Semiconductor diodes are fundamental components in modern electronics, with the PN junction diode being the most basic and widely used type. Its operation relies on the properties of a PN junction, formed by joining p-type and n-type semiconductor materials. Understanding the PN junction diode requires an exploration of its formation, working principles, electrical characteristics, and applications.

The PN junction forms when p-type and n-type semiconductor materials are brought into contact. P-type semiconductors are doped with acceptor impurities, creating an excess of holes, while n-type semiconductors are doped with donor impurities, resulting in an excess of electrons. When these materials are joined, the concentration gradient causes electrons from the n-side to diffuse into the p-side and holes from the p-side to diffuse into the n-side. This diffusion leaves behind immobile ionized donor atoms on the n-side and ionized acceptor atoms on the p-side, forming a region called the depletion layer. The depletion region is devoid of free charge carriers and acts as an insulator under equilibrium conditions. The fixed charges in this region create an electric field that opposes further diffusion, establishing a built-in potential barrier, typically around 0.7 V for silicon and 0.3 V for germanium.

Under forward bias, the external voltage is applied such that the positive terminal connects to the p-side and the negative terminal to the n-side. This reduces the built-in potential barrier, allowing majority carriers to cross the junction. Electrons from the n-side are injected into the p-side, and holes from the p-side move into the n-side. As the forward voltage increases beyond the threshold, current rises exponentially, as described by the diode equation: I = I₀(e^(V/nV_T) - 1), where I₀ is the reverse saturation current, V is the applied voltage, n is the ideality factor, and V_T is the thermal voltage. The forward bias characteristic is dominated by diffusion current.

Under reverse bias, the positive terminal connects to the n-side and the negative terminal to the p-side, increasing the depletion region width and the potential barrier. The majority carrier flow is blocked, and only a small reverse saturation current (I₀) flows due to minority carriers. This current remains nearly constant with increasing reverse bias until breakdown occurs. The reverse bias characteristic is dominated by drift current.

The current-voltage (I-V) curve of a PN junction diode illustrates its nonlinear behavior. In the forward direction, current remains negligible until the threshold voltage is reached, after which it rises sharply. In the reverse direction, current is minimal until breakdown. The I-V relationship is asymmetric, demonstrating rectification properties.

Breakdown in PN junction diodes occurs under high reverse voltage and can be either avalanche or Zener breakdown. Avalanche breakdown happens when the electric field in the depletion region accelerates minority carriers to high energies, causing them to collide with lattice atoms and generate electron-hole pairs. These new carriers are also accelerated, leading to a multiplicative effect and a sudden increase in current. Avalanche breakdown typically occurs at higher voltages (above 5 V) and depends on the doping concentration and material properties.

Zener breakdown occurs in heavily doped junctions where the depletion region is very narrow. The high electric field directly pulls electrons from the valence band to the conduction band via quantum tunneling, resulting in a rapid increase in current. Zener breakdown dominates at lower voltages (below 5 V). Both mechanisms are non-destructive if the current is limited externally.

PN junction diodes are primarily used in rectification circuits, converting alternating current (AC) to direct current (DC). A half-wave rectifier uses a single diode to pass only the positive or negative half-cycle of the input AC waveform, while a full-wave rectifier employs four diodes in a bridge configuration to utilize both halves. The output is then filtered using capacitors to produce a smoother DC voltage. Rectifier circuits are essential in power supplies for electronic devices.

Other applications include signal demodulation in communication systems, where diodes extract information from modulated carrier waves, and voltage clamping circuits, which limit voltage swings to protect sensitive components. Diodes also serve as simple switches in logic circuits and provide temperature sensing due to the temperature dependence of the forward voltage drop.

The performance of PN junction diodes depends on material parameters such as bandgap energy, doping levels, and carrier mobility. Silicon diodes are common due to their stable operation and moderate bandgap, while germanium diodes are used in low-voltage applications. Wide-bandgap materials like silicon carbide and gallium nitride enable high-temperature and high-power operation.

Understanding the physics of PN junction diodes provides a foundation for exploring more complex semiconductor devices. Their simplicity, reliability, and versatility make them indispensable in electronic systems, from basic power supplies to advanced communication networks. The principles governing their operation also extend to other devices, including bipolar junction transistors and solar cells, highlighting their importance in semiconductor technology.