What is a diode?

An electronic component that facilitates the directional flow of current is known as a diode. According to the direction of the flow of current, the resistance of a diode changes from zero to Infinity.

A diode is a semiconductor device that is mainly used for its current-voltage characteristic property. It has two terminals that are connected to the positive and negative terminals of the battery. The flow of current can be classified into a forward direction and reverse direction. When current flows in a direction that is aligned in the forward direction of the diode, then the diode offers a minimum resistance path. The maximum or infinite resistance path is offered when the current flows in the reverse direction.

PN junction diode

It is a widely used electronic component in practical electronics. PN junction diode is manufactured from two types of semiconductors. Silicon and germanium are the semiconductors used for the manufacturing process. These semiconductors can be used in their pure format then it is known as intrinsic semiconductors. However, for practical purposes and better efficiency of the diode, doped semiconductors are used.

The p-type and n-type semiconductors are sandwiched together to create a p-n junction diode.

P-type semiconductor

A P-type semiconductor is an extrinsic semiconductor. It is doped with impurities that are electron deficient. Adding such impurities will lead to the production of holes which are denoted as positive carriers. Holes will become the majority carriers in P-type semiconductors. The impurity added in the P-type semiconductor is trivalent. For instance, if a pure block of silicon is doped with boron, then the semiconductor will become a P-type semiconductor.

Since the number of holes is much higher than the number of electrons in a P-type semiconductor, the magnitude of conduction current will be very small compared to the magnitude of valence current.

N-type semiconductor

When a pure block of semiconductor is doped with a pentavalent impurity, then the extrinsic semiconductor generated will be an n-type semiconductor. Pentavalent impurities are electron-rich. Consequently, the majority of carriers will be electrons.

In presence of an external voltage, the positive terminal will attract the negatively charged electrons. The conduction current will be high compared to the valance current.

Forward biased P-N junction

When the positive terminal is connected to the p-side and the negative terminal is connected to the N-region of the semiconductor, then the element is in forward bias. The holes in P-region are pushed to the negative terminal and the electrons in N-region are pushed to the positive terminal. In forward bias, the majority carriers of each type will be pushed to the junction where electron-hole pairs recombine to neutralize the depletion layer. This will reduce the thickness of the P-N junction depletion region and finally will lead to a condition where the effect of the depletion region on the flow of current will be non-existent. The minority current will be very low in magnitude and only the majority current will flow through the semiconductor. The electrons from the n-type will diffuse with holes in the p-type when there is a negligible effect of the depletion region. The value of the positive voltage at which the depletion region allows the flow of current is known as knee voltage. After knee voltage, there will be a rapid increase in the flow of current. This voltage is also known as threshold or cut-in voltage.

Reversed biased P-N junction

When the negative terminal is connected to the P-region and the positive terminal is connected to the N-region of the semiconductor, then the semiconductor is in reverse bias. In reverse bias, the cathode voltage will have a higher magnitude compared to that of the anode. In reverse voltage the diode will provide high resistance to the flow of current hence the current flowing through the diode will be very low. Because of this type of connection, the majority of carriers in p-type and n-type are pulled away from the junction. The holes will be attracted to the negative terminal and the electrons will be attracted to the positive terminal. This prevents recombination and diffusion of carriers.

Zener diode

Being operated in the reverse bias condition, Zener is a special application of the P-N junction diode. It is a heavily doped device that is created to be operated in reverse bias. It is also known as a breakdown diode. When connected to forward bias, the Zener diode operates as a normal P-N junction. In reverse bias, the reverse current flowing will be very small in magnitude till it reaches the breakdown voltage. After the Zener breakdown voltage, the diode will break down and a large amount of current will flow through it. The property of the Zener effect is that when the breakdown voltage is achieved then the reverse current will remain constant regardless of the increase made in the reverse bias voltage. The range of Zener voltage is around 5 to 6 volt.

Zener diode is mainly used as a voltage regulator. Other applications of Zener diode are in over-voltage protection and clipping circuits.

Light emitting diode

Commonly known as LED light, it is a commercially popular diode. When current flows through the diode, then light is emitted. The color emitted by the semiconductor depends upon the bandgap and the type of semiconductor. A layer of light-emitting phosphor is deposited on the semiconductor device to create a glow. Red color LED was first developed and was shortly followed by other colors. The white LED is a combination of different layers of semiconductor material.

Application of these diodes ranges from indicators, remotes to decorative lighting.

Photo diode

When light falling on a diode is converted into electric current, then such a diode is known as a photodiode. It is a P-N junction diode that can absorb photons and generate electricity. A transparent window package is used to encapsulate the diode so that light can fall on it. The connection of photodiode is made in reverse bias.

Applications of photodiode range from photomultiplier tubes, charge-coupled devices to two consumer devices such as compact disc players and medical devices.

Solar cell

It is a P-N junction diode that converts sunlight into electricity. The photovoltaic effect is used to convert the photos received from the sun to electricity. It is manufactured using crystalline silicon grids. Solar cells can be connected to create a solar panel. Solar panels are the source of renewable clean energy. It holds a lot of scope for futuristic developments that can lead to a sustainable environment. It is the major source of power used by satellites and space telescopes.

Laser diode

It is a modified version of a light-emitting diode that emits laser beams. It generates lasing condition.

Avalanche diode

It is a reverse bias p-n junction diode that is manufactured to generate an avalanche effect. It is used to save the circuit from hotspots.

Schottky diode

It is a metal-semiconductor diode that is used faster for switching purposes.

Context and Application

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• Bachelors in Science (Physics)
• Masters in Science (Physics)
• Bachelors in Technology (Electronics)
• Masters in Technology (Electronics)

Practice Problems

1.Which of the following is used as an impurity in P-type semiconductors?

1. Boron
2. Germanium
3. Aluminum
4. Carbon

Answer- a

Explanation: Boron is a trivalent impurity used to create p-type semiconductors.

2. Which of the following is not used to dope n-type semiconductors?

1. Arsenic
2. Antimony
3. Phosphorus
4. Boron

Answer- d

Explanation: N-type semiconductor requires pentavalent impurity. Boron is a trivalent atom.

3. What is the bias condition when the cathode is connected to the positive terminal of the battery and the anode is connected to the negative terminal of the battery?

1. Forward bias
2. null bias
3. reverse bias
4. none of these

Answer- c

Explanation: In reverse bias, the cathode is connected to the positive terminal of the battery and the anode is connected to the negative terminal of the battery.

4. Which of the following is not operated in reverse bias?

1. Zener diode
2. Photodiode
3. None of these
4. Both a and b

Answer- c

Explanation: Both Zener diode and photodiode are manufactured to be operated in reverse bias.

5. Which of the following is a semiconductor?

1. Aluminum
2. Copper
3. Iron
4. Germanium

Answer- d

Explanation: Germanium is a semiconductor. It is used to manufacture diodes.

Common Mistakes

The most common mistakes made by students while working with diodes involve overheating them. Overheating diode will lead to irreversible destruction. Mistakes made in the calculation of the breakdown voltage and cut-off voltage will to improper use of diode.

Using the wrong diode for a particular application can alter the purpose of the circuit. This can lead to dangerous consequences and will be economically wasteful.

Related Concepts

• Rectifier
• Transistor
• Varistor
• Thyristor