P-N Junction Formation

A p-n junction is formed simply by joining an n-type and a p-type semiconductor crystal together. What happens at that boundary immediately afterwards -- carriers diffusing across, a self-limiting depletion region growing, and a barrier potential building up -- is the subject of the two sub-sections below. Two things happen essentially at once when the two crystals are joined: charge carriers begin diffusing across the boundary purely because of the concentration difference (9.3.1.1), and this diffusion immediately starts building up a region of fixed, immobile charge whose own electric field opposes further diffusion, eventually reaching a stable equilibrium characterised by a fixed barrier potential (9.3.1.2). Both effects are two sides of the same physical process -- one describing the region of space involved (the depletion layer), the other describing the electrical potential it produces -- and toge …