Q.What is p-n junction diode? Define Depletion layer.

When p-type and n-type semiconductor material are joined, the result is a p-n junction -- the structural basis of almost every modern semiconductor device. Immediately after joining, a large density gradient of each carrier type exists across the junction, driving DIFFUSION: electrons migrate from the electron-rich n-side into the p-side, and holes migrate from the hole-rich p-side into the n-side. This leaves behind FIXED, immobile ions near the junction -- positively charged donor ions on the n-side (where electrons departed) and negatively charged acceptor ions on the p-side (where holes departed). Because free carriers cannot survive within a region containing this exposed ionic charge, the region straddling the junction becomes essentially empty of mobile carriers -- the DEPLETION REGION -- and the accumulated ionic charge on both sides sets up a permanent POTENTIAL BARRIER (about 0.6-0.7 V for silicon, 0.3-0.35 V for germanium) that exists even with no external voltage connected, bringing the junction to electrostatic equilibrium with an internal field pointing from the n-side toward the p-side. …