Q. pn-Junction under Bias?

Let an external source be connected between the p- and n-regions, as shown in Figure (a). (b) shows the circuit representation of the pn-junction or diode, and its external circuit. The voltage source V, called the bias, either decreases or increases the potential barrier, thereby controlling the flow of carriers across the junction. With V = 0, the barrier is unaffected and the pn-junction has zero current. Positive values of V, known as forward biasing, decrease the potential barrier, thereby increasing the number of electrons and holes diffusing across the junction. The increased diffusion results in a net current, called the forward current, from the p-tothe n-region. With increased V, the forward current further increases rapidly because the barrier is reduced even further.Making V negative (reverse biasing), on the other hand, increases the potential barrier and reduces the number of carriers diffusing across the boundary. The drift component produced by the electric field from the n-tothe p-region causes a small current, called the reverse current (or saturation current) IS. The magnitude of the saturation current depends on the doping levels in the p- and n-type materials and on the physical size of the junction. Increasing the reverse bias, however, does not affect the reverse current significantly until breakdown occurs.