A PN junction is a semiconductor device formed by joining p-type and n-type materials. 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 region where electrons and holes can recombine, leading to the formation of a depletion zone. This depletion zone acts as a barrier to the flow of charge carriers, allowing current to flow in one direction but not the other. This property makes PN junctions essential components in various electronic devices, such as diodes and transistors, enabling functions like rectification and amplification.