Answer:SCR is a four layered PNPN switching device, having three junctions J1, J2 and J3 as shown in the figure.It has three external terminals namely the anode (A), cathode ( C or K ) and Gate (G). When the Anode is made positive with respect to the cathode ( C), then the junctions J1 and J3 are forward biased but the middle junction J2 becomes reverse bias. Only leakage current of negligibly small amount is flowing from anode to cathode, this is because of the drift of the mobile charge carriers. This current is not sufficient to make the device to conduct. This State is called Forward Blocking State of the SCR.
When the Voltage between the anode and cathode is increased, then at particular voltage depletion layer at junction J2 breaks down and SCR starts conducting, This phenomenon is known as Avalanche Breakdown. This type of conduction method may damage the device.
PNPN layer SCR can be represented as Two Transistors connected as shown in the figure. In this model the output of one transistor is fed to the input of the other transistor. When anode is made positive with respect to cathode no currents flows because neither of the transistors are conducting. When a positive voltage is applied from Gate to Cathode, holes are injected in to the base of the T2, this forward biases the base emitter junction of T2, increases ic2. This collector current ic2 is the base current for T1, so increase in the ic2 (ib1) will increase collector and emitter currents of T1 which results in increase in the base current (ib2). This increase in the base current of (ib2) once again increase the ic2 and the process continues. The net result is regenerative increase in the collector currents of each transistors.This regenerative process continues untill both the transistors are driven in to saturation condition making all the junctions forward biased.
Once the SCR is turned on the gate current can be removed, still the device will be conducting because of the above said regenerative action
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