Diode Selection Guide and Fundamentals

As a veteran semiconductor component, the diode, this article provides a detailed overview—from diode classification and naming conventions to the characteristics and selection criteria of commonly used diodes. Based on their electrical conductivity, materials are classified into conductors, insulators, and semiconductors. Semiconductors are substances with unique properties whose conductivity falls between that of conductors and insulators; hence the term “semiconductor.” Common semiconductor materials include silicon (Si) and germanium (Ge). The diode is one of the oldest members of the semiconductor family. Its most distinctive feature is its unidirectional conduction property: current can flow in only one direction—from the positive terminal to the negative terminal—but not in the reverse direction.

Introduction

As a veteran semiconductor component, the diode, this article provides a detailed overview—from diode classification and nomenclature to the characteristics and selection criteria of commonly used diodes.

Based on their electrical conductivity, materials are classified into conductors, insulators, and semiconductors. A semiconductor is a material with unique properties whose conductivity lies between that of a conductor and an insulator; hence the name “semiconductor.” Common semiconductor materials include silicon (Si) and germanium (Ge).

The diode is one of the oldest members of the semiconductor family, and its most distinctive characteristic is its unidirectional conductivity: current can flow in only one direction, from the positive terminal to the negative terminal.

I. Basic Knowledge

1. Classification of Diodes

There are many types of diodes. Based on the semiconductor material used, they can be classified as germanium diodes (Ge diodes) and silicon diodes (Si diodes). According to the chip structure, they can further be divided into point-contact diodes, surface-contact diodes, and planar diodes.

Depending on their intended applications, diodes can be classified into detector diodes, rectifier diodes, Zener diodes, switching diodes, Schottky diodes, light-emitting diodes, and others.

2. Diode Naming Conventions

(1) According to the domestic semiconductor device naming convention, a diode’s model designation consists of five parts: the primary designation, material and polarity, category, serial number, and specification code (indicating the product’s grade within the same category).

3. Characteristics of Several Common Diodes

(1) Rectifier diode

A diode that rectifies alternating current into direct current is called a rectifier diode; due to its large junction capacitance, it operates at low frequencies. Typically, diodes with a forward current (IF) of 1 ampere or greater are housed in metal packages to facilitate heat dissipation, while those with IF below 1 ampere are packaged entirely in plastic.

(2) Switching Diode

In pulse digital circuits, diodes used to switch circuits on and off are called switching diodes. They are characterized by short reverse recovery times, making them well suited for high‑frequency and ultra‑high‑frequency applications. Switching diodes come in several types, including contact‑type, planar‑type, and mesa‑type. For silicon switching diodes with a forward current rating of less than 500 mA, fully sealed epoxy or ceramic chip packages are commonly employed.

(3) Zener diode

A Zener diode is a surface‑junction crystal diode made of silicon. It is called a Zener diode because it can provide voltage regulation in circuits. It achieves this by exploiting the characteristic that, during reverse breakdown, the voltage across the PN junction remains essentially constant despite changes in current.

(4) Varactor Diode

Varactor diodes are nonlinear capacitive components that exploit the voltage‑dependent capacitance of a PN junction. They are widely employed in microwave circuits such as parametric amplifiers, electronic tuners, and frequency multipliers. To optimize their performance for specific applications, varactor diodes typically rely on careful structural design and advanced fabrication techniques to enhance the nonlinearity of the capacitance–voltage relationship and to increase the Q factor.

(5) TVS diode

A TVS diode (Transient Voltage Suppressor) is connected in parallel with the circuit being protected. When a transient voltage exceeds the circuit’s normal operating voltage, the diode undergoes avalanche breakdown, providing a low‑impedance path for the transient current and thereby preventing the internal circuitry from being damaged by overvoltage breakdown or overheating due to excessive current. Thanks to its relatively large junction area, the TVS diode can handle substantial transient currents, offering highly effective protection.

II. Selection of Diodes

1. Select according to the main parameters

(1) Rated Forward Operating Current

The rated forward operating current is the maximum forward current that a diode is permitted to carry under continuous operation over the long term.

(2) Maximum Surge Current

The maximum surge current is the excess forward current that is permitted to flow; it is not a normal operating current but rather an instantaneous current. Its value is typically around 20 times the rated forward operating current.

(3) Maximum Reverse Operating Voltage

When the reverse operating voltage applied across a diode exceeds a certain threshold, the device will undergo breakdown and lose its unidirectional conductivity. To ensure safe operation, a maximum reverse operating voltage is specified. For example, the 1N4001 diode has a reverse breakdown voltage of 50 V, while the 1N4007 has a reverse breakdown voltage of 1000 V.

(4) Reverse Current

Reverse current refers to the current that flows through a diode when it is subjected to a specified temperature and the maximum reverse voltage. The smaller the reverse current, the better the diode’s unidirectional conduction characteristics. Reverse current is closely related to temperature: for every 10°C increase in temperature, the reverse current approximately doubles. Silicon diodes exhibit superior thermal stability at elevated temperatures compared to germanium diodes.

(5) Reverse Recovery Time

When the voltage switches from forward to reverse, the current typically cannot turn off instantaneously; a slight delay occurs. This delay is known as the reverse recovery time, which directly affects the diode’s switching speed.

(6) Maximum Power

Maximum power is the product of the voltage across the diode and the current flowing through it. This limiting parameter is particularly relevant for Zener diodes and similar devices.

(7) Frequency Characteristics

Due to the presence of junction capacitance, when the frequency reaches a certain level, the capacitive reactance becomes so small that it effectively shorts the PN junction, causing the diode to lose its unidirectional conductivity and cease functioning. The larger the area of the PN junction, the greater its junction capacitance, and the more severely this limits its ability to operate at high frequencies.

2. Selection of Different Diodes

(1) Demodulation diode

For demodulation, point-contact germanium diodes are typically used. When selecting a diode, choose one whose operating characteristics—high frequency response, low reverse current, and sufficiently high forward current—meet the specific requirements of the circuit.

(2) Rectifier diode

Rectifier diodes are typically planar silicon devices, employed in a variety of power‑supply rectification circuits. When selecting a diode, the primary considerations include its maximum forward current, maximum reverse operating current, cutoff frequency, and reverse recovery time. In conventional series‑regulated power supply circuits, the requirements for cutoff frequency and reverse recovery time are relatively modest; it suffices to choose a rectifier diode whose maximum forward and reverse currents meet the circuit’s specifications.

( 3) Zener diode

Zener diodes are typically used in regulated power supplies as reference voltage sources or in overvoltage protection circuits as protective diodes. When selecting a Zener diode, it is essential to meet the key parameter requirements of the application circuit. The Zener diode’s stabilization voltage should match the reference voltage of the circuit, and its maximum stabilization current should be approximately 50% higher than the circuit’s maximum load current.

(4) Switching Diode

Switching diodes are primarily used in household appliances and electronic devices such as tape recorders, televisions, and DVD players, where they serve in switching circuits, detection circuits, and high-frequency pulse rectification circuits.

For medium-speed switching circuits and detection, standard switching diodes from the 2AK series can be used. For high-speed switching circuits, high-speed switching diodes from the PLS, 1SS, 1N, and 2CK series are suitable.

The specific model of the switching diode should be selected based on the key parameters of the application circuit, such as forward current, maximum reverse voltage, and reverse recovery time.

(5) Varactor Diode

When selecting a varactor diode, it is essential to ensure that its operating frequency, maximum reverse‑bias voltage, maximum forward current, and zero‑bias junction capacitance meet the requirements of the application circuit. Choose a varactor diode with a large junction capacitance variation, a high Q factor, and low reverse leakage current.