A PN junction is a type of semiconductor device consisting of a P-type and N-type semiconductor material that are joined together.
In a PN junction, the P-type material has an excess of positively charged carriers (holes), while the N-type material has an excess of negatively charged carriers (electrons). When the two materials are joined together, the excess carriers from each side diffuse across the junction, creating a depletion region with no free carriers. This creates a built-in potential barrier, preventing further diffusion of carriers.
In forward bias, the positive terminal of a voltage source is connected to the P-type material and the negative terminal is connected to the N-type material. This reduces the built-in potential barrier, allowing current to flow through the junction. In reverse bias, the positive terminal is connected to the N-type material and the negative terminal to the P-type material, increasing the potential barrier and preventing current flow.
In reverse bias, a small amount of current can still flow through the PN junction due to minority carriers (electrons in the P-type material and holes in the N-type material) that are thermally generated. This is known as reverse saturation current and is typically very small.
PN junctions are used in a variety of electronic devices, including diodes, transistors, solar cells, and LEDs. They also play a crucial role in the operation of integrated circuits.