Semiconductor Controlled Rectifier
SCR is a three terminal, three junction & four terminal device. It is used in power switching & various control circuits. The diff in b/w SCR & PNPN diode is that in SCR, there is a third terminal, GATE. This device can handle currents from few mA to hundred of Amp. The SCR can be used to regulate the amount of power delivered to a load.
The most important four layer device is SCR. In this figure, there is three terminal anode, cathode & gate terminal. When the SCR is biased in the forward blocking state, a small current is supplied to the gate can initiate switching to the conducting state.
As result, the anode switching voltage Vp decreases as the current ig is applied to the gate is increased. This type of turn on control makes the SCR a useful & a versatile device.
- Let us assume that the device is in forward blocking state with a small saturation current from anode to cathode.
- The positive gate causes the holes from gate into p2.
- This added the supply of holes & the accompanying injection of electrons from n2 into p2.
- The electrons are injected by j3 arrive at the center junction and are swept into n1.
- This causes an increase in hole injection of j1 and these holes diffuse across the base n1.
- Then the device is driven into the forward conducting state.
- The delay time of the SCR is less than 1 μs.