TERMINAL CHARACTERISTICS OF THYRISTORS
Thyristor is a four layer, three-junction, p-n-p-n semiconductor switching device. It has three terminals ; anode, cathode and gate. Fig. 4.1 (a) gives constructional details of a typical thyristor. Basically, a thyristor consists of four layers of alternate p-type and n-type silicon semiconductors forming three junctions J1, J2 and J3 as shown in Fig. 4.1 (a). The threaded portion is for the purpose of tightening the thyristor to the frame or heat sink with the help of a nut. Gate terminal is usually kept near the cathode terminal Fig. 4.1 (a). Schematic diagram and circuit symbol for a thyristor are shown respectively in Figs. 4.1 (b) and (c). The terminal connected to outer p region is called anode (A), the terminal connected to outer n region is called cathode and that connected to inner p region is called the gate (G). For large current applications, thyristors need better cooling ; this is achieved to a great extent by mounting them onto heat sinks. SCR rating has improved considerably since its introduction in 1957. Now SCRs of voltage rating 10 kV and an rms current rating of 3000 A with corresponding power-handling capacity of 30 MW are available. Such a high power thyristor can be switched on by a low voltage supply of about 1 A and 10 W and this gives us an idea of the immense power amplification capability (= 3 x 106) of this device. As SCRs are solid state devices, they are compact, possess high reliability and have low loss. Because of these useful features, SCR is almost universally employed these days for all high power-controlled devices.
An SCR is so called because silicon is used for its construction and its operation as a rectifier (very low resistance in the forward conduction and very high resistance in the reverse direction) can be controlled. Like the diode, an SCR is an unidirectional device that blocks the current flow from cathode to anode. Unlike the diode, a thyristor also blocks the current flow from anode to cathode until it is triggered into conduction by a proper gate signal between gate and cathode terminals.
For engineering applications of thyristors, their terminal characteristics must be known. In this article, their static V-I characteristics, dynamic characteristics during turn-on and turn-off processes and their gate characteristics are discussed.
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i got some some new points from it , if u can send me link of good semiconductor device then please send