p-n Junction — Definition
Definition
Imagine taking two different types of specially treated semiconductor materials, one called 'p-type' and the other 'n-type', and carefully bringing them together to form a single crystal structure. The point where they meet is called a p-n junction. This junction is the heart of many electronic devices, including the simple diode, which acts like a one-way valve for electricity.
Let's break down what happens at this junction. A p-type semiconductor has an excess of 'holes', which are essentially empty spaces where electrons could be, and they behave like positive charge carriers. An n-type semiconductor, on the other hand, has an excess of 'free electrons', which are negatively charged.
When these two types are joined, the free electrons from the n-side, being in a region of high concentration, tend to diffuse (spread out) into the p-side, which has a low concentration of free electrons.
Similarly, holes from the p-side diffuse into the n-side. As electrons move from the n-side to the p-side, they leave behind positively charged immobile donor ions on the n-side. When these electrons combine with holes on the p-side, they leave behind negatively charged immobile acceptor ions on the p-side.
This movement of charges creates a narrow region around the junction that becomes depleted of mobile charge carriers (both electrons and holes). This region is aptly named the 'depletion region' or 'depletion layer'.
Within this depletion region, the immobile positive ions on the n-side and negative ions on the p-side create an electric field. This electric field points from the n-side to the p-side and acts as a barrier, opposing further diffusion of electrons and holes across the junction.
This electric field also creates a potential difference across the depletion region, known as the 'barrier potential' or 'junction potential'. For silicon, this barrier potential is typically around $0.
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Once this barrier potential is established, the diffusion process largely stops, and a state of equilibrium is reached. The p-n junction is now ready to be used as a diode, where its electrical behavior can be controlled by applying an external voltage, a process known as 'biasing'. This biasing fundamentally alters the width of the depletion region and the height of the potential barrier, thereby controlling the current flow through the junction.