Functions and Classification of Semiconductor Diodes

Diodes play an important role in electronic circuits. To understand the circuits, we must first learn the function of various electronic components in the circuit.

I Classification According to the Structures

Semiconductor diodes mainly rely on PN junctions to work. And according to the characteristics of the surface structure of PN junctions, the semiconductor diodes are divided into the following types

1. Point-contact Diode

The point-contact diode is formed by pressing a metal pin on a single chip of germanium or silicon material and then performing the current method. Therefore, the PN junction has small electrostatic capacitance suitable for high-frequency circuits. However, compared with the junction type, the point contact diode has poor forward and reverse characteristics, which is not suitable for large current and rectification. And as its construction is simple, the price is cheap. Therefore, it is widely used for general purposes such as detection, rectification, modulation, mixing and slicing of small signals.

2. Bond-type Diode

The bond-type diode is formed by fusing a silver filaments on one chip of germanium or silicon. Compared with the point-contact type, although the capacitance of the PN junction in bond-type diode is slightly increased, it has excellent forward characteristic. So it’s usually used for switching and sometimes used for detection and power rectification (no more than 50mA). And bond-type diode fused with golden wires is referred to as golden-bond type diode, and that fused with the silver wire belongs to silver-bond type diode.

3. Alloyed Diode

A PN junction is formed by alloying a indium or aluminum metal on a germanium or silicon single chip of N-type. Its forward voltage drop is small, suitable for large current rectification. Since its PN junction has a large electrostatic capacity when it is reversed, it’s not suitable for high-frequency detection and rectification.

4. Diffused-type Diode

In a high-heat foreign gas of P-type, a N-type single chip of germanium or silicon is heated to make a portion of its surface become P-type, and the PN junction of diffused-type diode is obtained. Due to the small forward voltage drop of its PN junction, it is suitable for heavy current rectification. Recently, the mainstream of large current rectifiers has been transferred from a silicon-alloyed type to a silicon-diffused type.

5. Mesa-type Diode

   Its PN junction is made in the same manner as the diffused type, but its unnecessary portions are corroded. Thus the rest of the section is in the shape of a mesa. Initially, the mesa-type diode is made by diffusing the semiconductor materials, so it’s also called diffused mesa type diode. This type is mainly used for small current switches.

6. Planar Diode

On an N-type silicon single chip, a part of P-type foreign gas is diffused through the shielding effect of the oxide film on the surface, thus forming the PN junction of planar diode. Therefore, there is no need for corrosion to adjust the area of PN junction. Further, since the surface of the PN bond is flat and covered by the oxide film, it has good stability and long service life. It’s also called the epitaxial planar type diode at first for the semiconductor material is formed with epitaxial method. Planar diode is also primarily used for for small current switches.

7. Alloy-Diffused Diode

It is a type of alloyed diode. As alloy materials are easily diffused, if we subtly blend the impurities with the materials, then the mixture will diffused with the alloy to obtain an appropriate concentration distribution of the impurities in the formed PN junction. This method is suitable for manufacturing a high- sensitivity varactors.

8. Epitaxial-type Diode

It’s formed by making a PN junction through process of extent the length. It needs superior skills for manufacturing. Since the distribution of different concentrations of impurities can be controlled, it is also suitable for manufacturing a highly-sensitive varactor.

9. Schottky Diode

   Basic principle of Schottky diode is to block the reverse voltage with the formed Schottky on the contact surface of the metal (for example, lead) and the semiconductor (N-type silicon chip). Its pressure resistance is only about 40V. As the switching speed is very fast, and the reverse recovery time trr is particularly short. Therefore, a switching diode and a low-voltage high-current rectifier diode can be fabricated.

II Classification according to the Purpose

1. Detection Diode

In principle, the process of taking modulation signal from the input signal is a detection. In the detection diode, working frequency of germanium point-contact type diode is up to 400MHz, with small forward voltage drop, small junction capacity, high detection efficiency and good frequency characteristics. A diode for detection like point-contact type can be used for circuits such as amplitude limiting, clipping, modulation, mixing, and switching. There are also two diode assemblies with consistent characteristics specifically for frequency detection.

2. Rectifier Diode

In principle, the DC output taken from the input AC is rectification. Taking the magnitude of the rectified current (100 mA) as the boundary, the output current  greater than 100 mA is called rectification. It can subdivided into three types: silicon -semiconductor rectifier diode of 2CZ type, 2 silicon bridge rectifier of QL type, 2CLG-type rectifier diode used for high-voltage silicon stack for TV with working frequency of nearly 100KHz.

3. Amplitude-limiting Diode

Most diodes can be used for amplitude limiting. There are also dedicated  amplitude-limiting diodes like the high-frequency Zeners. In order to make these diodes have a particularly strong effect of limiting the sharp amplitude, they are usually made of silicon materials.There are also such components that a number of necessary rectifier diodes are connected in series to form a single unit according to the needs of limiting voltage.

4. Modulation Diode

It’s usually referred to as a dedicated diode for ring modulation, which is a combination of four diodes with consistent forward characteristics. They are usually directly used for frequency modulation.

5. Mixing Diode

When the mixing method of diode is used, Schottky type and point-contact type diodes are often used in the frequency range of 500 to 10,000 Hz.

6. Amplifying Diode

Usually, amplifying with diodes are indicating the amplification of the negative-resistance devices such as tunnel diodes and bulk-effect diodes, as well as the parametric amplification with varactors. Therefore, the diode for amplification generally refers to tunnel diode, bulk effect diode, and varactor diode.

7. Switching Diode

There is switching diode used for logic operations at low currents (such as 10 mA), and the switching diode used for magnetic core excitation at hundreds of milliamps. Low-current switching diode has point-contact type and bond type, and silicon-diffused type, mesa type and planar diode can work at high temperatures. Switching diode has fast switching speed. And schottky diode has an extremely short switching time, which makes it an ideal switching diode. Point-contact diode of 2AK type is often used for switching circuits at medium speed, while 2CK-type planar diode is used for high-speed switching circuits.

8. Varactor Diode

Small power diodes for automatic frequency control (AFC) and tuning are called varactor diodes. The electrostatic capacitance of its PN junction is changed by applying a reverse voltage. Therefore, it is widely used for automatic frequency control, scanning oscillation, frequency modulation, and tuning. Generally, a silicon diffused diode is used, but diodes of alloy diffused type, epitaxial type, or double diffused type can also be used because they have a particularly large changing rate of electrostatic capacitance in terms of the voltage.

9. Frequency Multiplication Diode

There is a frequency multiplication with the varactor and a frequency multiplication by the snap-off diode. The varactor diode for frequency multiplication is called a variable reactor. Although the variable reactor works the same as the varactor diode for automatic frequency control, its structure can withstand higher power. The snap-off diode is also called a step recovery diode, and its reverse recovery time from on to off is remarkably short. Therefore, if a sine wave is applied to the snap-off diode, the output waveform is suddenly pinched off, so that many high-frequency harmonics can be generated.

10. Zener Diode

   Zener Diode is usually made into diffused type or alloyed type of silicon, with a curve of reverse breakdown changing rapidly, used for control voltage and standard voltage. The terminal voltage (also known as Zener voltage) when the diode is working is from about 3V to 150V, and can be divided into many grades. In terms of power, there are also products ranging from 200mW to more than 100W. Generally 2CW-type zenzer diode works in the reverse breakdown state with a small dynamic resistance. And 2DW-type zener diode is the two complementary diodes reversely connected in series for reducing the temperature coefficient.