Current gain

Summary

The functional dependence of current gain on current before the turn on of the SCR is reached is determined from the theoretical considerations under somewhat idealized conditions.

The transport factors in both device bases are functions of the electric fields and diffusion. The internal field in the n base depends on the magnitude of the current flowing; consequently, the transport factor is strongly current dependent. The field in the p base has approximately a constant value, so the transport factor is independent of the current level within certain limits.

Because of the existence of the recombination-generation currents at the n emitter (cathode) and p emitter (anode), the emitter efficiencies at low-current levels, occurring before the turn-on, are strongly current-dependent and increase with increasing current density.

If the SCR is considered before the turn-on as an n-p-n transistor with a so-called hook collector, then it is possible to determine experimentally the values of current gains of both the n-p-n and the p-n-p transistors using only the three available SCR terminals.