Power semiconductor device with protective means

Abstract: In a semiconductor device where a protective Zener diode is formed through an insulation film to a silicon substrate with a power MOSFET formed, substantial breakdown strength of the insulation film is improved and withstand voltage of the Zener diode can be set large. A gate plate 111 connected electrically to an outer circumferential part of p-type diffusion region 104 is installed, and element parts 112a - 112c and equipotential plates 113a - 113c to constitute a Zener diode group 115 are formed. The equipotential plates 113a - 113c hold prescribe potential by Zener diode pairs 114 of the element parts 112a - 112c.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor device where a first semiconductor element formed on a main surface of a semiconductor substrate and a second semiconductor element formed on an insulation film covering the main surface are multiplied. 2. Description of the Related Art As such a semiconductor device, for example, when an inductive load is driven by a power semiconductor element such as a power MOSFET, in order to prevent that the power semiconductor element is subjected to application of overvoltage due to flyback of the load and is broken, in some constitution, a protective element is installed integral with the power semiconductor element. That is, in the case of constitution that an inductive load is driven by connecting a power MOSFET for example at low potential side, when the power MOSFET is turned off, flyback voltage is generated at the load and its positive voltage is applied to the power MOSFET. Then in some cases, the positive voltage may exceed the withstand voltage of the power MOSFET resulting in breakage of the element. Therefore in order to prevent an element from being broken due to such overvoltage, for example, in a power MOSFET, a Zener diode is connected between drain and gate, when definite voltage or more is applied between drain and source, the Zener diode is subjected to breakdown and the element is turned on thereby flyback energy generated in the load is absorbed at the ON-state of the element. Pig. 15 shows modeled section of a semiconductor device in constitution that a Zener diode having protective function as above described is installed integral with a power MOSFET 10. In Pig. 15, n-type silicon substrate 11 being drain region is provided at rear surface side with n-type region 11a of high impurity atom concentration and at side of a main surface 11b with a number of p-type diffusion regions 12 forming a cell on a center portion (the right side in the figure), and also p-type diffusion region 13 is formed so as to surround the number of p-type diffusion regions 12. In the p-type diffusion regions 12 respectively forming the cell, n-type diffusion region 14 is formed where n-type impurities to become source region are diffused at high concentration. Also in a peripheral portion of the main surface 11b, n-type diffusion region 15 of high concentration for contact used also as a channel stopper is formed. A gate oxide film 16 is formed on a surface between neighboring p-type diffusion regions 12, and a channel is formed in the p-type diffusion regions 12 by a gate electrode 17 of polycrystalline silicon provided on an upper portion of the gate oxide film 16. In regions from the p-type diffusion region 13 of the main surface 11b to the n-type diffusion region 15 of high concentration, a silicon oxide film 18 for insulation is formed so as to cover these regions. On the silicon oxide film 18, a connecting electrode 19 is formed and positioned at the inner circumferential side and a polycrystalline silicon thin film layer 20 is formed and positioned at the outer circumferential side. The polycrystalline silicon thin film layer 20 is formed as n-type, p-type, n-type regions 20a, 20b, 20c in that order from the inner circumferential side to the outer circumferential side, thereby two Zener diodes 21, 22 subjected to breakdown at prescribed voltage are constituted in series connection in reverse direction. The p-type diffusion region 12, the n-type diffusion region 14 and the p-type diffusion region 13 as above described are electrically connected by a surface electrode 23, and the connecting electrode 19 and the n-type region 20a of the polycrystalline silicon thin film layer 20 are electrically connected by a surface electrode 24, and the n-type region 20c of the polycrystalline silicon thin film layer 20 and the n-type diffusion region 15 are electrically connected by a surface electrode 25. A source terminal S is connected to the surface electrode 23, and a gate terminal G is connected to the gate electrode 17 and the connecting electrode 19 by the surface electrode 24, and a drain terminal D is connected to a surface electrode 26 formed at the rear surface side of the silicon substrate 11. According to the above-mentioned constitution, if the drain terminal D side is connected to the power source through an inductive load (not shown) and the gate terminal G is supplied with a gate signal and the ON/OFF drive control is carried out, the flyback voltage generating in the load at the OFF state of the power MOSFET 10 is applied between the drain terminal D and the source terminal S. Since this applied voltage is applied through the n-type diffusion region 15 of the silicon substrate 11 and the surface electrode 25 to the polycrystalline silicon thin film layer 20 constituting Zener diodes 21, 22, when the voltage is prescribed voltage or more, the Zener diode 22 is subjected to breakdown and the gate terminal G is supplied with voltage. Then the power MOSFET 10 is turned on and conduction is done between the drain terminal D and the source terminal S therefore the flyback energy of the load is absorbed. Thereby the power MOSFET 10 is prevented from the overvoltage breakdown. In conventional constitution as above described, however, since reliability of insulation characteristics of the silicon oxide film 18 is deteriorated on account of following reasons, circumstances exist that the breakdown voltage of the Zener diodes 21, 22 cannot be set large. That is, in the above-mentioned constitution, the voltage applied nearly to the drain terminal D as voltage applied to the n-type region 20c among the polycrystalline silicon thin film layer 20 constituting the Zener diodes 21, 22 is applied to the upper surface side of the silicon oxide film 18, and the voltage applied to the source terminal S as potential of the p-type diffusion region 13 is applied to the lower surface side of the silicon oxide film 18. Consequently the voltage applied to the thickness direction of the silicon oxide film 18 becomes maximum at the portion of the n-type region 20c, and the voltage becomes nearly the same value as that of the potential difference applied between the drain D and the source S of the power MOSFET 10. Since the voltage applied to the power MOSFET 10 is limited by the breakdown voltage of the Zener diodes 21, 22 as above described, if the breakdown voltage is to be set large, also the potential difference applied to the thickness direction of the silicon oxide film 18 becomes large. Therefore in the silicon oxide film 18, since the electric field intensity for the thickness direction becomes larger, reliability for the dielectric breakdown is deteriorated. In other words, the breakdown voltage of the Zener diodes 21, 22 is restricted in that it cannot be set to the dielectric breakdown strength of the silicon oxide film 18 or more. EP 0 372 820 A discloses a semiconductor device having high energy sustaining capability. SUMMARY OF THE INVENTION In view of the above-mentioned circumstances, an object of the present invention is to provide a semiconductor device wherein even if potential difference between a first semiconductor element formed at semiconductor substrate side and a second semiconductor element formed on an insulation film becomes large, excessive voltage is not applied to the insulation film thereby the reliability for the dielectric breakdown of the insulation film can be prevented from being deteriorated. A semiconductor device of the present invention is defined in claim 1. Also a ring-shaped diffusion region formed so as to surround the periphery of the impurity diffusion region with specified spacing is provided on the main surface of the semiconductor substrate, and the second semiconductor element may be formed and positioned on the upper portion of the ring-shaped diffusion region. According to the semiconductor device of the present invention, if the voltage of reverse bias is applied to the pn junction part of the first semiconductor element formed between the semiconductor substrate and the impurity diffusion region, the voltage is applied also to the second semiconductor element connected electrically to the first semiconductor element. Then in the inside of the semiconductor substrate, the depletion layer region having width corresponding to amount of the voltage is formed at the outside of the impurity diffusion region. On the other hand, since the voltage applied to the second semiconductor element has potential distribution, the potential is applied to the upper surface portion of the insulation film. Consequently, potential difference between the voltage applied to the terminal of the second semiconductor element on the insulation film and the voltage applied to the lower surface side of the insulation film becomes less than the voltage applied between the semiconductor substrate and the impurity diffusion region. Thereby since the potential difference in the thickness direction of the insulation film can be decreased, reliability for the dielectric breakdown of the insulation film can be substantially improved. In other words, the voltage applied to the second semiconductor element can be set further large. Also when the ring-shaped diffusion region is provided, the depletion layer region formed at the outside of the impurity diffusion region becomes wide, and if it attains to the ring-shaped diffusion region, the potential at that time is shared by the ring-shaped diffusion region. That is, the ring-shaped diffusion region shares the voltage having value between the voltage of the semiconductor substrate and the voltage of the impurity diffusion region. Consequently, the potential difference between the voltage applied to the terminal of the second semiconductor element on the insulation film and the voltage applied to the lower surface side of the insulation film becomes less than the voltage applied between the semiconductor substrate and the impurity diffusion region. Thereby since the potential difference in the thickness direction of the insulation film can be decreased, the reliability for the dielectric breakdown of the insulation film can be substantially improved. In other words, the voltage applied to the second semiconductor element can be set further large. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of main part of a first embodiment of the invention showing in modeled section; Fig. 2 is an electric constitution diagram of the first embodiment; Fig. 3 is a modeled longitudinal sectional side view of a simulation model of the first embodiment; Fig. 4 is a potential distribution diagram within a silicon substrate estimated by simulation; Fig. 5 is an electric field intensity distribution diagram of a silicon oxide film estimated by simulation; Fig. 6 is a withstand voltage characteristic (reverse direction current characteristic) diagram estimated by simulation; Fig. 7 is a diagram corresponding to Fig. 6 when power source voltages V1, V2 are optimized; Fig. 8 is a modeled longitudinal sectional side view of main part of a device not covered by the present invention; Fig. 9 is a diagram corresponding to Fig. 2 showing the above device; Fig. 10 is a diagram corresponding to Fig. 1 showing a second embodiment of the invention; Fig. 11 is a modeled longitudinal sectional side view of a simulation model of the second embodiment; Fig. 12 is a distribution diagram showing voltage sharing of each part estimated by simulation; Fig. 13 is a diagram corresponding to Fig. 10 showing a third embodiment of the invention; Fig. 14 is a longitudinal sectional side view along circumferential direction of element parts to constitute a Zener diode group; and Fig. 15 is a modeled longitudinal sectional diagram showing an example in the prior art. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described regarding a first embodiment when the present invention is applied to a semiconductor in constitution of a longitudinal type power MOSFET of n channels and a protective Zener diode connected between drain and gate of the power MOSFET referring to Figs. 1 to 7 as follows. Fig. 1 shows main part of a power MOSFET 101 as a first semiconductor element in modeled longitudinal section. In Fig. 1, a silicon substrate 102 as a semiconductor substrate formed in n type as first conduction type becomes a drain region, and has n-type region 102a of high impurity atom concentration at its rear surface side. A number of p-type diffusion regions 103 to form a cell at a center portion (the right side in the figure) are provided at side of a main surface 102b of the silicon substrate 102, and also p-type diffusion region 104 is formed so as to surround the number of p-type diffusion regions 103. At the outer circumferential side of the p-type diffusion region 104 p-type diffusion region 105 with diffusion depth being shallow in comparison with the inner circumferential side is formed in coupled state. In the inside of the p-type diffusion regions 103 to form the cell respectively, impurities of n type are diffused at high concentration and n-type diffusion region 106 becoming a source region is formed. Also at a peripheral portion of the main surface 102b, n-type diffusion region 107 of high concentration for contact serving also as a channel stopper is formed. A gate oxide film 108 is formed covering between the neighboring p-type diffusion regions 103 and between the p-type diffusion region 103 and the p-type diffusion region 104, and a gate electrode 109 of polycrystalline silicon is formed on the gate film 108. The gate electrode 109 forms a channel region on the upper layer portion of the p-type diffusion region 103 in response to given gate voltage. In the region from the p-type diffusion region 104 of the main surface 102b to the high concentration n-type diffusion region 107, a silicon oxide film 110 as an insulation film is formed so as to cover these regions. On the upper portion at the inner circumferential side of the silicon oxide film 110, a gate plate 111 of polycrystalline silicon is formed so as to extend from the p-type diffusion region 105 to the outer circumferential side in connected state to the gate electrode 109. Since the gate plate 111 becomes the same potential as that of the gate electrode 109, when reverse bias is applied to the pn junction composed of the p-type diffusion regions 104, 105 and the n-type silicon substrate 102, the gate plate 111 acts to hold the part of the main surface 102b side of the generated depletion layer to the same potential as that of the gate electrode 109 through the silicon oxide film 110, and if the applied reverse bias is sufficiently more than the potential of the gate electrode 109, the electric field at the main surface 102b side of the silicon substrate 102 is relieved. Also on the silicon oxide film 110 at the outer circumferential side of the gate plate 111, element parts 112a, 112b, 112c constituted by polycrystalline silicon thin film layers with Zener diode parts formed along the circumferential direction and equipotential plates 113a, 113b, 113c constituted by polycrystalline silicon thin film layers formed in ring shape along the outer circumference of the gate plate 111 are arranged alternately. These are formed along the circumferential direction and therefore arranged in parallel to the end portion of the p-type diffusion 105. In the element parts 112a, 112b, 112c respectively, impurities of n type and p type are formed alternately along the circumferential direction, and a plurality of Zener diode pairs 114 each composed of Zener diodes connected in series in reverse directions are formed and respective both terminals are electrically connected between the neighboring gate plate 111 and the equipotential plate 113a, 113b or 113c. A Zener diode group 115 is formed by these Zener diode pairs 114. In this case, in the Zener diode pair 114, for example, the breakdown voltage Vz is set to 6V, and in the Zener diode group 115 as a whole, the breakdown voltage VZ is set to 380V. On the silicon oxide film 110 and the gate oxide film 108, BPSG (boron phosphorus contained silicon glass) protective films 116 having insulation property are formed so as to cover the element parts 112a to 112c, the equipotential plates 113a to 113c, the gate plate 111 and the gate electrode 109. A window to take electric contact is formed in prescribed position of these BPSG protective films 116. The p-type diffusion region 103, the n-type diffusion region 106 and the p-type diffusion regions 104, 105 as above described are electrically connected by a surface electrode 117, and the gate electrode 109 and the gate plate 111 are electrically connected by a surface electrode 118, and the equipotential plate 113c and the n-type diffusion region 107 are electrically connected by a surface electrode 119. A source terminal S is connected to the surface electrode 117, and a gate terminal G is connected to the surface electrode 118, the gate electrode 109 and the gate plate 111, and a drain terminal D is connected to a surface electrode 120 formed at the rear surface side of the n-type region 102a of the silicon substrate 102. Fig. 2 shows electric constitution, where a Zener diode group 115 constituted by a plurality of Zener diode pairs 114 in series connection is connected between a drain terminal D and a gate terminal G of a power MOSFET 101. For example, in the state that the drain terminal D is connected through an inductive load to the positive power source terminal and the source terminal S is earthed, the power MOSFET 101 controls conduction or interruption to the load by the control voltage supplied to the gate terminal G. Next, functions of this embodiment will be described. In the power MOSFET 101, in the OFF-state that the control voltage is not supplied to the gate terminal G, the power source voltage is applied as drain voltage to the drain terminal D through an inductive load (not shown). The drain voltage is supplied also to the Zener diode group 115. The breakdown voltage Vz of the Zener diode group 115 is set to value higher than the power source voltage so that breakdown does not occur in this state. Consequently, the potential of the gate terminal G does not fluctuate due to the drain voltage. In the above-mentioned state, the power source voltage applied between the drain terminal D and the source terminal S is applied in reverse direction to the pn junction between the silicon substrate 102 and the p-type diffusion regions 103, 104, 105, and is applied also to the equipotential plates 113a to 113c and the element parts 112a to 112c to constitute the Zener diode group 115. In this case, since the equipotential plate 113c is connected through the surface electrode 119, the n-type diffusion region 107 and the silicon substrate 102 to the drain terminal D, nearly equal voltage is applied to the equipotential plate 113c. In the pn junction part between the silicon substrate 102 and the p-type diffusion regions 103, 104, 105, since the impurity atom concentration of the silicon substrate 102 is set low, the depletion layer having width corresponding to the drain voltage spreads largely to the silicon substrate 102 side in the direction away from the pn junction surface, and the depletion layer part shares the almost drain voltage. Then in the upper portion of the silicon substrate 102, since the potential corresponding to the connection stage number of each Zener diode pair 114 of the Zener diode group 115 is distributed, the equipotential plates 113a to 113c respectively share the intermediate potential in sequence from the potential applied to the drain terminal D from the equipotential plates 113c to 113a, and the potential difference becomes small for the potential distribution of the depletion layer formed at these lower portions through the silicon oxide film 110. Consequently in the silicon oxide film 110, since the voltage applied between the upper and lower surfaces of the region from the outer circumferential part of the p-type diffusion regions 104, 105 to the n-type diffusion region 107 is decreased, the electric field intensity can be lowered. Thereby the silicon oxide film 110 having the same dielectric breakdown strength as that of the prior art can substantially improve the reliability of the dielectric breakdown for the Zener diode group 115 constituted by a plurality of Zener diode pairs 114. In other words, the breakdown voltage Vz of the Zener diode group 115 can be set to further larger value. In the power MOSFET 101, if the control voltage of the ON-operation is applied to the gate terminal G, the voltage applied to the gate electrode 109 is applied through the gate oxide film 108 to the p-type diffusion region 103, and a channel is formed in this part and the conduction state is produced between the drain terminal D and the source terminal S thereby current flows through the inductive load. If the control voltage of the OFF-operation is applied to the gate terminal G, the channel of the p-type diffusion region 103 is lost and the interruption state is produced between the drain terminal D and the source terminal S. Thereby the current flowing to the inductive load is interrupted, and in this state, the flyback voltage generated in the inductive load is applied to the drain terminal D of the power MOSFET 101. If the flyback voltage becomes higher than the power source voltage and exceeds the breakdown voltage Vz of the Zener diode group 115, the voltage value of the flyback voltage subtracted by the breakdown voltage Vz is applied to the gate terminal G. Then the power MOSFET 101 is turned on again by the gate voltage and becomes the conductive state and absorbs the flyback energy of the inductive load. Thereby the power MOSFET 101 is prevented from the overvoltage breakdown due to the flyback voltage. According to this embodiment, in the constitution that the Zener diode group 115 comprising a plurality of Zener diode pairs 114 is installed and the power MOSFET 101 is prevented from the overvoltage breakdown, since the polycrystalline silicon element parts 112a to 112c to constitute the plural Zener diode pairs 114 of the Zener diode group 115 and the equipotential plates 113a to 113c to connect these element parts electrically are formed, the potential difference in the film thickness direction of the silicon oxide film 110 at the lower portion of the Zener diode group 115 can be lowered. Thereby the dielectric breakdown strength of the silicon oxide film 110 can be substantially improved. In other words, the breakdown voltage Vz of the Zener diode group 115 can be further enlarged. The inventor has confirmed effects of the withstand voltage improvement according to the above-mentioned constitution in carrying out simulation by a computer using structure model shown in Fig. 3. That is, a silicon substrate SUB (n-type impurity atom concentration is 4 x 10 14 atm/cm 3 and thickness is 65µm) in Fig. 3 is set as that corresponding to the silicon substrate 102, and p-type diffusion region region PA corresponding to the p-type diffusion regions 104, 105 is provided on the right end at the main surface side of the upper portion of the substrate SUB. Also n-type diffusion region NA corresponding to the n-type diffusion region 107 is provided on the left end at the main surface side of the upper portion of the silicon substrate SUB. A silicon oxide film SIO with thickness of 0.8µm corresponding to the silicon oxide film 110 is arranged on the main surface of the silicon substrate SUB, and electrodes E0 to E3 corresponding to the gate plate 111, the equipotential plates 113a to 113c are arranged on the upper portion of the main surface so that the equivalent potential to the power MOSFET 101 is given from the outside. In the actual power MOSFET 101, the Zener diode pairs 114 are formed in connected state between the equipotential plates 113a to 113c respectively, but in this simulation, the voltage supplied to each of the equipotential plates 113b and 113c by the Zener diode group 115 is supplied to the electrodes E1 and E2 by the power sources V1 and V2 having prescribed voltage respectively from the outside. In the simulation, the potential distribution within the silicon substrate SUB is calculated when voltage of 380V as V0 for example is applied between the terminal D of the silicon substrate SUB and the terminal S of the p-type diffusion region PA. Then the power sources V1 and V2 are set so that the potential difference becomes equal between the electrodes E0 - E1, between the electrodes E1 - E2 and between the electrodes E2 - E3. This result is shown in Fig. 4. That is, Fig. 4 shows the potential distribution within the silicon substrate SUB in the equipotential lines at intervals of 20V, where each equipotential line extends in the direction of spreading the depletion layer in the vicinity of the surface of the silicon substrate SUB and the potential difference between the upper and lower surfaces of the silicon oxide film SIO is reduced. It is found that the maximum value of the electric field intensity of the silicon oxide film SIO in this state is 1MV/cm or less as shown in Fig. 5, and this value is sufficiently small in comparison with 7 - 8 MV/cm being the value of the critical electric field intensity for the silicon oxide film to be subjected to the dielectric breakdown and in entirely problemless level from the viewpoint of the reliability. Also when the applied voltage V0 is increased in the above-mentioned constitution, it is found that the withstand voltage between the p-type diffusion region PA and the silicon substrate SUB is about 415V as shown in Fig. 6. As a result, when the withstand voltage between the drain D and the gate G of the power MOSFET 101 is sufficiently large in comparison with 380V being the clamp voltage by the Zener diode group 115 and moreover the voltage of 380V or more is applied between the drain D and the gate G, the pn junction composed of the p-type diffusion region PA and the silicon substrate SUB is not subjected to breakdown but the Zener diode group 115 is subjected to breakdown. The inventors further tried that in the above-mentioned simulation, values of the power source voltages V1 and V2 are optimized and the breakdown voltage of the pn junction between the p-type diffusion region PA and the silicon substrate SUB is raised, and for example, when ratio of the potential difference between the electrodes E0 - E1, the potential difference between the electrodes E1 - E2 and the potential difference between the electrodes E1 - E2 is made 12 : 19 : 26, it is found that the withstand voltage becomes 460V as shown in Fig. 7. This shows that when the connection number of the Zener diode pairs 114 of the Zener diode group 115 giving voltage corresponding to the power source voltages V1 and V2 is adjusted and the shared voltage of the element parts 112a to 112c is set to the optimum value, the above-mentioned voltage 460V can be obtained. According to this embodiment, in constitution that the Zener diode group 115 is provided and the power MOSFET 101 is prevented from the overvoltage breakdown, since the element parts 112a to 112c and the equipotential plates 113a to 113c by polycrystalline silicon to constitute the Zener diode group 115 are electrically connected and the applied voltage between the drain electrode D and the gate electrode G is properly distributed to the equipotential plates 113a to 113c and the potential is held, width of the depletion layer spreading on the main surface of the silicon substrate 102 is widened and the electric field intensity is lowered and the withstand voltage is improved, and the potential difference applied to the silicon oxide film 110 is lowered and the dielectric breakdown strength can be improved substantially. In this first embodiment, although the Zener diode group 115 is constituted to have Zener voltage of 380V, the invention is not limited to this but the number of the Zener diode pairs 114 may be properly increased or decreased and the desired Zener voltage VZ can be set. Figs. 8 and 9 show a device not of covered by the present invention, and this will be described as follows. Fig. 8 shows main part of a power MOSFET 31 as a first semiconductor element in modeled longitudinal section. In Fig. 8, a silicon substrate 32 as a semiconductor substrate formed in n type as first conduction type becomes a drain region, and has n-type region 32a of high impurity atom concentration at its rear surface side. A number of p-type diffusion regions 33 to form a cell at a center portion (the right side in the figure) are provided at side of a main surface 32b of the silicon substrate 32, and also p-type diffusion region 34 is formed so as to surround the number of p-type diffusion regions 33. At the outer circumferential part of the p-type diffusion region 34, a ring-shaped diffusion region 35 with p-type impurities diffused is formed so as to surround the p-type diffusion region 34 with prescribed spacing. In the inside of the p-type diffusion regions 33 to form the cell respectively, n-type diffusion region 36 (source region) with impurities of n type diffused at high concentration is formed. Also at a peripheral portion of the main surface 32b, n-type diffusion region 37 of high concentration for contact serving also as a channel stopper is formed. A gate ozide film 38 is formed covering between the neighboring p-type diffusion regions 33 and between the p-type diffusion region 33 and the p-type diffusion region 34, and a gate electrode 39 of polycrystalline silicon is formed on the gate film 38. The gate electrode 39 forms a channel region on the upper layer portion of the p-type diffusion region 33 in response to given gate voltage. In the region from the p-type diffusion region 34 of the main surface 32b to the high concentration n-type diffusion region 37, a silicon oxide film 40 as an insulation film is formed so as to cover these regions. On the silicon oxide film 40, a connecting electrode 41 is formed and positioned at the inner circumferential side, and a polycrystalline silicon thin film layer 42 is formed and positioned at the outer circumferential side. Also the polycrystalline silicon thin film layer 42 is formed by introducing impurities from the inner circumferential side towards the outer circumferential side in ring shape in sequence into n-type, p-type, n-type regions 42a, 42b, 42c thereby in constitution having the two pn junctions. By these two pn junctions, constitution of the two Zener diodes 43, 44 subjected to breakdown at the prescribed voltage Vz connected in series in reverse directions is taken. On the silicon oxide film 40 and the gate oxide film 38, BPSG (boron phosphorus contained silicon glass) protective films 45 having insulation property are formed so as to cover the polycrystalline silicon thin film layer 42 and the gate electrode 39 respectively. Windows 45a, 45b, 45c to take electric contact are formed in prescribed positions of these BPSG protective films 45. The p-type diffusion region 33, the n-type diffusion region 36 and the p-type diffusion region 34 as above described are electrically connected by a surface electrode 46, and the connecting electrode 41 and the n-type impurity region 42a of the polycrystalline silicon thin film layer 42 are electrically connected by a surface electrode 47, and the n-type impurity region 42c of the polycrystalline silicon thin film layer 42 and the n-type diffusion region 37 are electrically connected by a surface electrode 48. A source terminal S is connected to the surface electrode 46, and a gate terminal G is connected to the gate electrode 39 and the connecting electrode 41, and a drain terminal D is connected to a surface electrode 49 formed at the rear surface side of the silicon substrate 32. The p-type ring-shaped diffusion region 35 is provided in non-connected state to any of surface electrode 46, 47 or 48 and therefore in floating state electrically. Fig. 9 shows electric constitution, where a circuit constituted by Zener diodes 43 and 44 in series connection with reverse polarities is connected between a gate terminal G and a drain terminal D of a power MOSFET 31. For example, in the state that the drain terminal D is connected through an inductive load to the positive power source terminal and the source terminal S is earthed, the power MOSFET 31 controls conduction or interruption to the load by the control voltage supplied to the gate terminal G. Next, functions of this embodiment will be described. In the power MOSFET 31, in the OFF-state that the control voltage is not supplied to the gate electrode G, the power source voltage is applied as drain voltage to the drain terminal D through an inductive load (not shown). The drain voltage is supplied also to the series circuit of the Zener diodes 43, 44. The breakdown voltage Vz of the Zener diodes 43, 44 is set to value higher than the power source voltage so that breakdown does not occur in this state. Consequently, the potential of the gate terminal G does not fluctuate due to the drain voltage. In the above-mentioned state, the power source voltage applied between the drain terminal D and the source terminal S is applied in reverse direction to the pn junction between the silicon substrate 32 and the p-type diffusion regions 33, 34, and is applied also between the n-type region 42c and the p-type region 42b to constitute the Zener diode 44. In this case, nearly the same voltage as that of the drain terminal D is applied to the n-type region 42c through the surface electrode 48, the n-type diffusion region 37 and the silicon substrate 32. In the pn junction part between the silicon substrate 32 and the p-type diffusion regions 33, 34, the depletion layer having width corresponding to the drain voltage spreads largely to the silicon substrate 32 side in the direction away from the pn junction surface, and the depletion layer part shares the almost drain voltage. Then in the ring-shaped diffusion region 35, since potential is not fixed, the potential depends on potential in a part to which the depletion layer spreading from the pn junction surface between the silicon substrate 32 and the p-type diffusion region 34 attains. That is, the potential of the ring-shaped diffusion diffusion region 35 becomes value lower than the drain voltage (power source voltage) and higher than the source voltage (earth). Thereby voltage of nearly the ring-shaped diffusion region 35 is applied to the lower surface side of the silicon oxide film 40 positioned at upper side of the ring-shaped diffusion region 35. On the other hand, since nearly the drain voltage is applied to the n-type region 42c of the polycrystalline silicon thin film layer 42 positioned at upper surface side of the silicon oxide film 40 as above described, in this part, voltage applied in the film thickness direction of the silicon oxide film 40 becomes lower voltage than the drain voltage. Also since the p-type region 42b and the n-type region 42a of the polycrystalline silicon thin film layer 42 become nearly the earth potential, in this part, voltage applied in the film thickness direction of the silicon oxide film 40 becomes lower voltage than the drain voltage. When the arrangement position of the ring-shaped diffusion region 35 is properly selected, the voltage applied in the film thickness direction can be set to nearly intermediate value of voltage applied between the drain terminal D and the source terminal S. Thereby the silicon oxide film 40 having the same dielectric breakdown strength as that of the prior art can substantially improve the reliability of the dielectric breakdown for the Zener diodes 43, 44. In other words, the breakdown voltage Vz of the Zener diodes 43, 44 can be set to further larger value. In the power MOSFET 31, if the control voltage of the ON-operation is applied to the gate terminal G, the voltage applied to the gate electrode 39 is applied through the gate oxide film 38 to the p-type diffusion region 33, and a channel is formed in this part and the conduction state is produced between the drain terminal D and the source terminal S thereby current flows through the inductive load. If the control voltage of the OFF-operation is applied to the gate terminal G, the channel of the p-type diffusion region 33 is lost and the interruption state is produced between the drain terminal D and the source terminal S. Thereby the current flowing to the inductive load is interrupted, and in this state, the flyback voltage generated in the inductive load is applied to the drain terminal D of the power MOSFET 31. If the flyback voltage becomes higher than the power source voltage and exceeds the breakdown voltage Vz of the Zener diode 43, the voltage value of the flyback voltage subtracted by the breakdown voltage Vz is applied to the gate terminal G. Then the power MOSFET 31 is turned on again by the gate voltage and becomes the conductive state and absorbs the flyback energy of the inductive load. Thereby the power MOSFET 31 is prevented from the overvoltage breakdown due to the flyback voltage. According to this embodiment, in the constitution that the Zener diodes 43, 44 are installed and the power MOSFET 31 is prevented from the overvoltage breakdown, since the ring-shaped diffusion region 35 is formed in the main surface 32b of the silicon substrate 32 at lower portion of the polycrystalline silicon thin film layer 42 to constitute the Zener diodes 43, 44, potential of the ring-shaped diffusion region 35 can be set to potential between the drain terminal D and the source terminal S, and the potential difference in the film thickness direction of the silicon oxide film 40 at the lower portion of the polycrystalline silicon thin film layer 42 can be lowered. Thereby the dielectric breakdown strength of the silicon oxide film 40 can be substantially improved. In other words, the breakdown voltage Vz of the Zener diodes 43, 44 can be further enlarged. In this device, although the Zener diodes 43, 44 in the polycrystalline silicon thin film layer 42 are formed by the three regions 42a, 42b, 42c in n-type, p-type, n-type, constitution is not limited to this, but, for example, the two Zener diodes may be formed by three regions in p-type, n-type, p-type. Figs. 10 to 12 show a second embodiment of the present invention, and description regarding these figures is as follows. Fig. 10 is a perspective view in section showing main part of n-channel power MOSFET 50 of high withstand voltage. In Fig. 10, n-type silicon substrate 1 as a semiconductor substrate has n-type region 51a at a lower layer, and on a main surface 51b at upper surface side are formed a number of p-type diffusion regions 52 to form a cell and p-type diffusion region 53 so as to surround the diffusion regions 52. At the outer circumferential part of the p-type diffusion region 53, for example, four ring-shaped diffusion regions 54a to 54d with p-type impurities diffused are formed in parallel to the outer circumferential edge of the p-type diffusion region 53 so as to surround this with prescribed spacing. In this case, the four ring-shaped diffusion regions 54a to 54d are so-called guard ring provided to make the power MOSFET 50 high withstand voltage structure. In the inside of the p-type diffusion region 52 to form the cell respectively, impurities of n type are diffused at high concentration and n-type diffusion region 55 becoming a source region is formed. Also at a peripheral portion of the main surface 51b, n-type diffusion region 56 of high concentration for contact serving also as a channel stopper is formed. A gate oxide film 57 is formed covering between the neighboring p-type diffusion regions 52 and between the p-type diffusion region 52 and the p-type diffusion region 53, and a gate electrode 58 of polycrystalline silicon is formed on the gate film 57. The gate electrode 58 forms a channel region on the upper layer portion of the p-type diffusion region 52 in response to given gate voltage. In the region from the p-type diffusion region 53 of the main surface 51b to the n-type diffusion region 56, a silicon oxide film 59 as an insulation film is formed so as to cover these regions. On the silicon oxide film 59, a gate electrode 58 is positioned at the inner circumferential side and extended and formed as a connecting electrode 60, and at the outer circumferential side, a polycrystalline silicon thin film layer 62 to constitute a Zener diode group 61 as a second semiconductor element is formed. The polycrystalline silicon thin film layer 62 comprises a connecting part 62a formed on covering part between the p-type diffusion region 53 and the ring-shaped diffusion region 54a, between the ring-shaped diffusion regions 54a to 54d, between the ring-shaped diffusion region 54d and the high concentration n-type diffusion region 56 respectively, and an element part 52b formed on each of the ring-shaped diffusion regions 54a to 54d. The connecting part 62a of the polycrystalline silicon thin film layer 62 is formed in n-type region by impurities. Also in the element part 62b, the p-type region and the n-type region are provided in sequence so that, for example, 15 pairs of Zener diode pairs 61a each composed of Zener diodes connected in series in reverse directions are formed along the circumferential direction of the ring-shaped diffusion regions 54a to 54d respectively. In order to connect these element parts 62b in series further in sequence, each element part 62b is connected at both ends to the neighboring part 62a. Thereby the Zener diode group 61 is formed in constitution that the total 60 pairs of the Zener diode pairs 61a are connected in series. In this case, in the Zener diode pair 61a, for example, the breakdown voltage Vz is set to 6V, and in the Zener diode group 61 as a whole, the breakdown voltage is set to 360V. On the silicon oxide film 59 and the gate oxide film 57, BPSG (boron phosphorus contained silicon glass) protective films 63 having insulation property are formed so as to cover the polycrystalline silicon thin film layer 62 and the gate electrode 59. A window to take electric contact is formed in prescribed position of these BPSG protective films 45. The p-type diffusion region 52, the n-type diffusion region 55 and the p-type diffusion region 53 as above described are electrically connected by a surface electrode 64, and the connecting electrode 60 and the connecting part 62a at the inner circumferential side of the polycrystalline silicon thin film layer 62 are electrically connected by a surface electrode 65, and the connecting part 62a at the outer circumferential side of the polycrystalline silicon thin film layer 62 and the n-type diffusion region 56 are electrically connected by a surface electrode 66. A source terminal S is connected to the surface electrode 64, and a gate terminal G is connected to the gate electrode 58 and the connecting electrode 60, and a drain terminal D is connected to a surface electrode 67 formed at the rear surface side of the silicon substrate 51. The four p-type ring-shaped diffusion regions 54a to 54d are provided in non-connected state to any of surface electrodes 64, 65, 66 or 67 and therefore in electrically floating state. Electric constitution is carried out similarly to the first embodiment in the state that the Zener diode group 61 in series connection of 60 pairs of the Zener diode pairs 61a is connected between the rain terminal D and the gate terminal G of the power MOSFET 50. According to the above-mentioned constitution, if excessive voltage is applied between the drain terminal D and the source terminal S, the Zener diode group 61 is subjected to breakdown and voltage is applied to the gate terminal G, thereby the power MOSFET 50 is turned on and the overvoltage breakdown is prevented. When normal power source voltage is applied between the drain terminal D and the source terminal S, the Zener diode group 61 is in the state being not subjected to breakdown, and if the control voltage is not applied to the gate terminal G, the power MOSFET 50 holds the OFF-state. In this case, in the pn junction between the p-type diffusion regions 52, 53 and the silicon substrate 51, the depletion layer spreads to the silicon substrate 51 side due to the applied voltage, and since each of the ring-shaped diffusion regions 54a to 54d becomes potential when the depletion layer attains, the potential becomes large stepwise in sequence from the ring-shaped diffusion region 54a at the inner circumferential side towards the ring-shaped diffusion region 54a at the outer circumferential side as described later. On the other hand, in the polycrystalline silicon thin film layer 62 to constitute the Zener diode group 61 as above described, voltage applied to each element part 62b becomes large in sequence from the inner circumferential side towards the outer circumferential side. Then in the ring-shaped diffusion regions 54a to 54d arranged at lower side of each element part 62b, since potential becomes large stepwise in sequence, the potential difference applied between upper and lower surfaces of the silicon oxide film, that is, in the film thickness direction becomes less than the power source voltage. Consequently, margin is given to the strength for the dielectric breakdown of the silicon oxide film 59, and the dielectric breakdown strength is substantially improved. In other words, the withstand voltage of the Zener diode group 61 can be further raised. The inventor has carried out simulation regarding potential applied to each of the ring-shaped diffusion regions 54a to 54d as above described, using model shown in Fig. 11. That is, a silicon substrate Sub (n-type impurity atom concentration is 1.5 x 10 14 atm/cm 3 ) in Fig. 11 corresponds to the silicon substrate 51, and impurity atom concentration and geometric arrangement state are fitted respectively so that the p-type regions P0 to P4 correspond to the p-type diffusion region 53, the ring-shaped diffusion regions 54a to 54 respectively, and the n-type region N1 corresponds to the n-type diffusion region 56, and the electrode EQR corresponds to the surface electrode 66. In the simulation, the potential distribution in each point on the main surface of the silicon substrate Sub is calculated from the spread state of the depletion layer when voltage of 415V for example is applied between the terminal D of the silicon substrate Sub and the terminal S of the p-type region P0. As a result, it is found that on the main surface of the silicon substrate Sub, the potential becomes large stepwise in sequence towards the p-type regions P0 to P4. By optimizing intervals of the p-type regions P0 to P4, since the potential can be set to amount close to potential of each element part 62b of the polycrystalline silicon thin film layer 62, the potential difference applied in the film thickness direction of the silicon oxide film 59 in constitution of this embodiment can be decreased and the dielectric breakdown strength can be substantially improved. Consequently, in this setting, reversely, within range of the dielectric breakdown strength of the silicon oxide film 59, the breakdown voltage of the Zener diode group 61 can be further set to large value. According to the second embodiment, since each element part 62b of the polycrystalline silicon thin film layer 62 to constitute the Zener diode group 61 is arranged to upper side of the four ring-shaped diffusion regions 54a to 54d respectively, the potential difference applied in the film thickness direction of the silicon oxide film 59 interposed between the main surface 51b and the polycrystalline silicon thin film layer 62 can be reduced extremely, and the dielectric breakdown strength is substantially improved and the breakdown voltage VZ set by the Zener diode group 61 can be made further higher value. In the second embodiment, although the 60 pairs of Zener diode pairs 61a are provided as the Zener diode group 61, constitution is not limited to this, but the number of the Zener diode pairs 61a may be further increased or decreased. Also in the second embodiment, although the description has been carried out in the case of constitution that one Zener diode group 61 is provided, constitution is not limited to this, but, for example, a plurality of Zener diode group 61 may be arranged and formed in parallel throughout the whole circumference of the forming region of the ring-shaped diffusion regions 54a to 54d. In this case, since the current capacity of the Zener diode group 61 can be increased, utilization in the power MOSFET corresponding to the current capacity of large current becomes possible. Figs. 13 and 14 show a third embodiment of the present invention. Parts different from the second embodiment will be described as follows. In Fig. 13, difference from the second embodiment is in that at intermediate position of each element part 62b to form a Zener diode group 61, a conductive electrode 81 for connecting electrically each element part 62b to ring-shaped diffusion regions 54a to 54d positioned as the lower part through the silicon oxide film 59. Fig. 14 is a diagram showing one element part 62b in section along circumferential direction. For example, at intermediate position of the element part 62b, a contact hole 82 is formed so as to penetrate a BPSG protective film 63, a polycrystalline thin film layer 62 and a silicon oxide film 59, and the ring-shaped diffusion regions 54a to 54d formed within the silicon substrate 51 are exposed. Also at the polycrystalline thin film layer 62 positioned at both sides of the contact hole 82 is formed a contact hole 83 which opens and exposes the BPSG protective film 63. The conductive electrode 81 is formed by aluminium evaporation and photo lithography so as to cover both the contact holes 82 and 83. Thereby since each of the ring-shaped diffusion regions 54a to 54d becomes conductive state to the intermediate position of the element part 62b to constitute the Zener diode group 61 respectively through the conductive electrode 81, it is held to the same potential as that of the Zener diode pair 61a in that part. Consequently, according to the third embodiment, voltage shared by the Zener diode pair 61a in the part of installing the conductive electrode 81 is set equal to the voltage value obtained by simulation, thereby the potential difference applied in the film thickness direction of the silicon oxide film 59 can be suppressed to prescribed value or less and the reliability of the silicon oxide film 59 can be improved. Also the shared voltage is properly set, thereby reverse direction withstand voltage of the pn junction between the p-type diffusion region 53 and the silicon substrate, hence reverse direction withstand voltage between the drain D and the gate G can be adjusted and set to the optimum value. In each of embodiments as above described, although the description has been carried out regarding the case that the n-channel power MOSFET 31, 50 or 101 is used as the first semiconductor element, constitution is not limited to this, but, for example, application to the p-channel power MOSFET is possible, or application not only to the power MOSFET but also to the semiconductor element such as a bipolar transistor or IGBT (insulation gate type bipolar transistor) becomes possible.

Claims: Power semiconductor device, comprising: [a] a semiconductor substrate (102) of a first conductivity type (n), said semiconductor substrate (102) having a first part in which a power semiconductor element (101) is formed and a second part on which protective means for protecting said power semiconductor element is formed, said second part surrounding said first part; [b] an impurity diffusion region (103 to 105) of a second conductivity type (p) formed at a main surface (102b) of said first part of said semiconductor substrate (102), said power semiconductor element (101) including said impurity diffusion region (103 to 105); [c] a first electrode (120) electrically contacting said semiconductor substrate (102); [d] a second electrode (117) electrically contacting said impurity diffusion region (103 to 105), a PN junction between said semiconductor substrate (102) and said impurity diffusion region (103 to 105) being reverse biased upon the application of a potential between said first and second electrodes (120,117); [e] a third electrode (109) disposed on said impurity diffusion region (103 to 105) with an insulating layer (108) therebetween; and [f] an insulation film (110) interposed between said semiconductor substrate (102) and said protective means to cover a main surface of said second part of said semiconductor substrate (102), wherein said protective means includes: a semiconductor film arranged on said insulation film (110) and including plural strings (112a to 112c) each of which has several diodes (114), said plural strings (112a to 112c) being disposed on said second part in parallel to an end portion of said impurity diffusion region (103 to 105) and said plural strings (112a to 112c) comprising an innermost string (112a) and an outermost string (112c); and means for applying a potential equivalent to that applied to said first electrode (120) to said semiconductor film at one end remote from said impurity diffusion region (103 to 105) adjacent to the outermost string (112c), and wherein said semiconductor film is connected at another end adjacent to the innermost string (112a) to said third electrode (109) such that the semiconductor film has a potential distribution approaching a potential applied to said third electrode (109) from a potential applied to said first electrode (120). Semiconductor device according to claim 1, characterized in that it further comprises a field plate part (111) between said impurity diffusion region (103 to 105) and said innermost string (112a). Semiconductor device according to claim 1 or 2, characterized in that a potential distributed throughout said semiconductor film is matched with a potential distribution of a depletion layer extending from said PN junction towards the semiconductor substrate side. Semiconductor device according to claim 1, characterized in that the potential of each string (112a to 112c) is matched with a potential corresponding to the position of a depletion layer extending from said PN junction towards the semiconductor substrate side. Semiconductor device according to one of the claims 1, 3 or 4, characterized in that it further comprises a diffusion layer of the second conductivity type in an electrically floating state, said diffusion layer being located at the main surface of said second part of said semiconductor substrate (102) and beneath said semiconductor film. Semiconductor device according to claim 1, 3 or 4, characterized in that it further comprises diffusion layers of the second conductivity type in an electrically floating state, said diffusion layers being located at the main surface of said second part of said semiconductor substrate (102) and being formed independently and along said strings (112a to 112b). Semiconductor device according to claim 6, characterized in that each of said diffusion layers is located at a corresponding portion beneath said respective strings (112a to 112c). Semiconductor device according to claim 7, characterized in that each of said diffusion layers of the second conductivity type is provided with a potential of the semiconductor film in each string (112a to 112c). Semiconductor device according to one of the claims 1 through 8, characterized in that : the semiconductor device further comprises a source region (106) of the first conductivity type (n) formed at a main surface of said impurity diffusion region (103 to 105), the third electrode is a common gate electrode (109), and said second electrode (117) contacts to both said source region (106) and said impurity diffusion region (103 to 105). Semiconductor device according to one of the claims 1 through 9, characterized in that said means for applying to said semiconductor film at one end remote from said impurity diffusion region (103 to 105) a potential equivalent to that applied to said first electrode (120) is an electrode which connects a high concentration diffusion region (107) of said semiconductor substrate (102) with said semiconductor film at one end remote from said impurity diffusion region (103 to 105). Semiconductor device according to claim 9, characterized in that it further comprises a field plate part (111) between said impurity diffusion region (103) and said innermost string (112a) which connects said innermost string (112a) with said common gate electrode (109). Power semiconductor device according to claim 1, characterized in that said semiconductor film further comprises plural equipotential plates (113a to 113c), said plural strings (112a to 112c) and said equipotential plates (113a to 113c) are alternately disposed and connected to each other.

Inventors: KATO NAOHITO, OKABE NAOTO, TOYODA ETSUJI, KATO, NAOHITO, OKABE, NAOTO, TOYODA, ETSUJI
Applicants: DENSO CORP, DENSO CORPORATION
Application year: 1993
Main classification: H01L27
IPC codes: H01L21, H01L27, H01L29