Static induction transistor and static induction thyristor

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Static induction transistors (SIT) and electrostatic induction thyristors (SITH) are new high-frequency and high-power power electronic devices with many similarities between the two structures and principles. They are power switching devices that use the principle of electrostatic induction to control the operating current. SIT and SITH have low power consumption, high switching speed, high input impedance, and the advantages of gate voltage control switch. High-frequency high-power devices such as induction heating, ultrasonic processing, and broadcast transmission, as well as inverter power supply, switching power supply, and discharge equipment power supply. It has a strong advantage in the application of new power sources.

1. Static induction transistor (SIT)

A static induction transistor (SIT) is a junction field effect transistor that was developed in 1970. The structure of the SIT is shown in Figure 1a). On both sides of a N-type semiconductor with a high doping concentration, there is a P-type semiconductor thin layer, which leads to drain D, source S and gate G, respectively. When the voltage between G and S is UGS=0, the power supply US can flow through a wide N-zone (the majority of the carriers can conduct electricity), the equivalent resistance of the N-region channel is not large, and the SIT is in the on state. If a negative voltage is applied to both ends of G and S, that is, UGS<0, that is, the semiconductor N is connected to a positive voltage, the semiconductor P is connected to a negative voltage, and the two PN junctions of P1N and P2N are reversed, and two Depletion layers A1 and A2 (no carriers in the depletion layer, no conduction), narrowing the N region which can be electrically conductive, and increasing the equivalent resistance. When the reverse bias voltage between G and S is large enough to be a certain critical value, when the depletion layers on both sides are widened to be connected together, the conductive N region disappears, and between the drain D and the source S The equivalent resistance becomes infinite and turns SIT into an off state. Since the depletion layer is generated by the external reverse voltage formed by the applied reverse bias voltage, the on-off state of the tube is controlled by the action of the applied voltage to form an electrostatic field, so it is called the static induction transistor SIT. The switching effect of SIT in the circuit is similar to the normally closed contact of a relay. When there is no applied voltage UGS=0 at both ends of G and S, SIT is in the on state (closed) to close the circuit. After the applied voltage UGS, the SIT is connected. (closed) turns to off state (disconnected).

2. Static induction thyristor (SITH)

Static induction thyristor (SITH) is also known as Field Controlled Thyristor (FCT), and its on-off control mechanism is similar to SIT. The structural difference is only that SITH adds a PN junction to the SIT structure, and a triode is formed inside. The two triodes form a thyristor and become an electrostatic induction thyristor.

When the gate is not applied with voltage, SITH is in the same state as SIT, and it is switched from the on state to the off state when the gate negative voltage is applied. Since SITH has one more PN junction with less sub-injection than SIT, SITH belongs to two kinds of carrier-conducting bipolar power devices. In actual use, in order to make the device reliably turn on, a positive gate voltage of 5 to 6V is often used instead of a zero gate voltage to reduce the on-state voltage drop of the device. Generally, turning off SIT and SITH requires a negative gate voltage of several tens of volts.