11.1 INTRODUCTION TO THYRISTOR

Thyristors are able to switch large levels of power and are used in a wide variety of different applications. Thyristors find use in low-power electronics, where they are used in many circuits ranging from light dimmers to power supply over voltage protection. The idea of thyristor was first described by William Shockley in 1950. It was referred to as a bipolar transistor with a p-n hook collector. The mechanism for the operation was analysed further in 1952 by Jewell James Ebers and in 1956 John L. Moll investigated the switching mechanism of the thyristor.

The first silicon-controlled rectifiers became available in the early 1960s when it started to gain a significant level of popularity for power switching. It is a multilayered semiconductor device which requires a gate signal to turn it ‘ON’. Once it is turned ‘ON’, it behaves like a rectifying diode. Thus, the name silicon-controlled rectifier is justified by its working principle. In fact, the circuit symbol for the thyristor suggests that this device acts like a controlled rectifying diode.

Four layers are formed by alternating n-type and p-type semiconductor materials. Consequently, there are three p-n junctions formed in the device. It is a bistable device. The three terminals of this device are anode (A), cathode (K) and gate (G). The gate (G) terminal is the control terminal of the device. The circuit symbol is shown in the Figure 11.1.The current flowing through the device is controlled by electrical signal applied to the gate (G) terminal. The anode (A) and cathode (K) are the power terminals of the device which can handle large applied voltage and conduct the major current through thyristor. For example, when the device is connected in series with load circuit, the load current will flow through the device from anode (A) to cathode (K) but this load current will be controlled by the gate (G) signal applied to the device externally.

11.2 CONSTRUCTION: REALIZATION USING TRANSISTORS

Unlike the normal p-n junction diode, which is a two-layer semiconductor device, or the transistor, a three layer (p-n-p or n-p-n) device, the Thyristor is a four-layer (p-n-p-n) semiconductor device that contains three p-n junctions in series, as shown in Figure 11.2.