Reverse Biased p-n Junction Diode:
Electrons in the n-type half of the diode are repelled from the junction by the negative ions in the p-type region, and holes in p-type half are repelled by the positive ions in the n-type region. A space on either side of the junction boundary is left without either type of current carriers. It is known as the depletion layer. Since there are no current carriers in this layer no current may flow. The depletion layer is, in effect, an insulator. Now assume what would happen if we connected a small voltage to the diode. Connected in reverse bias as in Figure, it would attract the current carriers away from the pn junction boundary and make the depletion layer wider. Connected in forward bias as in Figure 1(a), it would repel the carriers and drive them towards the pn junction boundary, so decreasing the depletion layer. In neither case would any current pass because there would always be some depletion layer left. Now assume increasing the voltage. In reverse bias case, there is still no current because the depletion layer is even wider, but in the forward bias case, the layer disappears totally and current may flow. Above a certain voltage the diode behaves as a conductor. As electrons & holes meet each other at the junction they combine and disappear. The battery keeps the diode supplied along current carriers.
Maxwell-Boltzman statistics applied to diffusion of charge carriers may predict current density across the p-n junction in forward and reverse bias. We shall not get into details of Maxwell-Boltzman statistics. Rather directly give the relation among the current and voltage across a diode.
I = I0 ( exp (eV /KT) -1)
where I is the current through the diode (positive I refer the current is passing conventionally from the p-type to n-type semiconductors), k is the Boltzman constant (1.38 × 10- 23 J/K), T is absolute temperature, e is the electronic charge (1.6 × 10- 19 C), V is the bias voltage (positive indicating forward bias) and I0 is the leakage current in reverse bias.