Bring n-type and p-type material into contact and something interesting happens. Near the boundary, the surplus electrons on the n-type side diffuse across into the empty seats (holes) on the p-type side, and the two cancel each other out. The result is a layer around the boundary with no carriers left in it. This is called the depletion region.

Once the electrons have moved, they leave positive charge stranded on the n-type side and negative charge on the p-type side. An electric field therefore appears inside the depletion region, and it pushes back against any further diffusion. Eventually the two effects balance and the process stops. That balanced state is the resting state of a pn junction.

So what happens when you apply a voltage from outside?

In other words, a pn junction is a component that passes current in one direction only. This is the diode, and it is the basis of rectification — turning alternating current into direct current.

And the range of applications is wide. Shine strong light on it under reverse bias, and the light's energy knocks electrons loose, producing a current. That is the solar cell and the image sensor. Run it forward instead, and arrange for the energy released when electrons and holes recombine to come out as light, and you have an LED. A single pn junction explains rectification, light detection, and light emission all at once.