Fundamental Characteristics of Thyristors

The thyristor family of semiconductors consists of several very useful devices. The most widely used of this family are silicon controlled rectifiers (SCRs), triacs, sidacs, and diacs. In many applications these devices perform key functions and are real assets in meeting environmental, speed, and reliability specifications which their electro-mechanical counterparts cannot fulfill. This application note presents the basic fundamentals of SCR, triac, sidac, and diac thyristors so the user understands how they differ in characteristics and parameters from their electromechanical counterparts. Also, thyristor terminology is defined. Figure AN1001.1 shows the simple block construction of an SCR. The operation of a PNPN device can best be visualized as a specially coupled pair of transistors as shown in Figure AN1001.2. Figure AN1001.2 Coupled Pair of Transistors as a SCR The connections between the two transistors trigger the occurrence of regenerative action when a proper gate signal is applied to the base of the NPN transistor. Normal leakage current is so low that the combined hFE of the specially coupled two-transistor feedback amplifier is less than unity, thus keeping the circuit in an off-state condition. A momentary positive pulse applied to the gate biases the NPN transistor into conduction which, in turn, biases the PNP transistor into conduction. The effective hFE momentarily becomes greater than unity so that the specially coupled transistors saturate. Once saturated, current through the transistors is enough to keep the combined hFE greater than unity. The circuit remains "on" until it is "turned off" by reducing the anode-to-cathode current (IT) so that the combined hFE is less than unity and regeneration ceases. This threshold anode current is the holding current of the SCR.