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Essay on The P-N Junction
A p-n junction consists of two semiconductor regions with opposite doping type as shown in Figure-1. The region on the left is p-type with an acceptor density Na, while the region on the right is n-type with a donor density Nd. The dopants are assumed to be shallow, so that the electron (hole) density in the n-type (p-type) region is approximately equal to the donor (acceptor) density (Douglas, 2003).
Figure-1
To reach thermal equilibrium, electrons/holes close to the metallurgical junction diffuse across the junction into the p-type/n-type region where hardly any electrons/holes are present. This process leaves the ionized donors (acceptors) behind, creating a region around the junction, which is depleted of mobile carriers. I call this region the depletion region, extending from x = -xp to x = xn. The charge due to the ionized donors and acceptors causes an electric field, which in turn causes a drift of carriers in the opposite direction. The diffusion of carriers continues until the drift current balances the diffusion current, thereby reaching thermal equilibrium as indicated by a constant Fermi energy (Charles, 2003).
While in thermal equilibrium no external voltage is applied between the n-type and p-type material, there is an internal potential, i, which caused by the work function difference between the n-type and p-type semiconductors. This potential equals the built-in potential.
I now consider a p-n diode with an applied bias voltage, Va. A forward bias corresponds to applying a positive voltage to the anode (p-type region) relative to the cathode (n-type region)...............
