Thyristor, Construction and Operation

The Thyristor is a solid state electronic device which had four doped semiconductor layers. The layers were arranged in alternate manner to form a stack shaped structure. Three terminals were taken out from the thyristor, and named as Anode, Cathode and Gate.

Fig 1: Layered Structure of Thyristor and equivalent transistor model

The operation of a thyristor can be described easily, by using the two transistor model, if a current pulse applied to the gate terminal, it triggers the bottom transistor, and because of that, the top one automatically starts to conduct. This caused to keep the gate current until the supply stops or reversed (Anode cathode current). Operation of the thyristor is same as this. It won’t conduct till the gate received a current pulse, after that it will conduct till the removal or reversal of the external source.

Thyristor have three states:

1. Reverse blocking mode - Voltage is applied in the direction that would be blocked by a diode

2. Forward blocking mode - Voltage is applied in the direction that would cause a diode to conduct, but the thyristor has not yet been triggered into conduction

3. Forward conducting mode - The thyristor has been triggered into conduction and will remain conducting until the forward current drops below a threshold value known as the "holding current"

Fig 2: VI Characteristics of the thyristor

In a conventional thyristor, once it has been switched on by the gate terminal, the device remains latched in the on-state (i.e. do not need a continuous supply of gate current to conduct), providing the anode current has exceeded the latching current (I_L). As long as the anode remains positively biased, it cannot be switched off until the anode current falls below the holding current (I_H). The thyristor can be switched off if the external circuit causes the anode to become negatively biased. In some applications this is done by switching a second thyristor to discharge a capacitor into the cathode of the first thyristor. This method is called forced commutation.