[UPSC CSE 1988] Semiconductors/Solar cell (Q292, 60M)

What is a P.N. junction? Describe with energy level diagram the formation of double layer of charges which generates a potential difference (V) across, the junction. Hence show that the capacitance of the double layer. C=KV^{-1/2}, where K is a constant. State some practical use of this relation between C and V.