Q.Determine cathode current $I_K$ of SCR when $I_G$ = 0. Given $(\alpha_1 + \alpha_2)$ = 0.99 and $(I_{co1} + I_{co2})$ = 3 mA.

The silicon controlled rectifier (SCR), or thyristor, is a four-layer p-n-p-n, three-terminal controlled device — the device with which modern power electronics truly began. Its three terminals are the anode (on the top p layer), the cathode (on the bottom n layer) and the gate (connected to the lower p layer). Four layers meet at three junctions, J1, J2 and J3. An SCR can be turned on by a gate current pulse but cannot be turned off by the gate; it blocks voltage in both directions but conducts in only one.

The pnpn action is best understood by splitting the structure into two interconnected transistors — a pnp transistor Q1 and an npn transistor Q2 — that share the three junctions, with the collector of each driving the base of the other. This positive feedback is what gives the thyristor its latching behaviour.

With the gate open $(I_G = 0)$, the anode current is

where $\alpha_1,\alpha_2$ are the current gains of Q1 and Q2. When a gate current is applied $(I_K = I_A + I_G)$, it becomes

A large enough gate current drives $(\alpha_1 + \alpha_2)$ towards unity, switching the device on. These equations let you compute the anode or cathode current for given gains and leakage currents.