What are diodes?

A diode is a device which has two active electrodes and two terminals and allows the current to pass only in single direction. These devices are used to protect circuits by limiting the voltage and also turn AC into DC. Semiconductors like silicon and germanium are used to make most of the diodes. Even though they transmit current in a single direction, the way with which they transmit differs. There are different kinds of diodes and each type has their own applications.


Types of Diodes

Light Emitting Diode (LED)

When electric current between the electrodes passes through this diode, light is produced. In other words, light is generated when sufficient amount of forwarding current passes through it. In many diodes, this light generated is not visible as they are frequency levels that do not allow visibility. LEDs are available in different colors. There are tri-color LEDs which can emit three colors at a time. Light color depends on the energy gap of the semiconductor used.

Laser Diode

It is a different type of diode as it produces coherent light. It is highly used in CD drives, DVDs and laser devices. These are costly when compared to LEDs and are cheaper when compared to other laser generators. Limited life is the only drawback of these diodes.

Avalanche Diode

This diode belongs to a reverse bias type and operates using avalanche effect. When voltage drop is constant and is independent of current, the breakdown of avalanche takes place. They exhibit high levels of sensitivity and hence used for photodetection.

Zener Diode

It is the most useful type of diode as it can provide a stable reference voltage. These are operated in reverse bias and break down on the arrival of a certain voltage. If current passing through the resistor is limited, a stable voltage is generated. Zener diodes are widely used in power supplies to provide a reference voltage.

Schottky Diode

It has a lower forward voltage than other silicon PN junction diodes. The drop will be seen where there is a low current and at that stage, voltage ranges between 0.15 and 0.4 volts. These are constructed in a different way in order to obtain that performance. These are highly used in rectifier applications.


A photodiode can identify even a small amount of current flow resulted from the light. These are very helpful in the detection of the light. This is a reverse bias diode and used in solar cells and photometry. They are even used to generate electricity.

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Practise This Question

An electric bell has a rating of 500 W 100 V. It is used in a circuit having a 200 V supply. What resistance must be connected in series with the bulb so that it delivers 500 W?