What Are the Types of PN Junction Diodes?
In the semiconductor and electronic components industry, the diode is one of the most fundamental and widely used elements. Diodes based on the PN junction structure—often called "PN diodes" or "PN junction diodes"—are especially important, serving as key components for functions such as rectification, switching, voltage regulation, and signal processing in electronic devices. Depending on the manufacturing process and structural design, PN junction diodes can be classified into several types, each with its own advantages and trade-offs in terms of production complexity, electrical performance, and suitable applications. This article will first introduce the PN junction diode and then provide a systematic overview of the main types of junction diodes.
Catalog
I. What is a PN Junction Diode?
II. What Are the Types of PN Junction Diodes?
2. Alloy (or Fused) Junction Diode
I. What is a PN Junction Diode?
A PN junction diode is formed by joining P-type and N-type semiconductors through doping and various fabrication processes, creating a unified crystalline or bonded structure. The interface where the P and N regions meet is called the "PN junction." The P-region uses holes as its majority charge carriers, while the N-region uses electrons. Once these two semiconductor types are bonded, carrier diffusion and recombination occur at the interface, forming a depletion region—an area with very few free charge carriers—which establishes a built-in potential barrier. This structure gives the PN junction its unidirectional conduction property: when the diode is forward-biased (positive voltage applied to the P-side and negative to the N-side) beyond a certain threshold, carriers overcome the barrier and current flows. Under reverse bias (positive to N-side, negative to P-side), the barrier and depletion region prevent significant carrier movement, allowing almost no current. It is this one-way conduction that makes the PN junction diode a foundational component in circuits used for rectification (AC to DC conversion), switching, signal processing, protection, and voltage regulation.
It is important to note that a PN junction is not simply made by placing a piece of P-type semiconductor against an N-type piece. Instead, it is formed through precise semiconductor fabrication techniques—such as doping, crystal growth, diffusion, and epitaxy—which allow careful control over impurity concentration, junction depth, junction shape, junction area, and carrier distribution. These methods enable the creation of stable, reproducible, and performance-controllable PN junction structures. Different process routes have given rise to several distinct types of "junction diodes."
II. What Are the Types of PN Junction Diodes?
Based on the process used to form the PN junction—that is, the method of semiconductor crystal formation, doping, and junction formation—common PN junction diodes can be divided into the following main types: Grown Junction, Alloy (or Fused) Junction, Diffused Junction, and Epitaxial Planar Junction. Each type has its own typical manufacturing flow, structural features, and application scenarios, which will be detailed below.
1. Grown Junction Diode
The grown junction diode was an early manufacturing method in semiconductor technology. The process starts with a highly pure melt of germanium or silicon. First, a small amount of N-type impurity (such as antimony) is added to the melt, and a single crystal is slowly pulled from it. During the crystal growth process, the doping is switched to a high concentration of P-type impurity (such as indium). This creates an abrupt transition from N-type to P-type within a continuously grown single crystal, forming the PN junction.
Figure (a) and (b) above show three-dimensional and two-dimensional views, respectively, of a basic grown junction diode. Figure 1(c) shows the circuit symbol common to all PN diode types. The grown junction diode is made by first pulling a crystal from an extremely pure (impurity less than 1 in 10⁹) melt of germanium or silicon doped with N-type impurities. Shortly after, the doping is changed by introducing a large amount of P-type impurity into the melt, switching the crystal growth from N-type to P-type.
This results in a continuous crystal that is partly P-type with a junction in the middle. The large crystal formed this way is then sliced into many small wafers, each containing one P-region and one N-region with a junction between them. Each section is polished and etched to remove surface impurities, after which non-rectifying (ohmic) electrodes are deposited on both ends of the slice. Wires are then soldered to these terminal electrodes. The entire assembly is etched, coated with moisture-resistant grease, mounted in a suitable mechanical structure, and finally sealed in a small glass envelope with leads protruding through the base. Opaque paint is usually applied to the outside of the glass envelope to block incident light.
2. Alloy (or Fused) Junction Diode
The alloy junction process was a primary manufacturing method for early point-contact diodes and some power diodes. As shown in the figure below, the process is straightforward and clever: a small pellet of indium (a trivalent element, acting as a P-type impurity) is placed on a thin wafer of N-type germanium. The assembly is then heated in a hydrogen or vacuum environment at around 500°C. This temperature is above the melting point of indium but below that of germanium. The indium pellet melts and dissolves a small amount of the germanium beneath it, forming an indium-germanium eutectic molten alloy.
When the assembly cools, the molten alloy recrystallizes. Due to the high concentration of indium, the recrystallized region becomes a P-type semiconductor, forming a PN junction with the original N-type germanium wafer underneath. The indium ball itself naturally serves as the ohmic contact electrode for the P-region, while an N-region electrode is formed on the bottom of the wafer through sintering or soldering. This diode structure is simple, has a relatively large junction area, and can handle high forward currents, making it suitable for early rectifier devices. However, control over junction depth and geometry is limited, and its operating frequency is typically not very high.
3. Diffused Junction Diode
The diffusion process was a revolutionary technology in modern semiconductor manufacturing, enabling precise control over junction depth and impurity distribution. As shown in the figure below, the process begins with a lightly doped silicon wafer (for example, N-type). First, a dense silicon dioxide protective layer is grown on its surface through high-temperature oxidation. Using photolithography, specific windows are etched into the SiO₂ layer. The wafer is then placed in a high-temperature diffusion furnace (e.g., 1000°C) and exposed to a gaseous source containing boron (a P-type impurity). Boron atoms diffuse through the windows into the silicon wafer, forming the P-type region.
At this high temperature, silicon atoms gain kinetic energy, creating pathways for impurity atom diffusion. By precisely controlling temperature and time, the impurity concentration gradient and junction depth can be accurately regulated. After diffusion, an oxide layer is grown again for surface passivation and protection. Aluminum electrodes are then formed using photolithography and metallization processes. Diffused junction diodes offer excellent reproducibility, stable parameters, and relatively high operating frequencies, making them widely used in switching, voltage regulation, and general rectification applications.
4. Epitaxial Planar Diode
The epitaxial planar process is a core technology for modern high-performance diodes and integrated circuit manufacturing. As shown in the figure below, it combines the dual advantages of epitaxial growth and planar processing. First, on a heavily doped silicon substrate (which serves as both support and electrode, e.g., N⁺ type), a layer of single-crystal silicon with lower impurity concentration and precisely controlled thickness (e.g., N-type) is epitaxially grown via chemical vapor deposition. This layer perfectly continues the crystal lattice structure of the substrate.
Then, on this high-quality epitaxial layer, a planar process similar to that used for diffused junctions is applied: thermal growth of silicon dioxide, photolithographic window opening, selective diffusion to form the P-region, and finally, metal contact electrode formation. The junction formed by the planar process is covered by the SiO₂ layer, resulting in a stable surface, very low leakage current, and high reliability. The epitaxial layer provides an ideal low-resistance current path and precisely defined space for depletion layer expansion. This structure not only gives the diode extremely fast switching speed and consistent high-frequency performance but is also naturally suited for integrating multiple devices on the same chip. It forms the manufacturing foundation for Schottky diodes, PIN diodes, and nearly all modern silicon integrated circuits.
III. Conclusion
From the initial attempts with grown junctions, to the practical application of alloy junctions, followed by the precise control offered by diffused junctions, and finally evolving to the highly integrated and high-performance epitaxial planar process, the history of PN junction diode manufacturing types is essentially a condensed history of semiconductor industry advancement. Each diode type, due to its unique manufacturing process, exhibits distinct characteristics in key parameters such as reverse recovery time, maximum current, breakdown voltage, junction capacitance, and cost. This enables them to meet the demanding requirements of various applications, from power-frequency rectification to microwave communications.
