What is a diode?

Diodes are the two-terminal components used in many electronics and electrical applications. The current flow in such components is unidirectional. A diode provides very low resistance in one direction, but in the opposite direction, it provides very high resistance. One of the common functions of a diode is to allow the current flow in the forward direction and resist the current flow in the reverse direction. However, besides this simple functioning of diodes, diodes generally show non-linear current-voltage characteristics. For instance, the semiconductor diodes conduct current only when a certain threshold voltage, or preferably cut-off voltage is reached in the forward direction. They are known as forward-biased diodes. Also, upon reaching a certain voltage, known as the breakdown voltage, the high resistance provided by diodes in the reverse direction reaches a low resistance value.

A silicon diode.
CC BY-SA 2.5 | Image credits: https://commons.wikimedia.org | Morcheeba

The variation of the voltage drop across the diode's terminals with the change of current flow is negligible, it rather depends largely on temperature variations. This characteristic behavior makes the diode act as temperature sensors or voltage reference devices. Voltage reference devices are those devices that produce a constant voltage signal upon change in the input current, power supply variations, temperature changes, and so on. A diode can be engineered to serve various applications by tailoring the current-voltage characteristics of the diodes. This can be achieved by selecting suitable semiconductor materials and the nature of doping introduced during manufacturing. The diodes that are manufactured by this technique are known as special-purpose diodes. Zener diodes (diodes that are used to regulate voltage), avalanche diodes (diodes that are used to control voltage surges), diodes to emit light (light-emitting diodes or LEDs), diodes to generate radio frequency oscillations (tunnel diodes), rectifier diodes, power diodes, and so on.

Working of a diode

In this section, a brief review has been provided about the working of a diode. When a diode allows current flow, it is known as forward-bias, when it resists the current flow, it is known as a reverse-bias. Diodes in their basic form act as a P-N junction diode, which permits current flow due to applied forward potential. In their simple form, diodes are doped with trivalent and pentavalent impurities in their P-side and N-sides. This is done to ensure controlled quantities of donor and acceptor impurities. The P-type region is doped with a trivalent impurity, while the N-region is doped with a pentavalent impurity. Usually, germanium or silicon are commonly chosen materials. The below figure shows the structure of a P-N junction diode.

Architecture of P-N junction diode.
CC BY-SA 3.0 | Image credits: https://commons.wikimedia.org | Raffamaiden

In the above figure, it is seen that the P-type semiconductor material is merged with the N-type semiconductor material. A junction is formed in between the two regions known as the P-N junction. The P-region has a majority of holes and electrons as its minority, whereas the N-region has a majority of electrons and holes as its minority. The general working of a diode can be divided into three categories, these are discussed below:

Unbiased condition

It is a condition of the diode when no voltage or potential difference is applied across the terminals of the diode. Under such conditions, the holes from the P-type region and electrons from the N-type region get accumulated at the junction, which creates immobile ions at the diode's junction and forms a depletion layer. The width of the depletion region is fixed as the accumulation of the majority carriers creates an electric field at the junction, which resists further movement of majority carriers from the P-type and N-type junctions.

Forward-biased condition

In the forward biased state, the P-side of the diode is connected with the positive terminal of the power supply, while the N-side is connected with the negative terminal. When the source is turned on, the holes and electrons in the P-side region and N-side region respectively experience a repulsive force. The holes and electrons are pushed towards the junction side. But the holes and the electrons do not cross the junction because of barrier potential. Once this barrier potential exceeds, there is a net movement of the majority charge carriers across the P-N junction region. This net movement gives rise to a current flow known as the majority current. A reduction in the width of the depletion region, in this case, can be observed.

Reverse-biased condition

The reverse bias state is obtained when the P-side is connected to the negative terminal of the source and the N-side is connected to the positive terminal of the source. Due to this arrangement, the majority of charge carriers are pulled away from the junction and thus, this condition widens the depletion region. The diode acts as a non-conducting component in this condition. However, the minority of charge carriers are still inside the regions which get accumulated in the depletion region giving rise to small signals of current.

Diode as a half-wave and full-wave rectifier

One of the fundamental applications of a diode is that it can be used to convert an AC voltage into an equivalent DC voltage. The device which converts AC into DC voltage is known as a rectifier, and the diodes used in such devices are known as rectifier diodes. Based on the circuit and nature of rectification, a single or group of rectifier diodes are used. This can be achieved by using diodes as the main component.

Diode as a half-wave rectifier

In the half-wave rectifier, only one diode is used in the rectifier circuit. These diodes conduct during the half cycles of the AC signal. When an AC signal is passed through the diode, the diode acts as a forward bias during the positive AC cycle and allows the positive half cycle of the signal to pass. During the negative half cycle, the diode behaves as a reverse bias and blocks the AC signal. Thus, an output signal is obtained with only the positive parts. Additional filter circuits are attached to smoothen the output signal.

Diode as a full-wave rectifier

Full-wave rectifier circuits are those that convert both the halves of an AC circuit into a DC signal. This circuit makes use of multiple diodes for the purpose. The full-wave rectifier can be further classified into center-tapped full-wave rectifiers and full-wave bridge rectifiers based on the number of diodes and nature of resistive loads.

Types of diodes for different applications

Based on different applications, the different types of diodes are represented below:

Zener diode

Zener diodes are heavily dopped P-N junction diodes. These diodes are primarily used in the reversed bias condition. These diodes are used in voltage regulation, switching and clipping operations, meter protection, and so on.

Tunnel diode

These diodes are densely dopped and are highly conducting. They show negative resistance characteristics. It is used for many amplifier and oscillator applications due to its fast response.

PIN diode

The PIN diodes are majorly used for many microwave and radar applications. It has an intrinsic layer that is placed between the P-region and N-region of the diode. This provides high resistivity and allows the process of small signals.

Varactor diode

These diodes are made up of a combination of different capacitors. It is a reverse bias diode whose function solely depends upon the transition capacitance of the capacitors. These diodes are used for high-frequency applications.

Context and Applications

This topic is taught in many undergraduate and postgraduate degree courses like:

  • Bachelors of Technology (Electrical Engineering)
  • Bachelors of Technology (Electronics Engineering)
  • Bachelors of Technology (Electronics and Electronics Engineering)

Practice Problems

Q 1. Which of the following diode is used for voltage regulation?

  1. Zener diodes
  2. Power diodes
  3. Tunnel diodes
  4. Rectifier diodes

Answer: Option a

Explanation: The Zener diodes are used for voltage regulating applications.

Q 2. By tuning which of the following characteristics different diodes can be obtained to serve different applications?

  1. Current-voltage characteristics
  2. Voltage-power characteristics
  3. Current-dopping characteristics
  4. None of these

Answer: Option a

Explanation: By regulating the current-voltage characteristics different diodes can be obtained to serve different applications.

Q 3. Which of the following component is/are used in a varactor diode?

  1. Capacitor
  2. Resistor
  3. Both capacitor and resistor
  4. Resistor and transistor

Answer: Option a

Explanation: Only capacitors are used in the varactor diodes.

Q 4. What happens to the current flow when the diode acts as a reverse bias?

  1. The diode allows the flow of current.
  2. The diode does not allow the flow of current.
  3. The diode allows little flow of current.
  4. The diode gets heated.

Answer: Option b

Explanation: When the diode acts as a reverse bias, it does not allow the flow of current.

Q5. Which of the following is a densely dopped diode?

  1. Tunnel diode
  2. Zener diode
  3. PIN diode
  4. Power diode

Answer: Option a

Explanation: The tunnel diodes are densely dopped.

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