In this blog post, we’ll explain the structure and operating principles of a PN junction—as well as some simple applications—in an easy-to-understand way.
Why Should You Learn About PN Junctions?
We enjoy the benefits of 21st-century technology, such as smartphones, laptops, and computers. While the semiconductor industry has played a major role behind these dramatic advancements, many people do not fully understand how semiconductors work or which components are key. One of the most fundamental and important electronic components is the PN junction. In this post, I’ll explain step by step what a PN junction is and why it’s important.
Basics of Semiconductors and Doping
Materials are classified as conductors, semiconductors, or insulators based on their electrical conductivity. The primary semiconductor materials used are silicon (Si) and germanium (Ge), which belong to Group 14 of the periodic table. Since these atoms have four electrons in their outermost shell, they have a strong tendency to form covalent bonds rather than exchange electrons with one another.
The process of partially replacing atoms in a semiconductor material with external atoms is called doping. If an atom with five valence electrons is introduced through doping, free electrons remain; if an atom with three valence electrons is introduced, holes remain. A semiconductor with excess electrons, which carries a negative charge, is called an N-type semiconductor, while a semiconductor with excess holes, which carries a positive charge, is called a P-type semiconductor.
Formation of a PN Junction and Equilibrium State
When an N-type and a P-type semiconductor are joined, charge carriers begin to move between the two regions. The electrons on the n-type side attempt to combine with the holes on the p-type side, and conversely, the holes on the p-type side attempt to combine with the electrons on the n-type side. This movement of charge carriers generates a diffusion current in the early stages of the junction.
As initial diffusion proceeds, ionized donors and acceptors remain near the junction, forming a region with fixed charge. These fixed charges create an electric field, which hinders the further diffusion of electrons and holes. The region where carrier concentrations have been depleted is called the depletion region, and the electric field generated in this region pulls the carriers in the opposite direction, causing them to move. Consequently, the current generated by this electric field is called the drift current.
At equilibrium, the diffusion current and the drift current are equal in magnitude but opposite in direction, so the total current cancels out. For this reason, no net current flows through the PN junction when no external power source is applied.
Operation and Applications Based on Bias
When an external voltage is applied to a PN junction, the height of the depletion region and the junction barrier changes, altering the current behavior. Under forward bias—where the P region is connected to the anode (+) and the N region to the cathode (−)—the junction barrier lowers, allowing electrons to move easily into the P region and holes into the N region, resulting in current flow. Utilizing this property, a PN junction functions as a diode, performing rectification (allowing current to flow in only one direction).
Conversely, under reverse bias (typically when a negative (−) voltage is applied to the P region and a positive (+) voltage to the N region), the barrier height increases and the depletion region widens. As a result, diffusion is further suppressed, and the junction blocks current flow.
Due to the characteristic of the depletion region widening, a PN junction exhibits behavior similar to that of a capacitor, and the property whereby the junction’s capacitance varies with bias is utilized in various ways in electronic circuits.
By utilizing these bidirectional bias characteristics, a PN junction is used as a rectifier or as a tuning device (e.g., a varactor) that takes advantage of changes in the junction’s capacitance. Furthermore, the characteristics of the PN junction form the basis for the design of various types of semiconductor devices, enabling them to perform diverse roles in circuits alongside batteries and other electronic components.
Ultimately, while the PN junction may appear simple, it embodies the core operating principles of electronic devices and serves as an essential element in the modern electronics industry, forming the foundation that enriches our lives.
