The present invention relates to a gate turn-off thyristor, and more particularly, to a gate turn-off thyristor which is improved in its current cut-off performance by minimizing the current amplification factor of one of the transistor elements in the thyristor structure.
The gate turn-off thyristor (referred to as simply a "GTO" thyristor, hereinafter) is a thyristor which is capable of cutting off current by a gate control in which current is drawn out from the gate and is therefore put into practical use in, for example, an inverter device for controlling the speed of a motor.
The turn-off gain G.sub.off (i.e. the ratio between the on-state current flowing when the thyristor is conductive and the gate turn-off current required for cutting off the current) of the thyristor is, as is well known, given by the following formula: ##EQU1##
In the above formula, .alpha..sub.32 represents the current amplification factor of one of the transistor elements in the thyristor structure which has a gate control terminal connected to its base, while .alpha..sub.12 represents the current amplification factor of the other transistor element of the thyristor structure.
In this specification, a P base type thyristor, which is more general, will be described hereinafter, and the current amplification factors .alpha..sub.32 and .alpha..sub.12 will be respectively referred to as ".alpha.(n-p-n): and ".alpha.(p-n-p)".
It is necessary to improve the current cut-off performance of thyristors. More specifically, in order to increase the turn-off gain G.sub.off of the GTO thyristor, as will be understood from the above-described formula, it is necessary to increase the current amplification factor .alpha..sub.32, that is, the current amplification factor .alpha.(n-p-n) and minimize the current amplification factor .alpha..sub.12, that is, the current amplification factor .alpha.(p-n-p).
There are two conventional methods of minimizing the current amplification factor .alpha.(p-n-p) as described in Hitachi Review, Vol. 29 (1980), No. 3, pp. 127-130, entitled "Gate Turn-Off Thyristor and Drive Circuits".
The first method features doping with a heavy metal, such as gold. Doping with a heavy metal is done with the intent of reducing the carrier life in the N.sub.B layer and thereby decreasing the coefficient of transportation of carriers to the base and, as a result, the current amplification factor .alpha.(p-n-p) is minimized.
The second methods of minimizing the current amplification factor .alpha.(p-n-p) features shunting the P emitter. This structure is equivalent to a structure in which the emitter and base of a p-n-p transistor are shunted to each other through a resistor. It is intended by the equivalent shunt resistor to reduce the effective emitter injection efficiency in the p-n-p transistor, thereby minimizing the current amplification factor .alpha.(p-n-p).
The heavy-metal doping method advantageously makes it possible to improve the current cut-off performance of a thyristor element without impairing its fundamental functional aspect as a thyristor. On the other hand, the heavy-metal doping method is disadvantageous in that a relatively short carrier lifetime in the N.sub.B layer involves increases in the on-state voltage and the leakage current and in that doping with a heavy metal may cause various problems at high temperature including a lowering in the performance of the thyristor element.
The second method in which the P emitter is shortcircuited advantageously makes it possible to overcome the disadvantage of the first method since it is possible to maintain the carrier lifetime at a high level. The second method, however, involves a disadvantage in that the thyristor loses the function of blocking the reverse voltage since the P.sub.E layer is shunted by the N.sup.+ -type layers.
In the early days of its application, the GTO thyristor was mainly applied to a voltage-type inverter device. In this device, only a forward voltage is applied to a thyristor element therein, that is, no reverse voltage is applied to the thyristor element. Therefore, early GTO thyristors did not need the capability of blocking a reverse voltage.
However, as the range of application of the GTO thyristor has been widened, the thyristor has been applied to devices other than the voltage type inverter, such as a current type inverter, converter and chopper. In these devices, a reverse voltage equal in magnitude to a forward voltage is applied to a thyristor element.
For this reason, in the case of a high-performance GTO thyristor with the P emitter short-circuited, it is necessary to insert a diode in series, which fact disadvantageously results in causing an increase in the size of the device and a reduction in the efficiency thereof.