Types of diodes and their applications

There are different types of diodes used in electronic circuits. The following are the most common ones.

Semiconductor diode

One type of diode is the p-n junction diode as described in the nrevious section. This is also referred to as a semiconductor diode. Most semiconductor diodes are made up of silicon or germanium. Semiconductor diodes are used for very switching and microwave applications. Figure 5.20 shows a picture of a semiconductor diode and its symbol.

Figure 5.20: Germanium diode

Light Emitting Diode

A Light-Emitting Diode (LED) is a semiconductor diode that emits light when an electric current is applied in the forward direction of the diode. LEDs are made from a variety of semiconductor materials depending on the wavelength of the light required. The most commonly used materials for visible LEDs are gallium phosphide and gallium arsenic phosphide. Figure 5.21 shows LEDs and their curcuit symbol.

Figure 5. 21: Light Emitting Diodes and their circuit symbol

LEDs have a wide range of applications; these include mobile phones, large advertising billboards, simple indicator lamps, large display screens, optical fibre communication links and as standard light sources in electrical equipment Unlike traditional light sources, which convert only some of the electrical energy into light, the rest being wasted as heat. LEDs convert most of the electrical energy into light, resulting in efficient light production and hence minimal electric power consumption.

Zener diode

A Zener diode, shown in Figure 5.22 with its circuit symbol, is a semiconductor diode designed to be operated in the reverse breakdown voltage, called the Zener voltage. Every Zener diode is manufactured for specific reverse breakdown voltage.

Figure 5.22: Zener diode audits circuit symbol

Zener diodes are among the main building blocks of electronic circuits and are widely used as voltage regulators in different electrical circuits.

Laser Diode

A laser diode is similar to LED because it converts electrical energy into light energy. Laser diodes, however, produce coherent light, as opposed to LEDs. This coherent light is generated using a process termed as “Light Amplification by Stimulated Emission of Radiation”, abbreviated as LASER. Laser diodes are used in optical communication, laser pointer, CD drives, barcode readers and laser printer. Figure 5.23 shows a laser diode and the respective circuit symbol.

Figure 5.23: Laser diode and its circuit symbol

Avalanche Diode

An Avalanche diode is a p-n junction diode that operates in the avalanche breakdown zone. Avalanche breakdown occurs when enough reverse voltage is given to the p-n junction. As a result, the minority carrier ionizes and initiates a strong current flow in the reverse direction. Avalanche diode works electrically similar to the Zener diode. However, a Zener diode has a higher doping concentration than an Avalanche diode. Figure 5.24 shows Avalanche diodes and the circuit symbol.

Figure 5.24: Awlanche diodes and their circuit symbol

Photodiode

A photodiode is a semiconductor device with a p-n junction that converts photons of light into electrical current. It is a type of light sensor that transforms light into electrical energy. Photodiodes are used in many applications such as safety electronics like fire and smoke detectors, cameras and photosensors. They are also widely used in a variety of medical applications, including sample analysis equipment, computed tomography detectors, and blood gas monitors. Figure 5.25 shows a photodiode and its circuit symbol. The photodiode symbol is similar to that of a LED, except the arrows in the photodiode point inwards rather than outwards as in the LED. It should be noted that a photodiode operates under reverse bias mode.

Schottky diode

The Schottky diode is a metal-semiconductor junction diode that has smaller forward voltage drop than a semiconductor p-n junction diode. In contrast to a p-n junction diode, a Schottky diode is formed by replacing a p-type semiconductor material with a metal like gold or platinum. When a metal is joined with n-type semiconductor, a junction is formed between the metal and n-type semiconductor, forming a metal­semiconductor junction diode called Schottky diode. The n-type semiconductor material acts as cathode terminal and the metal acts as the anode as illustrated in Figure 5.26. Schottky diodes provide small forward voltage drop and a very fast switching action. It is good to note that a Schottky diode conducts current in the forward direction when suffient forward voltage is applied. Schottky diodes are used as rectifiers and switching regulators.

Figure 5.26: Schottky diode and its circuit symbol

The I-V charactensuc curve of a Schottky diode is almost similar to that of a p-n junction diode except the knee voltage. A Schottky diode starts to conduct at much lower voltage compared to a p-n junction diode as illustrated in Figure 5.27.

Figure 5.27: I-V characteristics curve of Schottky diode

Tunnel diode

A tunnel diode is a heavily doped p-n junction diode in which the electric current that flows through it decreases with an increase in voltage. In this diode, electric current is caused by tunneling. The tunnel diode is used as a very fast switching device computers and in high frequency oscillators an amplifiers. A tunnel diode and its circuit symbol is shown in Figure 5.28.

Figure 5.28: Tunnel diode and its circuit symbol