Insulated gate type bipolar-transistor with overvoltage protection

Abstract: An insulated gate type bipolar-transistor (IGBT) incorporates an excess voltage protecting function and drain voltage fixing function in a monolithic structure. Impurity concentration ND and the thickness of an n⁻ type drain layer (3) is set so that a depletion region propagating from a p type base layer (7) reaches a p⁺ type drain layer at a voltage (V DSP ) lower than a voltage (V DSS ) at which avalanche breakdown is caused within the IGBT element when voltage is applied between the source and the drain.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an insulated gate type bipolar-transistor (hereafter referred to as "IGBT") integrating excess voltage protecting function or drain voltage fixing function. 2. Description of the Related Art Upon turning ON a power switching element employed as an inverting device for a motor circuit or a non-interruption power source circuit, a high voltage can be induced on an inductive load or a floating inductance in the circuit due to abrupt variation of circuit current flowing through the circuit. This high voltage is applied to the power switching element as a spike voltage to cause fatigue or breakage of the power switching element., As a means for protecting the power switching element from excess voltage, a voltage clamp circuit including a constant voltage diode based on the operation principle of avalanche breakdown for turning ON the constant voltage diode before breakdown occurs in the power switching element because of applied spike voltage and thus fixing the level of the spike voltage within a safe operation range, is employed. In the case that IGBT is employed as the power switching element, a similar means can be applied. In this case, the constant voltage diode is externally connected between the drain and gate of the IGBT element. However, it causes an increase in of the assembling cost and also increases the overall circuit dimension. It can form the constant voltage diode integrally with the IGBT element by depositing a polycrystalline silicon layer on the substrates formed thereon the IGBT element to form the constant diode therein. However, this increases the necessary number of photo-masks in the element fabrication process leading to an increase in the production cost. Furthermore, in order to form the constant voltage diode on the surface of the IGBT element, it becomes necessary to make the area for a cell region smaller or increase the chip dimension. As a solution for this problem, Japanese Patent Publication JP-A-181270 proposes a method for integrating the constant voltage diode, in which the constant voltage diode having an operational principle of avalanche breakdown is incorporated in the IGBT element so that the constant voltage diode is disposed between the drain and the source on the equivalent circuit. This method solves problems, such as narrowing of the chip area of the IGBT element. The proposed construction is shown in Fig. 6. In Fig. 6, the reference numeral 61 denotes a source electrode, 62 denotes a drain electrode, 63 denotes a gate electrode. The upper layer of a substrate is composed of an insulating gate of DSA (diffusion self-alignement) construction, p + base layer 65, n + source layer 66 and forms a p type channel immediately below the gate. On the other hand, observing the vertical structure, the substrate has a npnp four layer structure including n + source layer 66, p + base layer 65, n - drain layer 67 and p + drain layer 64 between n + layer 68 and drain electrode 62. The p + drain layer 64 at the side of the drain electrode 62 is separated into respective small segments and arranged parallel in a honeycomb fashion. This forms a structure, in which a diode with a pn junction is disposed between the collector and the emitter of a pnp transistor parallel thereto. The diode has a portion 69 of n + layer projecting into the n - layer 67 to be positioned closer to the p + layer 65 to provide avalanche type diode characteristics and thus protect the IGBT element from excess voltage. However, in order to incorporate the constant voltage diode, it is essential to provide n + layer 68 at the drain side of the substrate. Because of the presence of n + layer 68, hole injection from the drain side upon conducting the IGBT is restricted to make the ON voltage higher. In addition, for fabricating the p + layer in the honeycomb configuration at the side of the drain electrode 62 and to extend the part 69 of the n + layer 68 into the n - drain layer 67, the substrate structure becomes complicated and thereby increases costs for fabrication of the wafer and costs for production. Further information regarding the prior art can be found in S. M. Sze, "Physics of Semiconductor Devices", 2nd. edn., New York: Wiley & Sons, 1981, pp.194-195, a reference book that details many of the physical phenomena of semiconductor devices including punchthrough and avalanche breakdown. IEEE Electron Devices Letters, vol. EDL-7, no.12, December 1986, pp. 686-688 (Baliga) demonstrates that the output characteristics of insulated gate transistors (IGT's) are determined by the variation in the gain of the wide-base transistor in the device structure and that improvements in differential output resistance can be achieved either by decreasing the lifetime via electron irradiation or by using a punchthrough base design. This is illustrated on the basis of a insulated gate type bipolar-transistor having features similar to those of the preamble of claim 1. Patent Abstracts of Japan, vol. 10, no. 129 (E-403) [2186], May 14, 1986, corresponding to JP-A-60 260 152 proposes the formation of a high speed MOS gate bipolar transistor according to a simple construction by a method wherein a reversely conductive high concentration region is formed selectively in a single conductive substrate, and a reversely conductive low concentration layer is formed on the whole surface. In particular, an N+ type layer to suppress implantation of holes is provided between a P + type substrate and an N- type layer. The N+ type layers thereof can be formed easily by adhering phosphorus, antimony, arsenic, etc. according to diffusion or the ion implantation method, etc. to the P+ type substrate, prior to growing the N- type layer. According to this construction, implantation of unnecessary carriers is suppressed, and turn-off is performed at a high speed. Moreover, a difficulty in regard to the process as the usual method is reduced sharply. DE-A-3 519 389 discloses a MOSFET with alterable conductivity comprising a semiconductor substrate of a first conductivity type, a semiconductor layer of a second conductivity type formed on the substrate and having a high resistivity, a base layer of the first conductivity type created in the semiconductor layer, a source layer of the second conductivity type formed in the base layer, a gate electrode that is formed on a gate isolating layer that itself is formed on a channel zone that, on the other hand, is created in a surface of the base layer between the semiconductor layer and the base layer, a source electrode that is in ohmic contact with the source layer and the base layer, and a drain electrode that is formed on the surface of the semiconductor substrate opposite the semiconductor layer. This MOSFET is characterized in that its saturation current is smaller than a latch-up current when a predetermined gate voltage is applied to the gate electrode. SUMMARY OF THE INVENTION In view of the problems set forth above, it is an object of the present invention to provide an IGBT element that incorporates excess voltage protection and drain voltage fixing function in a monolithic structure with a function for clamping a drain-source voltage without causing an increase in the ON voltage. In order to accomplish the aforementioned and other objects, instead of incorporating the constant voltage diode having an operational principle of avalanche breakdown in the IGBT element, the present inventors propose a structure in which avalanche breakdown is suppressed within an IGBT element under a desired clamping condition of drain-source voltage and a minority carrier is injected from p + substrate to n - layer. In accordance with the present invention, this is achieved by the insulated gate type bipolar-transistor in accordance with claim 1. Further embodiments of the invention are detailed in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be more clearly understood from the description set forth below with reference to the accompanying drawings, wherein: Fig. 1 is a section showing a unit cell and an outer peripheral guard ring portion of an IGBT; Fig. 2 is a section showing a unit cell and an outer peripheral guard ring portion of an IGBT according to the invention; Fig. 3 is an electric characteristic chart of the IGBT of Fig. 1; Fig. 4 is an electric characteristic chart of the IGBT of Fig. 2; Fig. 5 is a section at the pn junction 5; and, Fig. 6 is a partially sectioned perspective view of the conventional IGBT element integrally incorporating a constant voltage diode having an operational principle of avalanche breakdown. DESCRIPTION OF THE PREFERRED EMBODIMENT Hereafter, an operation of a n channel type IGBT will be discussed such that the first conductivity type is a p type and the second conductivity type is a n type. When a positive voltage is applied to a drain electrode relative to a source electrode, a pn junction formed between the second n type semiconductor layer and the third p type semiconductor layer is in a reverse biased condition. Then, a depletion region propagates from this pn junction. Here, if the second n type semiconductor layer has lower impurity concentration in comparison with the third p type semiconductor layer, the depletion region propagates within the second n type semiconductor layer toward the first p type semiconductor layer according to an increase of the voltage between the source electrode and the drain electrode. When the depletion region reaches a pn junction formed between the first p type semiconductor layer and the second n type semiconductor layer, it reduces a potential barrier formed by diffusion potential at the pn junction. By this, from the first p type semiconductor layer to the second n type semiconductor layer, a hole as a minority carrier is injected. The hole flows through the void formed in the second n type semiconductor layer to reach the third p type semiconductor layer and then flows to the source electrode. Furthermore, the hole flows to the source electrode via the third p type semiconductor layer as a drift current because of the electric field within the depletion region. With the mechanism set forth above, a current rapidly flows between the source electrode and the drain electrode to suppress an increase of the voltage between the source electrode and the drain electrode. This phenomenon is known as a punch through phenomenon in the bipolar-transistor to form a void in all regions of the base layer and create the current between the collector and emitter. The present invention applies this phenomenon to the IGBT. Here, the voltage V DSP between the source electrode and the drain electrode, at which the current starts to flow, is a charge voltage, at which the end of the depletion region reaches the first p type semiconductor layer through the second n type semiconductor layer. Therefore, the level of the voltage V DSP can be set by selecting the thickness and impurity concentration of the second n type semiconductor layer. Namely, by setting the thickness and impurity concentration of the second n type semiconductor layer so that the V DSP becomes within the safe operation region of the element (first feature), it becomes possible to protect the IGBT element from excess voltage. Furthermore, by a rapid increase of the drain current, the source-drain voltage becomes fixed in the vicinity of a particular voltage. As can be appreciated herefrom, with the construction, the function preventing the source-drain voltage from rising across a given voltage and, at the same time, fixing the voltage at a particular voltage range, can be internally incorporated. In addition to the above-mentioned first feature, by providing the fifth n type semiconductor layer having a high impurity concentration and a given pattern leaving pn junction area, at or in the vicinity of the pn junction between the first p type semiconductor layer and the second n type semiconductor layer (second feature), an injection amount of carrier (hole) to be injected into the second n type semiconductor layer via the pn junction when the depletion region reaches the first p type semiconductor layer, can be restricted and thus can restrict the formation of a conductivity modulated region and the reduction of resistance associated with injection of the hole. As a result, at the rising of the drain current, fluctuation of drain voltage because of conductivity modulation-caused by an injection of a minority carrier can be suppressed to make the drain voltage more stable upon starting the current flow through the element. As set forth above, according to the present invention, by setting the impurity concentration and thickness of the second semiconductor layer at predetermined values, avalanche breakdown can be internally suppressed in the IGBT element at a desired condition corresponding to a desired clamp voltage between the drain and the source. Furthermore, since the invention injects a minority carrier from p + substrate to n - substrate, it becomes unnecessary to incorporate the constant voltage diode having an operation principle of avalanche breakdown. Therefore, the present invention can achieve a remarkable advantage in that the IGBT element incorporating excess voltage protection and a drain voltage fixing function in a monolithic structure with a function for clamping drain-source voltage without causing an increase in the ON voltage. Fig. 1 is a section of a unit cell and a guard ring of an IGBT. The structure will be discussed in order of fabrication. At first, with a monocrystalline silicon substrate, a p + drain layer 4 (first semiconductor layer) is prepared. On the p + drain layer 4, a high resistance n - drain layer 3 (second semiconductor layer) is formed by growing monocrystalline silicon by way of vapor deposition or so forth. This n - drain layer 3 has a predetermined impurity concentration ND and a predetermined thickness t e , as set out later. Next, for a depth of 3 to 6 µm, a p type base layer 7 (third layer) and a p type layer 13 are formed simultaneously by way of selective diffusion. Here, the p type layer 13 is a guard ring formed for providing high breakdown voltage. Then, within the p type base layer 7, an n + source layer 8 (fourth layer) is formed by way of selective diffusion. It should be noted that, during the fabrication process set forth above, the p type base layer and the n + type source layer 8 are self-alignment using DSA (Diffusion Self Alignment) technology using a gate electrode 10 formed on a gate oxide layer 11 formed by oxidation of the surface of the n - drain layer 3 as a mask, and thus forms a channel region. Subsequently, an interlayer insulation layer 12 is formed. Thereafter, to establish an ohmic contact formed between the p type base layer 7 and the n + source layer 8, contact holes are formed through the gate oxide layer and the interlayer insulation layer 12. Then , through deposition of several µm of aluminum and selective etching, a source electrode 9 and gate electrode pad (not shown) are formed. Then, a metal layer is deposited on the reverse side of the p + type drain layer 4 to form a drain electrode 1. Here, the impurity concentration ND and the thickness t e from the lower surface of the p type base layer 7 of the n - drain layer 3 are set so that a depletion region propagates from the p type base layer 7 and reaches the p + type drain layer 4 through the n - drain layer 3 at a voltage smaller than an avalanche breakdown voltage of pn junction between the p type base layer 7 and the n - drain layer 3. Namely when a voltage is applied between the source and the drain, the depletion region propagating from the p type base layer 7 reaches the p + drain layer at a voltage V DSP lower than a voltage V DSA at which an avalanche breakdown occurs in the element. Here, a pn - junction of a one-side abrupt junction, a relationship between a width W of void layer and a reverse bias voltage V R can be expressed by the following equation (1) W = V R +  B V R 2 K S ε 0 V R qN D where B is diffusion potential of pn - junction; KS relative dielectric constant, in case of Si 11.7; ε 0 is the permittivity in vacuum q is the elementary charge; ND is the impurity concentration in n - drain layer 3; and Here, since VR >> B, the foregoing equation is approximated by: W = 2 K S ε 0 V R qN D For example, assuming ND = 2.0 x 10 14 cm -3 , from the foregoing equation (2), when V R = 350V, W = 48 µm. Accordingly, by setting the impurity concentration ND and the thickness te of the n - type drain layer 3 of the IGBT structure to N D = 2.0 x 10 14 cm -3 and t e = 48 µm, the voltage V DSP to cause carrier injection from the p + drain layer 4 as the substrate, can be set at 350V. An excess voltage protecting function and a drain voltage fixing function in the IGBT and constructed as set forth above will be discussed herebelow. When positive voltage V D is applied to the drain electrode 1 while maintaining the potential at the source electrode 9 and the gate electrode 10 at the ground level, a depletion region is formed in n - type drain layer 3 because of reverse bias at the pn junction 2 between the p type base layer 7 and the n - drain layer 3. The depletion region propagates within the n - drain layer 3 toward the p + type drain layer 4 according to an increase in the voltage V D . When the voltage V D reaches V DSP , the end of the depletion region reaches the p + drain layer 4. At this time, the electric charge in the IGBT element becomes a maximum value Emax at a flat portion of an interface (pn junction 2) between the p type base layer 7 and n - type drain layer 3. The maximum value Emax can be expressed by the following equation (3): E max = qN D W K S ε 0 = 1.5 x 10 5 On the other hand, at the impurity concentration of 2.0 x 10 14 cm -3 , the critical field Ecrit at avalanche breakdown can be expressed by: Ecrit = 2.3 x 10 5 [V/cm] Accordingly, from the equations (3) and (4), since E max < E crit , avalanche breakdown can be successfully suppressed. At this time, the potential barrier at the pn junction 5 formed between the p + type drain layer 4 and the n - drain layer 3 is reduced, injection of a hole from the p + drain layer 4 to n - drain layer is initiated, the hole flows to the source electrode 9 through the p type base layer 7 as a drift current by the effect of an electric field in the depletion region. By this, a current rapidly flowing between the source and the drain to suppress a rise in the drain voltage. Therefore, at the bias condition, in which avalanche breakdown is eliminated, an excess voltage protecting function in terms of the drain current can be realized. On the other hand, because of the increase in the drain current, the voltage between the source and drain can be fixed in the vicinity of the particular voltage V DSP . Therefore, the drain voltage fixing function can be realized. It should be noted that for the foregoing equations (1) through (4), written reference is made by A. S. Grove, translated by Yasuo Tarui, "Foundation for Semiconductor Device" McGlaw-Hill, pages 176 - 179 and 215. The discussion in this publication is herein incorporated by reference for the sake of disclosure. Fig. 2 shows a section of a unit cell and a guard ring of an IGBT according to the present invention. The shown embodiment is differentiated from that illustrated in Fig. 1, in that an n + layer having a cyclic stripe pattern is provided in the vicinity of the pn junction 5 of the substrate. The n + layer 6 is formed at or in the vicinity the on junction 5 by selective diffusion of an impurity on the surface of the p + type drain layer 4, or, as an alternative, by forming a n - layer with a certain thickness on the surface of the p + type drain layer 4, subsequently performing selective diffusion of an impurity, and then performing the fabrication process discussed with respect to the Fig. 1. It should be noted that similar elements are represented by the same reference numerals as in Fig. 1. Discussion will be provided for an excess voltage protecting function and the voltage fixing function in terms of drain voltage with respect to the IGBT element constructed as set forth above. When a positive voltage V D is applied to the drain electrode 1 while maintaining the potential at the source electrode 9 and the gate electrode 10 at the ground level, the depletion region is formed in n - type drain layer 3 because of reverse bias at the pn junction 2 between the p type base layer 7 and the n - drain layer 3. The depletion region propagates within the n - drain layer 3 toward the p + type drain layer 4 according to an increase in the voltage V D . When the end of the depletion region reaches the position where the n + layer 6 is formed, propagation of the depletion region is blocked at the region where the n + layer 6 is selectively formed. On the other hand, in the region where the n + region is not formed, the depletion region reaches the pn junction to reduce the potential barrier thereat to cause injection of a hole in substantially the same manner as that discussed in the first embodiment. Therefore, in the structure of the shown embodiment of IGBT, by selectively forming the n + type layer, the area to inject a hole can be restricted. The effect of this structure will be discussed in comparison with the structure that does not have the n + type layer 6. In the case that the n + type layer 6 is not provided, when the depletion region reaches the p + type drain layer to start injection of a hole to the n - type drain layer 4, a region where the minority carrier is increased, is formed in the vicinity of pn junction 5 of the substrate. In such a region, the conductivity is lowered (conductivity modulation) to cause a lowering of resistance between the source electrode and the drain electrode. At this time, the relationship between the drain current versus the drain voltage shows negative characteristics to decrease the drain voltage according to an increase in the drain current. When the drain current is further increased, as propagation of the conductivity modulated region is restricted and thus the resistance between the source electrode and the drain electrode becomes fixed to cause an increase in the drain voltage again. Therefore, as shown in Fig. 3, the characteristics becomes I-V characteristics with a slight fluctuation of the drain voltage at the initiation of a current flow. Namely, the fluctuation of the drain voltage at the region of initiation of a flow of drain current is caused by a reduction of the element resistance due to the formation of the conductivity modulated region in the vicinity of the pn junction 5 by an injection of a minority carrier (hole) to the n - drain layer 3. In contrast to this, in the construction where the n + type layer 6 is provided as shown in Fig. 2, when the depletion region reaches the p + type drain layer 4, the region where injection of the hole to the n - type drain layer 3 via the pn junction 5 of the substrate, is restricted. Accordingly, an area forming the conductivity modulated region, and a reduction of the element resistance are restricted. Therefore, the drain voltage will never fluctuate even at the initiation of a flow of the drain current, and thus can be stably fixed. In addition, by forming the n + type layer 6 as a cyclic repeated pattern over the overall surface of the element, it becomes possible to make the current density flowing through the element uniform. It should be appreciated that, although Fig. 2 shows the example in which the n + type layer 6 is formed at the interface (pn junction 5) of the p + type drain layer 4 and the n - type drain layer 3, the equivalent effect can be obtained even when the n + type layer is formed at an upper or lower position from the interface surface of the p + type drain layer 4 and the n - type drain layer 3. Also, the pattern of the n + type layer 6 is not specified as a stripe pattern but can be any pattern, such as a net pattern or so forth. Fig. 5 shows the section at pn junction 5, and Fig. 5(a) shows an embodiment where the n + type layer 6 is formed as a stripe pattern and Fig. 5(b) shows an embodiment where the n + type layer 6 is formed as a net. Furthermore, although the foregoing embodiments have been discussion in terms of an n channel type IGBT, in which the first conduction type is a p type and the second conductivity type is a n type, the present invention is equally applicable for a p channel type IGBT, in which the conductivity types are reversed.

Claims: An insulated gate type bipolar-transistor comprising: a first semiconductor layer (4) of a first conductivity type; a second semiconductor layer (3) of a second conductivity type having an interface with said first semiconductor layer (4) and a main surface opposite said interface, a first pn junction being formed at said interface; a third semiconductor layer (7) of said first conductivity type formed in said second semiconductor layer (3) adjoining said main surface and forming with said second semiconductor layer (3) a second pn junction terminating at said main surface; a fourth semiconductor layer (8) of said second conductivity formed in said third semiconductor layer (7) adjoining said main surface and forming with said third semiconductor layer (7) a third pn junction terminating at said main surface; a gate electrode (10) formed on a channel region adjacent to the surface of said third semiconductor layer (7) extending between said second semiconductor layer (3) and said fourth semiconductor layer (8) via a gate insulation layer (11); a source electrode (9) directly contacting both of said third and fourth semiconductor layers (7 and 8); a drain electrode (1) directly contacting said first semiconductor layer (4); wherein said second semiconductor layer (3) has such an impurity concentration and such a thickness that a depletion region extending from said second junction between said third semiconductor layer (7) and said second semiconductor layer (3) toward the inside of said second semiconductor layer (3) reaches said first semiconductor layer (4) through said second semiconductor layer (3) at a voltage applied between said drain electrode (1) and said source electrode (9) lower than a drain-source voltage at which a critical electric field causing partial avalanche breakdown at or in the vicinity of said second semiconductor layer (3), and minority carriers for said second semiconductor layer (3) are injected from said first semiconductor layer (4) to said second semiconductor layer (3), and flow to said source electrode (9); characterized in that a fifth semiconductor layer (6) of said second conductivity type is disposed at or in the vicinity of the first pn junction between said first semiconductor layer (4) and said second semiconductor layer (3), said fifth semiconductor layer (6) having a higher impurity concentration than said second semiconductor layer (3) and having a pattern leaving a contact surface between said first and second semiconductor layers (4, 3) whereby carriers, can be transferred therethrough. An insulated gate type bipolar transistor as set forth in claim 1, characterized in that said pattern of said fifth semiconductor layer (6) has a periodic pattern at or in the vicinity of the interface between said first and second semiconductor layers (4 and 3). An insulated gate type bipolar transistor as set forth in claim 2, characterized in that said periodic pattern of said fifth semiconductor layer (6) is a net shaped or stripe shaped pattern. An insulated gate type bipolar transistor as set forth in claim 1, characterized in that said third semiconductor layer (7) has a higher impurity concentration whereby a step pn junction is formed with said second semiconductor layer (3); said fourth semiconductor layer (8) is spaced from said second semiconductor layer (3) by a given distance; and the impurity concentration ND of said second semiconductor layer (3) and a thickness t e between the second pn junction between said second semiconductor layer (3) and said third semiconductor layer (7) and the first pn junction between said second semiconductor layer (3) and said first semiconductor layer (4) are determined so that a condition expressed by the following equation is satisfied: t c = 2 K S ε 0 V R qN D wherein K S is the relative dielectric constant of the material semiconductor layers; V R is a drain-source voltage V R lower than a voltage causing avalanche breakdown at the second pn junction between said third semiconductor layer (7) and said second semiconductor layer (3); ε 0 is the permittivity in vacuum; and q is the elementary charge. An insulated gate type bipolar-transistor as set forth in claim 4, characterized in that the material of the semiconductor layers is silicon, said second semiconductor layer (3) has an impurity concentration ND of 2.0 x 10 14 cm -3 , and a thickness t e of 48 µm. An insulated gate type bipolar transistor as set forth in claims 1 to 5, characterized in that a guard ring (13) is disposed adjoining said main surface in said second semiconductor layer (3) for providing a high breakdown voltage. An insulated gate type bipolar transistor as set forth in claims 1 to 6, characterized in that said first semiconductor layer is a drain layer (4); said third semiconductor layer is a base layer (7) and said fourth semiconductor layer is a source layer (8).

Inventors: KATO NAOHITO, OKABE NAOTO, TOKURA NORIHITO, KATO, NAOHITO, OKABE, NAOTO, TOKURA, NORIHITO
Applicants: DENSO CORP, DENSO CORPORATION
Application year: 1992
Main classification: H01L29
IPC codes: H01L29