A PN junction is a semiconductor device formed by joining two types of materials: P-type and N-type semiconductors. The P-type material has an abundance of holes (positive charge carriers), while the N-type material has excess electrons (negative charge carriers). When these two materials are combined, they create a boundary where charge carriers can move, leading to the formation of a depletion region. This junction is crucial in many electronic devices, such as diodes and transistors. When a voltage is applied, the PN junction allows current to flow in one direction while blocking it in the opposite direction, making it essential for controlling electrical signals in circuits.