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.
Basic Structure:
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.
I-V Characterstics:
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.