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 known as electrons. When these two materials are joined, they create an electric field that allows current to flow in one direction but not the other. This unique property of the P-N junction is essential in many electronic devices, such as diodes and transistors. In a diode, for example, the P-N junction allows current to pass when forward-biased and blocks it when reverse-biased, making it a crucial component in controlling electrical signals.