An LED is a semiconductor diode. It consists of a chip of semiconducting material treated to create a structure called a p-n (positivenegative) junction.

When connected to a power source, current flows from the p-side or anode to the n-side, or cathode, but not in the reverse direction. Charge-carriers (electrons and electron holes) flow into the junction from electrodes. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon (light).