The thyristor is the most important type of the power semiconductor devices. They are used in very large scale in power electronic circuits. The thyristor are known also as Silicon Controlled Rectifier (SCR). The thyristor has been invented in 1957 by general electric company in USA.
The thyristor consists of four layers of semiconductor materials (p-n-p-n) all brought together to form only one unit. Figure 1 shows the schematic diagram of this device and its symbolic representation. The thyristor has three terminals, anode A, cathode K and gate G as shown in Figure 1.The anode and cathode are connected to main power circuit. The gate terminal is connected to control circuit to carry low current in the direction from gate to cathode.
 |
| Figure1: Schematic diagram of SCR and its circuit symbol. |
The operational characteristics of a thyristor are shown in Figure 2 In case of zero gate current and forward voltage is applied across the device i.e. anode is positive with respect to cathode, junction J1 and J3 are forward bias while J2 remains reverse biased, and therefore the anode current is so small leakage current. If the forward voltage reaches a critical limit, called forward break over voltage, the thyristor switches into high conduction, thus forward biasing junction J2 to turn thyristor ON in this case the thyristor will break down. The forward voltage drop then falls to very low value (1 to 2 Volts). The thyristor can be switched to on state by injecting a current into the central p type layer via the gate terminal. The injection of the gate current provides additional holes in the central p layer, reducing the forward break over voltage. If the anode current falls below a critical limit, called the holding current IH the thyristor turns to its forward state.
 |
| Figure2: Thyristor V-I characteristics. |
If the reverse voltage is applied across the thyristor i.e. the anode is negative with respect to cathode, the outer junction J1 and J3 are reverse biased and the central junction J2 is forward biased. Therefore only a small leakage current flows. If the reverse voltage is increased, then at the critical breakdown level known as reverse breakdown voltage, an avalanche will occur at J1 and J3 and the current will increase sharply. If this current is not limited to safe value, it will destroy the thyristor.
The gate current is applied at the instant turn on is desired. The thyristor turn on provided at higher anode voltage than cathode. After turn on with IA reaches a value known as latching current, the thyristor continuous to conduct even after gate signal has been removed. Hence only pulse of gate current is required to turn the Thyristor ON.
