A p-n junction is a boundary formed between two types of semiconductor materials: p-type and n-type. The p-type semiconductor has an abundance of positive charge carriers called holes, while the n-type has an excess of negative charge carriers called electrons. When these two materials are joined, they create an electric field that allows current to flow in one direction but not the other, making it essential for many electronic devices. This unique property of p-n junctions is crucial in the operation of components like diodes, transistors, and solar cells. In a diode, for example, the p-n junction allows current to pass when forward-biased and blocks it when reverse-biased, enabling control of electrical signals in circuits.