Page 997 - Basic Electrical Engineering

keep on increasing the voltage between the terminals A and K, a stage comes
when the depletion layer at junction, J will breakdown due to a large voltage
2
gradient across the depletion layer. This phenomenon is called avalanche
breakdown, the other two junctions, J and J being forward biased, there will
1
3
be free movement of charge carriers resulting in a large current flow through
the device. The SCR is said to be in conducting state.

Figure 14.35 A biased SCR

For the reverse biasing, the cathode is made positive with respect to the

anode by connecting the negative terminal of the dc source to the anode and
the positive terminal to the cathode. Junctions J and J are reverse biased and
3
1
junction J is forward biased. No current, except some negligible leakage
2
current will flow. However, when the reverse voltage is gradually increased,
the width of the depletion layers at junctions J and J will become thin and a
3
1
breakdown voltage will be reached when free charge carriers will be flowing
through the device resulting in a large current flow in the reverse direction.

The V–I characteristic of an SCR is shown in Fig. 14.36. At a forward
voltage of V AK = V BOF , the SCR starts conducting. The voltage across the

device falls to a low value and a large amount of current starts flowing. This
current is controlled by an external resistance placed in the circuit. The

voltage at which the SCR starts conducting is called its triggering voltage. By
gate control, it is possible to reduce this triggering voltage.

In Fig. 14.36 it is seen that the application of gate current I reduces the
g1
voltage at which the device starts conducting. A further increase in gate