Silicon Controlled Rectifier (SCR)

Silicon Controlled Rectifier or SCR is one of the oldest type of solid state power device. It was invented in 1975 by the General Electric Research Laboratory. It has the highest power handling capacity of all the power semiconductor device. It has four layer construction with three user accessible terminals. SCR is a latching type device that can be turned ON by the gate terminal but once turned ON, the Gate loses control on it.

Important Features

1. It is latching type device.
2. It can handle a very large power.
3. It is a current controlled device because the gate current controls the SCR.
4. It acts as an open or closed switch.
5. The ON state voltage drop is very low.
6. It can handle thousands of ampere of current.

Construction

It is a four layer PNPN device with three terminals brought out for the user, namely Anode(A), Cathode(K) and Gate(G). The Gate terminal is used in ON process. It can be split into two sections of NPN and PNP as shown below,

It has three junctions J1, J2 and J3. The anode and cathode are connected to the main power circuit. The gate terminal carries a low level gate current in the direction of gate to cathode. Normally, the gate terminal is provided at the P layer near the cathode as shown in above figure. This is known as cathode gate.