film 4 acts as a channel formation region 6. Regions in the thin silicon film 3 which are adjacent to the channel formation region 6 act as drain and source regions 7 and 8. With such a SOI structure, the buried oxide film 2 having a poor thermal conductivity (which is about 1 / 10 as much as that of silicon) is provided as described above. For this reason, heat generated when a surge voltage is applied is stored so that permanent destruction of a semiconductor device formed on the SOI substrate is easily caused by thermal runaway. Fig. 6 is a graph showing a discharge characteristic of an NMOS transistor which is obtained at the time of reverse bias connection. The graph of Fig. 6 indicates details of the thermal runaway caused at the time of the reverse bias connection, for example, when an input voltage IN (a surge voltage SV) is applied to a drain of an NMOS transistor Q1 having a source and a gate grounded as shown in Fig. 7. A process of the thermal runaway shown in Fig. 6 includes partial processes P1 to P5. When a positive surge voltage SV which is much greater than an ordinary voltage is applied to the drain of the NMOS transistor Q1 shown in Fig. 7, a drain voltage is quickly raised (P1). Since the NMOS transistor Q1 is reversely biased during the partial process P1, a current hardly flows. When a breakdown induced voltage is reached, the NMOS transistor Q1 causes avalanche breakdown to perform parasitic bipolar operation. Consequently, the drain voltage is dropped up to a holding voltage (P2). Then, the drain voltage is raised again with an internal resistance of the NMOS transistor Q1 (P3). When a thermal breakdown voltage is reached, a silicon portion forming a part of the NMOS transistor Q1 is brought into a fusing state so that a resistance value between a source and a drain is rapidly reduced so that the voltage is dropped (P4). As a result, a current concentrates on the NMOS transistor Q1 having a resistance reduced between the source and the drain, that is, positive feedback is caused (P5). Consequently, the NMOS transistor Q1 completely breaks down. For example, a large defective portion 10 is generated on the gate electrode 5 (or the channel formation region 6) by thermal breakdown as shown in Fig. 8. Thus, transistor operation cannot be performed. In the case where NMOS transistors are usually reversely biased as an input - output protecting circuit on the SOI substrate, NMOS transistors having a channel width W are connected in parallel between an input (voltage) IN and a ground level as shown in Fig. 9. In an example of Fig. 9, six NMOS transistors T1 to T6 having gates grounded are provided in parallel between the input IN and the ground level as shown in Fig. 10. The NMOS transistor T1 is formed by a gate electrode 51, a drain region 61 and a source region 71. The NMOS transistor T2 is formed by a gate electrode 52, the drain region 61 and a source region 72. The NMOS transistor T3 is formed by a gate electrode 53, a drain region 62 and the source region 72. The NMOS transistor T4 is formed by a gate electrode 54, the drain region 62 and a source region 73. The NMOS transistor T5 is formed by a gate electrode 55, a drain region 63 and the source region 73. The NMOS transistor T6 is formed by a gate electrode 56, the drain region 63 and a source region 74. A surge voltage is applied from a signal terminal 30. Thus, the six NMOS transistors T1 to T6 are provided as the input - output protecting circuit on the SOI substrate in parallel between the input IN and the ground level. Consequently, in the case where a current flows between the input IN and the ground level, it splits and flows into the six NMOS transistors T1 to T6. However, if one of the NMOS transistors T1 to T6 gets an avalanche breakdown voltage, a resistance between a source and a drain of the same transistor is reduced. Consequently, a current concentrates on the transistor which has caused avalanche breakdown. Thus, it is impossible to prevent the same transistor from getting a thermal breakdown voltage for thermal breakdown. Fig. 12 is a graph showing a discharge characteristic of an NMOS transistor which is obtained at the time of forward bias connection. The graph of Fig. 12 indicates details of the thermal runaway caused at the time of the forward bias connection, for example, when an input voltage IN (a surge voltage SV) is applied to a drain and a gate of an NMOS transistor Q2 having a source grounded as shown in Fig. 11. Fig. 12 is a graph showing the discharge characteristic of the NMOS transistor which is obtained at the time of the forward bias connection in Fig. 11. As shown in Fig. 12, a snap back phenomenon in which a drain voltage is dropped when a breakdown induced voltage is reached is not caused at the time of the forward bias connection. As shown in Fig. 13, accordingly, a plurality of NMOS transistors T11 to T16 provided in parallel between an input IN and a ground level cause avalanche breakdown almost uniformly with application of the surge voltage SV, and are brought into a thermal runaway state in the same manner as the reverse bias connection when a thermal breakdown voltage is reached by further application of the surge voltage SV. When a part of the NMOS transistors connected in parallel as shown in Fig. 13 causes the avalanche breakdown, the drain voltage is raised so that the other transistors also cause the avalanche breakdown. As a result, all the transistors cause the avalanche breakdown and a current uniformly flows into all the transistors. Consequently, thermal breakdown is caused at a thermal breakdown voltage which is higher than the breakdown induced voltage. Thus, a phenomenon in which a current concentrates on a part of the transistors that have caused the avalanche breakdown does not occur at the time of the forward bias connection differently from the time of the reverse bias connection. Consequently, a comparatively excellent ESD resistance can be obtained. Fig. 15 is a graph showing a discharge characteristic of a diode which is obtained at the time of the reverse bias connection. The graph of Fig. 15 indicates details of thermal runaway obtained at the time of the reverse bias connection, for example, when the surge voltage SV is applied to a cathode of a diode D1 having an anode grounded as shown in Fig. 14. The diode does not perform parasitic bipolar operation. Therefore, a voltage is successively raised after the avalanche breakdown, resulting in thermal breakdown. A thermal breakdown voltage which is higher than a breakdown induced voltage is effective in prevention of current concentration in the same manner as in the NMOS transistors which are forward bias connected. However, the breakdown induced voltage itself of the diode is high (which is about twice or three times as much as that of the MOS transistor). Therefore, power dissipation represented by a product of a voltage and a current is increased (that is, heat is also increased). As a result, the ESD resistance is deteriorated. If a junction breakdown voltage of the diode is dropped by a high concentration junction or the like, the breakdown induced voltage can be dropped. Consequently, the ESD resistance can be improved. Fig. 17 is a graph showing a discharge characteristic of the diode which is obtained at the time of forward bias connection. The graph of Fig. 17 indicates details of thermal runaway caused at the time of the forward bias connection, for example, when a surge voltage SV is applied to an anode of a diode D2 having a cathode grounded as shown in Fig. 16. Forward connection of the diode can discharge a great current at a low voltage. Therefore, a desired discharge characteristic for the ESD resistance can be obtained. As shown in Fig. 17, the avalanche breakdown is caused with an increase in a voltage. In the same manner as in other elements, when a thermal breakdown voltage is reached, permanent destruction of the element is caused by the thermal runaway. As described above, even if any of a transistor and a diode is used, nonuniformity of a current can be prevented more easily and the ESD resistance can be enhanced more when the forward connection is utilized. <Structure and Operation of First Mode> Fig. 1 is a circuit diagram showing a structure of an input - output protecting circuit according to a first mode of the present invention. As shown in Fig. 1, a signal terminal 30 is connected to one of ends of a protecting resistor 36, the other end of the protecting resistor 36 is connected to one of ends of an internal resistor 37 through a signal line (signal node) 81, and the other end of the internal resistor 37 is connected to an internal element 31. Between a power supply 32 and a ground level 33, NMOS transistors Q11 and Q12 are connected in series and PMOS transistors Q21 and Q22 are connected in series. The NMOS transistor Q11 has a gate connected to the ground level 33, a drain connected to the power supply 32, and a source connected to the other end of the protecting resistor 36 (one of ends of the internal resistor 37). The NMOS transistor Q12 has a gate and a source connected to the ground level 33, and a drain connected to the source of the NMOS transistor Q11. The PMOS transistor Q21 has a gate and a source connected to the power supply 32, and a drain connected to the other end of the protecting resistor 36. The PMOS transistor Q22 has a gate connected to the power supply 32, a source connected to the other end of the protecting resistor 36, and a drain connected to the ground level 33. The NMOS transistors Q11 and Q12 and the PMOS transistors Q21 and Q22 may be formed by parallel connection of a plurality of MOS transistors as shown in Figs. 10 and 13. With such a structure, if a positive surge voltage is applied to the signal terminal 30, a voltage which is greater than a voltage obtained by adding threshold voltages of the PMOS transistors Q21 and Q22 to a supply voltage is applied to the signal terminal 30. Consequently, the surge voltage can be discharged into the power supply 32 and the ground level 33 through the PMOS transistors Q21 and Q22 which are brought into a forward bias state for the signal terminal 30, respectively. On the other hand, if a negative surge voltage is applied to the signal terminal 30, a voltage which is smaller than a voltage obtained by subtracting threshold voltages of the NMOS transistors Q11 and Q12 from a ground level is applied to the signal terminal 30. Consequently, the surge voltage can be discharged into the power supply 32 and the ground level 33 through the NMOS transistors Q11 and Q12 which are brought into the forward bias state for the signal terminal 30, respectively. During normal operation, a normal operation signal applied to the signal terminal 30 mainly has a voltage ranging from a ground level to a supply level. Therefore, all the NMOS transistors Q11 and Q12 and the PMOS transistors Q21 and Q22 are reversely biased and brought into an OFF state. The normal operation signal is sent to the internal element 31 through the protecting resistor 36 and the internal resistor 37 without a leakage to the power supply 32 or the ground level 33 through the MOS transistor Q11, Q12, Q21 or Q22. In particular, if the voltage of the normal operation signal is limited to the range from the ground level to the power level, the NMOS transistors Q11 and Q12 and the PMOS transistors Q21 and Q22 are surely turned off. Thus, even if any of the positive and negative surge voltages is applied to the input - output protecting circuit according to the first mode, either of the NMOS transistors Q11 and Q12 and the PMOS transistors Q21 and Q22 are always brought into the forward bias state for the surge voltage. Consequently, it is possible to obtain an input - output protecting circuit having an excellent ESD resistance. The NMOS transistors Q11 and Q12 and the PMOS transistors Q21 and Q22 are connected so as to be brought into a reverse bias state when the normal operation signal is input to the signal terminal 30. Therefore, the normal operation is not affected. The input - output protecting circuit uses two P-channel MOS transistors and two N-channel MOS transistors. As shown in Fig. 20, CMOS inverters including PMOS and NMOS transistors are often formed as the internal element 31. By changing structures of two CMOS inverters a little, the input - output protecting circuit according to the first mode can easily be manufactured. <Structure and Operation of Second Mode> As the second mode, the following structure may be used. More specifically, the NMOS transistor Q12 and the PMOS transistor Q21 are replaced with diodes D11 and D12, a cathode of the diode D11 is connected to the other end of the protecting resistor 36 and an anode of the diode D11 is connected to the ground level 33, and an anode of the diode D12 is connected to the signal line 81 and a cathode of the diode D12 is connected to the power supply 32 as shown in Fig. 2. In the second mode, when a positive surge voltage is applied to the signal terminal 30, it can be discharged into the power supply 32 and the ground level 33 through the diode D12 and the PMOS transistor Q22 in the forward bias state for the signal terminal 30. On the other hand, when a negative surge voltage is applied to the signal terminal 30, it can be discharged into the power supply 32 and the ground level 33 through the NMOS transistor Q11 and the diode D11 in the forward bias state for the signal terminal 30. Thus, if the positive or negative surge voltage is applied to the input - output protecting circuit according to the second mode of the first embodiment, either of the NMOS transistor Q11 and diode D11 and the diode D12 and PMOS transistor Q22 are always brought into the forward bias state for the surge voltage. Consequently, it is possible to obtain an input - output protecting circuit having an excellent ESD resistance. The NMOS transistor Q11, the diodes D11 and D12 and the PMOS transistor Q22 are connected so as to be always brought into an OFF state when the normal operation signal is input to the signal terminal 30. Therefore, the normal operation is not affected. According to the second mode, furthermore, MOS transistors of a conductivity type are used one by one with balance kept. Therefore, manufacture can be performed comparatively easily in the same manner as in the first mode.

Claims: A semiconductor having an input-output protecting circuit section including a signal node (81) connected to a signal terminal (30), and an internal circuit (31) connected to said signal node, which are formed on a SOI substrate (1 to 3), the semiconductor device comprising: a power supply node (32) to be connected to a supply potential; and a reference potential node (33) to be connected to a reference potential, said input-output protecting circuit section including: a first transistor as a first discharge element (Q11) connected between said power supply node (32) and said signal node (81), the control region thereof being connected to said reference potential node (33), and being reversely biased when a normal operation signal with a potential between the supply potential and the reference potential is applied to said signal terminal (30), said first discharge element (Q11) being forward biased when a surge voltage having a first polarity and a potential lower than the reference potential is applied to said signal terminal (30); a second transistor as a second discharge element (Q22) connected between said signal node (30) and said reference potential node (33), the control region thereof being connected to said power supply node (32) and being reversely biased when said normal operation signal is applied to said signal terminal (30), said second discharge element (Q22) being forward biased when a surge voltage having a second polarity and a potential higher than the supply potential is applied to said signal terminal (30); a third discharge element (Q21, D12) connected in parallel to said first discharge element (Q11) between said power supply node (32) and said signal node (81), said third discharge element (Q21, D12) being reversely biased when said normal operation signal is applied to said signal terminal (30), said third discharge element (Q21, D12) being forward biased when said surge voltage having the second polarity is applied to said signal terminal (30); and a fourth discharge element (Q12, D11) connected in parallel to said second discharge element (Q22) between said signal node (81) and said reference potential node (33), said fourth discharge element (Q12, D11) being reversely biased when said normal operation signal is applied to said signal terminal (30), said fourth discharge element (Q12, D11) being forward biased when said surge voltage having the first polarity is applied to said signal terminal (30). The semiconductor device according to claim 1, wherein said first and second discharge element (Q11, Q22) include MOS transistors of a first and second conducting type, respectively; said third discharge element includes a third MOS transistor (Q21) of the second conductivity type, said first and fourth MOS transistors (Q11, Q12) have control electrodes connected to said reference potential node (33) respectively, and said second and third MOS transistors (Q21, Q22) have control electrodes connected to said power supply node (32) respectively, said first MOS transistor (Q11) has a first electrode connected to said power supply node (32) and a second electrode connected to said signal node (81), and third MOS transistor (Q21) has a first electrode connected to said signal node (81) and a second electrode connected to said power supply node (32), said fourth MOS transistor (Q12) has a first electrode connected to said signal node (81) and a second electrode connected to said reference potential node (33), and said second MOS transistor (Q22) has a first electrode connected to said reference potential node (33) and a second electrode connected to said signal node (81). The semiconductor device according to claim 1, said first discharge element includes a MOS transistor (Q11) of a first conductivity type and has a control electrode connected to said reference potential node (33), said third and fourth discharge elements include diodes (D12, D11), and said second discharge element includes a MOS transistor (Q22) of a second conductivity type and has a control electrode connected to said power supply node (32), said first MOS transistor (Q11) has a first electrode connected to said power supply node (32) and a second electrode connected to said signal node (81), said second MOS transistor (Q22) has a first electrode connected to said reference potential node (33) and a second electrode connected to said signal node (81). The semiconductor device according to claim 2 or 3, wherein said supply potential is higher than said reference potential, said first conductivity type is N, said second conductivity type is P, said first polarity is negative, and said second polarity is positive. The semiconductor device according to one of claims 1 to 4, further comprising: a first internal resistor (36) inserted between said signal terminal (30) and said signal node (81); and a second internal resistor (37) inserted between said signal node (81) and said internal circuit (31).

Inventors: IPPOOSHI TAKASHI, YAMGUCHI YASUO, IPPOOSHI, TAKASHI, YAMGUCHI, YASUO
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1998
Main classification: H01L27
IPC codes: H01L21, H01L29, H01L27