Semiconductor device comprising a high speed switching bipolar transistor

Abstract: A semiconductor device, comprising a transistor, a constant voltage diode having a first end of a first conductivity type connected to an emitter of the transistor and a second end of a second conductivity type, a reverse current preventive diode having a first end of the first conductivity type connected to a collector of the transistor and a second end of the second conductivity type connected to the second end of the constant voltage diode, and a high speed diode reverse-bias connected between the transistor collector and the emitter of the transistor.

Description: The present invention relates to a semiconductor device comprising a transistor, such as a power transistor or an insulated gate bipolar transistor (IGBT), for use as a high speed semiconductor power switch. Discussion of the Related Art Typically, when a transistor, such as a power transistor or an IGBT, is used as a high speed power switch, a diode, such as a free wheeling diode (FWD), is connected across the transistor's emitter and collector to provide a current path from the transistor's emitter to its collector. For example, as shown in Figure 3, there is provided a transistor 1 having a high speed diode 2 reverse-bias connected between a collector 12 and emitter 11 of the transistor 1. As shown in Figure 2, such a circuit arrangement, which is known from EP-A-0 379 346, has application in an inductive load circuit, wherein a plurality of power rectifiers 22 and transistor inverters 23 control the speed of a motor 21. In the circuit arrangement shown in Figure 3, it is often desirable that the high speed diode 2 not only provide a current path from the transistor's emitter to its collector, but also function as a constant voltage diode to protect the transistor 1 against voltage surges at the collector 12. Thus, it is often desirable that the high speed diode 2 have the characteristics of, for example, a Zener or avalanche diode, as exemplified by Zener diode 3 shown in the equivalent circuit of Figure 4. Such a circuit is disclosed in JP-A-59-74728. It is known in the art that a reach-through Zener diode having a cross sectional structure shown in Figure 5 may be used as the Zener diode 3 shown in the circuit of Figure 4. As shown in Figure 5, the reach-through Zener diode comprises a cathode electrode 36, an N⁺-type layer 32 formed on the cathode electrode 36, an N⁻-type layer 31 formed on the N⁺-type layer 32, a P⁺-type layer 33 diffused in the N⁻-type layer 31, an oxide film 34 formed on the P⁺-type layer 33, and an anode electrode 35 formed on the P⁺-type layer 33 through an opening in the oxide film 34. The Zener voltage of the reach-through Zener diode is determined by the specific resistance which is largely a function of the width W1 of the N⁻-type layer 31. Because t rr of such a Zener diode is long, however, the diode will not have the desired characteristics of the high speed diode 2 shown in Figure 3. Furthermore, if a lifetime Killer is used to control t rr of the Zener diode to shorten t rr , the Zener voltage will not be stable. Where, t rr indicates a time in which a reverse current flowing a diode when a voltage applied to the diode is changed from a fixed forward voltage to a fixed reverse voltage, recovers to 10% of the maximum value. SUMMARY OF THE INVENTION Accordingly, it is the object of the present invention to provide a semiconductor device comprising a transistor and a diode arrangement, wherein the diode arrangement functions as a bypass circuit capable of high speed switching across the transistor's emitter and collector and as a voltage regulator circuit for protecting the transistor from irregular voltages applied to its collector. To achieve the foregoing object, and in accordance with the invention a semiconductor device comprises the features as claimed in claim 1. In an embodiment, a semiconductor device is provided comprising a transistor including a first layer having a first conductivity, a second layer diffused in the first layer and having a second conductivity, and a third layer diffused in the second layer and having the first conductivity, a high speed diode including a fourth layer diffused in the first layer and having the second conductivity, and a lifetime killer for controlling carrier rebinding within the high speed diode, a constant voltage diode including a fifth layer diffused in the first layer and having the second conductivity, and a preventive diode including a Schottky barrier electrode formed on the first layer. The present invention provides a high speed diode, for example, an FWD, connected in parallel with a constant voltage diode between a transistor's emitter and collector to ensure that the high speed switching property of the FWD and the stable constant voltage characteristics of the constant voltage diode are made compatible. The present invention also provides a reverse current preventive diode connected reversely in series with the constant voltage diode to allow a reverse voltage to be applied to the constant voltage diode and to prevent a forward current from flowing therethrough. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates an equivalent circuit of the present invention; Fig. 2 illustrates a motor speed control circuit with which the teachings of the present invention can be incorporated; Fig. 3 illustrates an equivalent circuit of a high speed diode and transistor; Fig. 4 illustrates an equivalent circuit of a Zener diode and a transistor; Fig. 5 illustrates a cross-sectional view of the Zener diode of Fig. 4; and Fig. 6 illustrates a cross-sectional view of an embodiment of the present invention. As illustrated in Figure 1, there is provided a transistor 1 having a base 10, an emitter 11, and a collector 12, a high speed diode 2, a constant voltage diode 3, and a reverse current preventive diode 4. As also shown in Figure 1, the high speed diode 2 is reverse-bias connected between the emitter 11 and collector 12, and the constant voltage diode 3 and reverse current preventive diode 4 are connected in series between emitter 11 and collector 12 such that the anode of the high speed diode 2 is connected to the emitter 11, the cathode of the constant voltage diode 3 is connected to the cathode of the reverse current preventive diode 4, and the anode of the reverse current preventive diode 4 is connected to the collector 12. As shown in Figure 6, in an embodiment, the transistor 1, high speed diode 2, constant voltage diode 3, and reverse current preventive diode 4 are all integrated into a single device. As shown in the figure, the transistor 1 includes a P⁺-type layer 41 as its base, an N⁺-type layer 44 as its emitter, and an N⁻-type layer 40 as its collector. The P⁺-type layer 41, the P⁺-type layer 42, and the P⁺-type layer 43 are formed, for example, as a result of first selective diffusion using an oxide film mask. The transistor 1 also includes a base electrode 47 connected to the P⁺-type layer 41, an emitter electrode 48 connected to the N⁺-type layer 44, and a metallic layer 37, as a collector electrode, connected to the N⁻-type layer 40 via an N⁺-type layer 46. The high speed diode 2 includes the P⁺-type layer 42 as its anode and the N⁻-type layer 40 as its cathode. Further, a lifetime killer 45 comprising, for example, gold can be introduced into the N⁻-type layer 40 beneath the P⁺-type layer 42 by, for instance, selective diffusion, to promote carrier recombination, thereby increasing the operating speed of the high speed diode 2. The high speed diode 2 is also provided with an anode electrode 49 connected to the P⁺-type layer 42 and to the emitter electrode 48 of the transistor 1. The constant voltage diode 3 and the reverse current preventive diode 4 include a P⁺-type layer 43 diffused into the N⁺-type layer 40, a well P⁺-type layer 33 formed by rediffusing impurities in part of the P⁺-type layer 43, and the metallic layer 37. Although not shown in the figure, the well P⁺-type layer 33 and the P⁺-type layer 43 can have the same area. The constant voltage diode 3 is constituted by the well P⁺-type layer 33 and the N⁻-type layer 40, and the reverse current preventive diode 4 is constituted by a Schottky diode including the N⁻-type layer 40 and the metallic layer 37 as a Schottky barrier electrode. The constant voltage diode 3 and the reverse current preventive diode 4 are also provided with an anode electrode 35 which is connected to the well P⁺-type layer 33, the emitter electrode 48 of the transistor 1, and the anode electrode 49. As shown in Figure 6, the metallic layer 37 contacts the N⁻-type layer 40 of the transistor 1 and the high speed diode 2 via the N⁺-type layer 46 and, therefore, simultaneously acts as the collector electrode of the transistor 1 and the cathode of the high speed diode 2. Moreover, the base electrode 47 of the transistor 1 contacts the P⁺-type layer 41, the emitter electrode 48 contacts the N⁺-type layer 44, the anode electrode 49 of the high speed diode 2 contacts the P⁺-type layer 42, and the anode electrode 35 of the constant voltage diode 3 contacts the P⁺-type layer 43. Because the electrodes 48, 49, and 35 are mutually connected, the equivalent circuit shown in Figure 1 is thus arranged. In this structure, the width W1 of the N⁻-type layer 40 between the well P⁺-type layer 33 and the metallic layer 37 is approximately 35µm, provided the Zener voltage of the constant voltage diode 3 is, for example, 400 V. Further, the width W2 of the N⁻-type layer 40 underneath the P⁺-type layer 41 and the P⁺-type layer 42 is approximately 60 µm. Although the transistor 1 shown in Figure 10 is an ordinary bipolar transistor, the transistor 1 can also be, for example, an IGBT by forming a P⁺-type source layer on the surface of the N⁺-type layer 44 and providing an MOS structure thereon. In addition, the conductive type of the transistor 1 may be reversed. According to the present invention, a transistor is provided with a diode arrangement including a high speed diode which functions as a bypass circuit capable of high speed switching across the transistor's emitter and collector, and as a constant voltage diode and reverse current preventive diode for protecting the transistor from irregular voltages applied to its collector. Moreover, the constant voltage diode and the reverse current preventive diode are integrated into a single device, thereby simplifying the circuit arrangement, in accordance with the present invention. Further, the transistor and high speed diode, as well as the constant voltage diode and the reverse current preventive diode, are integrated into a single device, thereby further simplifying the circuit arrangement, in accordance with the present invention. Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative devices, and illustrative examples shown and described.

Claims: A semiconductor device formed in a semiconductor substrate (40) of a first conductivity type having an upper and a lower major surface, a highly doped layer (46) of said first conductivity type being exposed at said lower major surface and a metallic layer (37) being disposed on said lower major surface and contacting said highly doped layer (46), said semiconductor device comprising: a vertical bipolar transistor (1) including a first layer formed by a portion of said semiconductor substrate (40), a second layer (41) being a base region, located in said first layer, exposed at said upper major surface and having a second conductivity type opposite to the first conductivity type, a third layer (44) located in said second layer (41), exposed at said upper major surface and having said first conductivity type, a fourth layer formed by a portion of said highly doped layer (46) first and second electrodes (47,48) in contact with the second and third layers (41,44) respectively; a high speed, free-wheel diode (2) including a fifth layer formed by a portion of said semiconductor substrate (40) which comprises a lifetime killer (45), whereby carrier recombination is controlled within said high-speed diode (2), a sixth layer (42) located in said fifth layer, exposed at said upper major surface and having said second conductivity type, a seventh layer formed by a portion of said highly doped layer (46), a third electrode (49) in contact with said sixth layer (42) and connected to said second electrode (48); a constant voltage diode (3) such as a Zener diode, including an eighth layer formed by a portion of said semiconductor substrate (40), a ninth layer (33,43) located in said eighth layer, exposed at said upper surface and having said second conductivity type, a fourth electrode (35) in contact with said ninth layer (33,43) and connected to said second and third electrodes (48,49); and a Schottky barrier diode including said eighth layer, and a portion of said metallic layer (37) forming a Schottky barrier with said eighth layer. The semiconductor device of claim 1, characterized in that said first conductivity type is n and said second conductivity type is p, said metallic layer (37) forming a collector electrode of said bipolar transistor (1) and a cathode electrode of said high-speed diode (2). The semiconductor device of claim 1, characterized in that said lifetime killer comprises gold. The semiconductor device of claim 1 characterized in that the bipolar transistor (1) is an insulated gate bipolar transistor having a tenth layer being a source layer of said second conductivity type on the surface of said third layer (44), and a MOS structure thereon.

Inventors: SHIGEKANE HISAO, SHIGEKANE, HISAO, Shigekane, Hisao, c/o Fuji Electric Co.,Ltd.
Applicants: FUJI ELECTRIC CO LTD, FUJI ELECTRIC CO., LTD.
Application year: 1991
Main classification: H01L27
IPC codes: H01L29, H01L21, H01L25, H01L27