Q. Physical operation of the junction of diode?

The physical operation of the junction can be described in terms of the charge-flow processes. Usually there is a greater concentration of holes in the p-region than in the n-region; similarly, the electron concentration in the n-region is greater than that in the p-region. The differences in concentration establish a potential gradient across the junction, resulting in a diffusion of carriers, as indicated in Figure (a). Holes diffuse from the p-region to the n-region, electrons from the n-region to the p-region. The result of the diffusion is to produce immobile ions of opposite charge on each side of the junction, as shown in Figure (b), and cause a depletion region (or space-charge region) in which no mobile carriers exist.

The immobile ions (or space charge), being of opposite polarity on each side of the junction, establish an electric field because of which a potential barrier is formed and drift current is produced. The drift current causes holes to move from the n-tothe p-region and electrons to move from the p-tothe n- region, as shown in Figure (c). In equilibrium and with no external circuit, the drift and diffusion components of current are equal and oppositely directed. The potential barrier established across the depletion region prohibits the flow of carriers across the junction without the application of energy from an external source.