What is a Diode?
A diode is a two-terminal electronic component that conducts electricity primarily in one direction. It has high resistance on one end and low resistance on the other end. These devices are used to protect circuits by limiting the voltage and to also transform AC into DC. Semiconductors like silicon and germanium are used to make the most of 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 its own applications.
Diode Construction and Symbol Explained
A standard diode is represented as shown in the diagram, a triangle adjoining the line. The terminal entering the flat edge of the triangle represents anode and the other end the cathode. The current always flows from anode to cathode, but never the other way round.
Diodes can be made of either of the two semiconductor materials, silicon and germanium. When the anode voltage is more positive than the cathode voltage, the diode is said to be forward-biased and it conducts readily with a relatively low-voltage drop. Likewise, when the cathode voltage is more positive than the anode, the diode is said to be reverse-biased. The arrow in the diode symbol represents the direction of conventional current flow when the diode conducts.
Types of Diodes
- Light Emitting Diode
- Laser diode
- Avalanche diode
- Zener diode
- Schottky diode
Light Emitting Diode (LED)
When electric current between the electrodes passes through this diode, light is produced. In other words, light is generated when a 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 colours. There are tri-colour LEDs which can emit three colours at a time. Light colour depends on the energy gap of the semiconductor used.
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.
This diode belongs to a reverse bias type and operates using the 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.
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.
It has a lower forward voltage than other silicon PN junction diodes. The drop will be seen where there is 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. Schottky diodes are highly used in rectifier applications.
A photo-diode 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 photometer. They are even used to generate electricity.
Following are the characteristics of the diode:
- Forward-biased diode
- Reverse-biased diode
- Zero biased diode
There is a small drop of voltage across the diode when the diode is forward-biased and current is conducting. For silicon diode, the forward voltage is 690mV and for germanium, 300mV is the forward voltage. The potential energy across the p-type material is positive and across the n-type material, the potential energy is negative.
A diode is said to be reverse-biased when the battery’s voltage is dropped completely. For silicon diode, the reverse voltage is -20μA and for germanium, -50μA is the reverse voltage. The potential energy across the p-type material is negative and across the n-type material, the potential energy is positive.
When the diode is zero-biased, the voltage potential across the diode is zero.
Following are the applications and uses of the diode:
- Diodes as a rectifier
- Diodes in the clipping circuit
- Diodes in clamping circuits
- Diodes in logical gates
- Diodes in reverse current protection