A Beginner’s Guide to Diode (Definition, symbol & working)

A diode is an electronic device that allows current to flow easily in one direction but restricts current flow in the opposite direction. Semiconductor diodes are widely used in modern electronic circuits for rectification, voltage regulation, signal modulation, and more. This blog post is all about diode- definition, its working, types, and applications.

Diodes are also known as Rectifiers. The term diode is reserved for small signal devices which consume a maximum current of 1 A, while the term rectifier is used for power devices.

Diode in a Rectifier circuit
Diodes in a Rectifier circuit

Given above is a rectifier circuit. A rectifier circuit converts the AC voltage to stable DC voltage with the help of diodes and filters. Diodes used in this circuit convert the input AC waveform into fluctuating DC waveform. This fluctuating DC waveform is then made stable using a capacitor as a filter.

What’s the symbol of a Diode?

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The standard symbol for diodes is given below. Two terminals of the diode are the anode and cathode. The arrowhead represents the flow of current during the forward bias.

What is a diode? Symbol
Diode symbol

How does a Diode work in a circuit?

A diode can easily allow or block the flow of electric current based on the way it is connected in a circuit. Consider two cases that are discussed below with the help of a figure.

Case 1: When the anode is connected to the positive pole of the battery

A diode in forward-bias mode
A diode in forward-bias mode

In this orientation, the diode is forward biased. The diode offers little resistance to the electric current in this state, and hence the bulb glows which is connected to the circuit.

Case 2: When the anode is connected to the negative pole of the battery

A diode in reverse-bias mode
A diode in reverse-bias mode

In this orientation, the diode is reverse biased. In this state, the diode offers very high resistance to the flow of electric current. The diode does not conduct, due to which the bulb connected to the circuit remains OFF.

Note: A diode conducts when it is connected in a particular way (forward-biased) in a circuit. Hence diode is a type of unilateral device.

Water-flow analogy

The unilateral behavior of the diode can be understood with the water-flow analogy. Consider a gate attached with a spring mechanism as shown in the figure below.

Case 1: When water flow is from left to right

Gate opens and allow water flow
The gate opens and allows water to flow

When the water pressure exceeds a certain threshold, it pushes the gate, and the water flows easily. This condition is similar to the forward biasing of the diode.

Case 2: When water flow is from right to left

Gate remains closed and no water flows
Gate remains closed and no water flows

In this case, irrespective of the water pressure, the gate does not open. Thus, no water flows in this case. This condition is similar to the reverse biasing of the diode.

Led diodes in an LED Bulb
Led diodes in an LED Bulb

Given above is the PCB board of an LED bulb. Several LEDs are connected together in order to output high-intensity light. Please note that an LED works on DC voltage only but the LED bulb is AC. The circuit inside the bulb converts the input AC voltage to a suitable DC voltage for the LEDs.

Construction of diode

A PN Junction diode is a two-terminal semiconductor device whose one side is doped with P-type material (acceptor) and another with N-type material (donors) as shown in the figure below. Electrons and holes are the majority carriers in the N-type and P-type regions respectively.

Representation of PN junction diode
Representation of PN junction diode

The formation of the depletion region

These electrons in the N region and holes in the P region recombine due to concentration differences. This results in positive ions in the N region and negative ions in the P region. These ions are collected at the junction. And this region is known as the depletion region.

Bias is the external voltage applied to the PN Junction. When no voltage is applied to the diode then it is zero bias. Depending upon the voltage potential applied to the diode, it is operated in two modes; forward biased mode and reverse biased mode.

Diffusion Current

The flow of charge carriers across the junction area is known as diffusion. Hence the current at no bias condition is known as diffusion current.

Barrier Potential

The electric field formed in the depletion region acts as a barrier. External energy must be applied to get the electrons to move across the barrier of the electric field. The potential difference required to move the electrons through the electric field is called the barrier potential.

The barrier potential of a P-N junction depends on the type of semiconductor material, amount of doping, and temperature. This is approximately 0.7V for silicon and 0.3 V for germanium.

Working: Forward biased

It is the condition where the positive terminal of the battery is connected to the P-type material and the negative terminal of the battery is connected to the N-type material of the diode. The figure below shows a diode in the forward-biased mode.

Forward biasing of a diode
Forward biasing of a diode

The electric field created by the applied voltage forces the electrons in the n-side of the diode to move towards the p-side, while also forcing the holes in the p-side to move towards the n-side. This movement of electrons and holes creates a current through the diode. In this condition, the width of the depletion region decreases. 

Knee voltage

As the supply voltage increases above barrier potential (the knee voltage), the majority of charge carriers flow through the circuit and it makes the diode conducting.

Working: Reverse biased

In reverse biased conditions, the positive terminal of the battery is connected to the N region, and the negative terminal of the battery is connected to the P region. A diode in reverse bias mode is shown in the figure below.

Reverse biasing of a diode
Reverse biasing of a diode

In this case, the negative terminal of the battery attracts the holes in the p-side away from the edge of the junction barrier. On the other hand, the positive terminal attracts the electrons in the n-side away from the edge of the barrier. This prevents current from flowing through the diode, as there are no carriers available to carry it, and hence the width of the depletion layer increases.

Leakage current and breakdown voltage

Usually, a small current flows when a diode is reverse-biased, known as the leakage current. This is due to the movement of minority charge carriers across the junction. However, at normal operating temperatures, this small current may be neglected.

Read also: What is a Potentiometer? 15 Things You Must Know

Graphical representation

The PN junction diode offers very little resistance to current flow in the forward-bias direction but maximum resistance to current flow when reverse biased. A good way of illustrating this point is by plotting a graph of the applied voltage versus the measured current. The figure below shows a plot of this voltage-current relationship (characteristic curve) for a typical PN junction diode.

Diode VI characteristics
Diode VI characteristics

Different types of diode

Different types of diodes are available in the market, which come in handy in various applications. Some of them are listed below.

Light Emitting Diode(LED)

When an electric current passes through this diode, light is emitted. This happens when the electrons in the semiconductor recombine with holes, due to which energy is released in the form of photons. The emitted light color depends on the energy gap of the semiconductor used.

LED (Light Emitting Diode)
LED (Light Emitting Diode)

LEDs are used for indicators( such as power on/off lights) on electronic devices. Other applications of the LEDs are in electronic signs, clock displays, and flashlights. The LEDs consume less energy than the old bulb, CFL, so it is largely used for lighting purposes.

Schottky Diode

The name Schottky comes from the German physicist Walter H. Schottky. The Schottky diode is also known as the Schottky barrier diode or hot carrier diode. It has a lower forward voltage than other silicon PN junction diodes.

Schottky diode
Schottky diode

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 differently to obtain that performance. Schottky diodes are highly used in rectifier applications.

Shockley Diode

It is a four-layer semiconductor diode known as a PNPN diode. The Shockley diode is named after its inventor William Shockley. This type of diode is made with a set of interconnected bipolar transistors.

Shockley diode
Shockley diode

The last NP part of the PNP transistor is connected with the NP part of the NPN transistor. The Shockley diode is used as a trigger switch for silicon control rectifier, relaxation oscillator(sawtooth oscillator), and also it is used as an audio amplifier.

Laser Diode

Laser diode
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 as compared to LEDs but cheaper as compared to other laser generators. Limited life is the only drawback of these diodes. Read more

Avalanche Diode

Avalanche diode
Avalanche diode

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 avalanche breakdown takes place. They exhibit high levels of sensitivity and hence are used for photo detection.

Zener Diode

Zener diode
Zener diode

It is the most helpful 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 the 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.

Photo Diode

Photodiode
Photodiode

A photo-diode is a light-sensitive diode. It starts conducting, as soon as a beam of light is incident on it. This is a reverse bias diode and is used in solar cells and photometers. They are even used to generate electricity.

Varactor Diode

Varactor diode
Varactor diode

Varactor is a type of PN junction diode that has a capacitance that varies with varying the reverse-biased voltage. It operates only in reverse-biased voltage and acts as a variable capacitor. The varactor diode is also known as varicap diode, tuning diode, and variable capacitance diode. The varactor diode is used in frequency multipliers and also it is used in parametric amplifiers and voltage-controlled oscillators.

Similar: 10 Types of Potentiometer: How to choose & Applications

Tunnel Diode

Tunnel diode
Tunnel diode

A tunnel diode is a heavily doped PN junction diode that has negative resistance. For this negative resistance the current decreases with increasing the voltage across the diode. The tunneling diode was invented by Leo Esaki when he was working in quantum mechanics about the electron tunneling effect. The diode is used as a fast switching device in computers and as a logic storage memory device. It is used in high-frequency oscillators, amplifiers, and FM receivers.

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