the p-n junction

The p-n junction is what you get when you press a p-type semiconductor against an n-type one, and it is the single most important building block in electronics: a one-way valve for electric current. Push current one way and it flows freely; push it the other way and it is blocked. From this simple asymmetry come the diode, the LED, the solar cell, and, in pairs, the transistor.

At the boundary, electrons from the electron-rich n-side diffuse into the p-side and holes diffuse the other way; they recombine near the interface and leave behind a depletion region stripped of mobile carriers but full of fixed, charged dopant ions. Those fixed charges set up a built-in electric field and a potential barrier V_bi across the junction. Equilibrium is reached when the diffusion of carriers down their concentration gradient is exactly cancelled by their drift back in the built-in field, a balance signalled by a single flat Fermi level throughout. Applying a forward bias (making the p-side positive) lowers the barrier and current rises steeply, following the Shockley diode equation I = I_0 [exp(eV / k_B T) - 1]; a reverse bias raises the barrier, widens the depletion region, and chokes the current off to a tiny saturation value I_0.

The p-n junction is the elemental part from which rectifiers, light-emitting and photovoltaic diodes, and bipolar transistors are constructed. The subtle point to hold onto is that the built-in field does not, by itself, drive any net current in equilibrium; drift and diffusion cancel precisely, and it is only when an external bias upsets that balance that the junction conducts or blocks.