Why is it called a pn junction?

The term "pn junction" is derived directly from the types of semiconductor materials that are joined together to create the device. The "p" stands for positive, referring to p-type semiconductor material, which is doped with impurities that create an excess of holes (positive charge carriers). The "n" stands for negative, referring to n-type semiconductor material, which is doped with impurities that create an excess of electrons (negative charge carriers). When these two distinct types of materials are brought into contact, a boundary or interface is formed between them. This interface is the "junction." At this specific point, electrons from the n-side diffuse into the p-side, and holes from the p-side diffuse into the n-side, creating a depletion region that is crucial for the diode's ability to control the flow of electric current in one direction. The name simply describes the physical composition of the two halves that meet at this critical interface, making it a fundamental building block of modern electronics like transistors, solar cells, and LEDs.