GATE TURN-OFF THYRISTOR

Abstract: A gate turn-off thyristor is provided with a plurality of gate electrodes (40a to 40d) with substantially the same areas formed on a second base layer (14). The gate electrodes (40a to 40d) are arranged so as to respectively surround a plurality of cathode electrodes (18l to 18n).

Description: Gate turn-off thyristor The present invention relates to a gate turn-off thyristor and, more particularly, to a gate turn-off thyristor with a reduced gate turn-off loss. In the gate turn-off thyristor (to be referred to as a GTO thyristor for brevity hereinafter), when a positive voltage is applied across the anode and the cathode of the thyristor, if a positive potential is applied to the gate thereof to cause a gate current to flow, then the anode-cathode path is conductive to allow an anode current to flow therethrough. When the anode current flows in the thyristor, if a negative potential is applied to the gate electrode, the anode current is pulled out of the gate electrode, so that the thyristor is turned off. Time required for the GTO thyristor to be switched from an ON state to an OFF state, that is, a gate-turn-off time, limits service frequencies of apparatuses using GTO thyristors. For this reason, the gate turn-off time is one of the important electric properties of GTO thyristors. With a recent trend toward an increased power capacity of the GTO thyristor, diameter and thickness of the silicon wafer used for the GTO thyristor increase, resulting in elongation of the gate turn-off time of the thyristor. This is undesirable for high frequency applications. Fig. 1 is a plan view of a prior GTO thyristor as viewed from the cathode electrode side and Fig. 2 shows a cross section taken on line A-A' in Fig. 1. The GTO thyristor contains four continuous layers, a p-type first emitter layer 10, an n-type first base layer 12, a p-type second base layer 14, and an n-type second emitter layer 16 (161 to 16n). The second emitter layer 16 is divided into a plurality of layers (n layers, for example). Cathode electrodes 181 to 18n are provided on the second emitter layers 161 to 16n, respectively. The gate electrode 20 is formed on the second base layer- 14, so as to surround the cathode area. An anode electrode 22 made of tungsten, for example, is provided on the first emitter layer 10. Two circles as indicated by dotted lines schematically indicate that the cathode electrodes 181 to 18n are arranged circularly. Fig. 3 is a circuit diagram for explaining the operation of the GTO thyristor. When the switch 24 in an on-gate circuit is turned on, a closed circuit consisting of an on-gate power source 26, a switch 24, a circuit impedance 28 and a GTO thyristor 30 has an on-gate current circulating therein. The GTO thyristor 30 is triggered by the on-gate current and rendered conductive. The result is that a main current flows in a closed circuit consisting of a main power source 32, a load 34, and the GTO thyristor 30, in other words, the anode current IA of the GTO thyristor 30. In order to gate turn off the GTO thyristor 30 when it is in a turn-on state, the switch 36 of an off-gate circuit is turned on. Then, a gate current ig having a direction opposite to that occurring when the GTO thyristor is gate turned on, flows in a closed circuit consisting of an off-gate power source 38, the switch 36, the circuit impedance 28, and the GTO thyristor 30. The gate current ig turns off the GTO thyristor 30. Figs. 4A, 4B and 4C show waveforms for explaining the operation of the GTO thyristor 30 when it is gate turned off. When the GTO thyristor 30 is conductive, if the gate turn-off switch 36 is closed at time to, the off-gate current ig increases toward the negative (Fig. 4B). At time tl, the anode current IA starts to decrease and at the same time the voltage VA across the anode and the cathode also starts to increase (Fig. 4A). At time t2, the junction between the gate and the cathode is restored, and the gate-cathode voltage vg reaches a negative maximum. The gate current ig reaches a negative maximum before the gate-cathode voltage vg reaches the negative peak value, and the gate current ig then sharply reduces toward the positive (Fig. 4B). At time t2, the anode current IA has a current value based on the residual charge in the GTO thyristor. The current with an initial value It and flowing after time t2 is called a tail current (Fig. 4A). In the GTO thyristor operation as mentioned above, a turn-off time toff is a time period from to to t2 (toff = t2 - t0). The anode current IA flowing after time t2 continues to flow until time t3 when the stored charge in the GTO thyristor disappears (Fig. 4A). In Fig. 4B, when the voltage EG of the off-gate power source 38 is lower than the gate-cathode reverse breakdown voltage Vsl of the GTO thyristor (EG < Vsl), the gate current ig reaches its negative peak and decreases as indicated by a solid line. On the other hand, when the voltage EG of the off-gate power source 38 is larger than the gate-cathode breakdown voltage VJ1 (EG > VJ1), the gate current ig further increases toward the negative and than decreases as indicated by a broken line. Fig. 4C illustrates a power loss P (= VAxIA) at the time of the gate turning off. Fig. 5 illustrates a relationship between the anode current IA and an off-gate charge amount EMI3.1 In the graph of Fig. 5, the same relationship is illustrated with parameters A to D of an off-gate current rising rate (dig/dt). The parameters are in the order of A < B < C < D. As shown, QG changes linearly with respect to IA and this is little influenced by the off-gate current rising rate (dig/dt). This implies that to turn off an anode current, a substantially constant off-gate charge quantity QG is required irrespective of the-rising rate of the off-gate current. Fig. 6 shows a relationship of the turn-off time toff with respect to an off-gate charge quantity discharged per unit time when the anode current is constant. As seen from the graph, to shorten the turn-off time toff, the off-gate charge quantity QG discharged per unit time must be increased. The circuit impedance 28 in the off-gate circuit has only a negligible amount of DC resistance, and a rising rate dig/dt (; EG/L) of the off-gate current is determined by the inductance L. Therefore, the off-gate charge QG is given approximately as: QG = l/20EG/L.(toff)2 (1) Accordingly, to make the QG larger while the gate turn-off time toff is kept small, there is no other way than increasing the EG if the inductance L is a minimum value usually obtained. As indicated by a broken line in Fig. 4B, if the off-gate power source voltage EG is larger than the gate-cathode reverse breakdown voltage VJ1 of the GTO thyristor (VJ1 < EG), a Zener current flows, causing power loss as given by the product of the Zener current and the gate-cathode voltage restored up to VJ1. The heat generated remarkably reduces the reliability of the GTO thyristor. For the purpose of shortening the turn-off time, heavy metal is doped into the wafer, or the first base layer and the first emitter are exposed at one main surface and shortened by the anode electrode. These measures, however, bring about increasing both the forward voltage drop and the latching current. Accordingly, an object of the present invention is to provide a gate turn-off thyristor with reduced power loss. Another object of the present invention is to provide a gate turn-off thyristor which may shorten a gate turn-off time without increasing the latching current and the forward voltage drop. To achieve these objects, in the present invention, a plurality of gate electrodes are provided on the second base layer. Each of the gate electrodes surrounds at least one cathode electrode. With the plurality of the gate electrodes of the GTO thyristor, a current of Ik/m, which is obtained by dividing the cathode current Ik by the number of the gate electrodes m, flows as the cathode current into each cathode electrode surrounded by a corresponding gate electrode. This is equivalent to an arrangement of a plurality of smaller GTO thyristors each with a volume l/m of the original GTO thyristor connected in parallel. Since the cathode current Ik is substantially equal to the anode current IA, if the current Ik becomes smaller, the off-gate charge QG also becomes smaller, as seen from Fig. 5. Accordingly, the equation (1) shows that if the gate turn-off time toff is equal to that of the prior thyristor, the off-gate voltage EG may be smaller. Further, by making the EG smaller, the tail current becomes smaller, so that the power loss P at the time of turn-off may be reduced. Further, with the smaller EG, the Zener current flowing after the turn off may be prevented, improving the reliability of the GTO thyristor. Further, when the off-gate voltage EG is equal to that of the prior GTO thyristor, the gate turn-off time toff may be reduced, as seen from the equation (1). By way of example and to make the description clearer, reference is made to the accompanying drawings, in which: Fig. 1 shows a plan view of a prior GTO thyristor as seen from the cathode electrode side; Fig. 2 shows a cross sectional view taken on line A-A' in Fig. 1; Fig. 3 is a circuit diagram for explaining the operation of the circuit of Fig. 3; Figs. 4A to 4C show waveforms for explaining the operation of the circuit shown in Fig. 3; Fig. 5 shows a graph illustrating a relationship between an anode current IA and an off-gate charge quantity QG; Fig. 6 shows a graph depicting a relationship between an off-gate charge quantity dQG/dt discharged per unit time and a turn-off time toff; Fig. 7 shows a plan view of a GTO thyristor as viewed from the cathode side according to an embodiment of the present invention; and Fig. 8 shows a cross sectional view taken on line B-B' of the thyristor of Fig. 7. Fig. 7 shows a plan view of a GTO thyristor as viewed from the cathode side according to an embodiment of the present invention and Fig. 8 shows a cross sectional view taken on line B-B' in Fig. 7. In Figs. 7 and 8, like symbols are used for designating like portions in Figs. 1 and 2. A different point of the GTO thyristor in the present embodiment from that of Figs. 1 and 2, resides in the provision of a plurality of gate electrodes. In this embodiment, the gate electrode 40 consists of four gate electrodes 40a, 40b, 40c and 40d The number of the gate electrodes may properly be selected, for example, at more or less than 4. When m number of gate electrodes are used, the GTO thyristor is equivalent to m number of thyristors each having l/m volume of the GTO thyristor arranged in parallel. When a turn-on pulse is applied to the gate electrodes 40a to 40d of the GTO thyristor at the same time, the GTO thyristor is conductive. The cathode current of each cathode electrode surrounded by the corresponding gate electrode 40a, 40b, 40c or 40d is 1/4 of the cathode current of the whole thyristor. At the time of turning off, if all gate electrodes 40a to 40d are simultaneously applied with an off pulse, these gate electrodes 40a to 40d pull out current from the thyristor and the thyristor is turned off. The charge quantity pulled out by each of the gate electrodes is 1/4 of the whole current. At this time, a rising rate of the off-gate current (dig/dt) is approximately expressed by dig/dt = EG/L for each gate electrode 40a to 40d where the gate power source voltage is EG and the total inductance of the gate circuit is L. An off-gate charge quantity QG per gate is also given, like the equation (1), QG = l/2.EG/L.(toff)2. In this embodiment, the QG pulled out by each gate electrode is smaller than that of the prior GTO not with a plurality of gate electrodes. Accordingly, when the same turn-off time toff as that of the prior GTO thyristor is employed, the Eg may be smaller than the prior one. The reduced Eg makes the tail current initial value It smaller, and the power loss P = (VAxIA) at the time of the gate turning off may also be reduced. The smaller Eg may lessen a probability that the Zener current flows after the turn off. This improves the reliability of the GTO thyristor. When the L and EG are the same as those of the prior art, the toff may be made smaller. Therefore, unlike the prior art, there is no need for the heavy metal doping or the special structure. Therefore, the latching current and the forward voltage drop are never increased. The GTO thyristor with such a structure may be realized by the same fabricating process steps as those of the prior art. Specifically, the cathode electrodes 181 to 18n shown in Figs. 7 and 8 and the gate electrodes 40a to 40d are formed in a manner such that soft metal, e.g., aluminum is uniformly deposited over the main surfaces of the cathode and the gate of the semiconductor, and unnecessary soft metal portions are etched away using a resist mask having gate electrode patterns. Therefore, no additional cost is required for reducing the invention to practice. If the gate electrodes 40a to 40d are separated from one another by 100 pm or more, a sheet resistance of the second base layer 14 is normally several tens #/o, and hence the gate electrodes are electrically separated to a satisfactory extent, thus attaining the effect by this invention. It is evident that the present invention is not limited to the above-mentioned embodiment any way. For example, the present invention is applicable for the GTO thyristor with a structure such that a part of the first base layer is exposed to the main surface of the anode surface and the first emitter layer and the first base layer are shorted by the anode electrode. It should be understood that the present invention may be variously changed and modified within the scope of the present invention.

Claims: Claims: 1. A gate turn-off thyristor comprising a first emitter layer of a first conductivity type, a first base layer of a second conductivity type formed on said first emitter layer, a second base layer of the first conductivity type formed on said first base layer, a plurality of second emitter layers of the second conductivity type formed on said second base layer, a plurality of cathode electrodes formed on said second emitter layer, a gate electrode disposed on said second base so as to surround a pair of said second emitter layers and said cathode layer, and an anode electrode formed on said first emitter layer of the first conductivity type, characterized in that said gate electrode is comprised of a plurality of electrodes (40a, 40b, 40c, 40d) with substantially the same areas, each of said gate electrodes surrounding at least a pair of said second emitter layers (161 to 16n) and said cathode electrode (181 to 18n). 2. A gate turn-off thyristor according to claim 1, characterized in that said plurality of electrodes (40a to 40d) are disposed on said second base layer (14) and arranged symmetrical with respect to the center of said second base layer (14). 3. A gate turn-off thyristor according to claim 2, characterized in that each of said plurality of gate electrodes (40a to 40d) is shaped like a fan.

Inventors: AZUMA, MAKOTO, TAKIGAMI, KATSUHIKO
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1982
Main classification: H01L29
IPC codes: H01L29