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A diode is an electronic device that conducts current in only one direction under specific voltage. It is a two-terminal device, which depicts the positive and negative terminal.

An ideal diode has zero resistance in a particular direction. We can also say that it has high resistance in the other direction. Here, infinite resistance means zero current flow through the circuit. Hence, we can say that an ideal diode conducts current in one only direction due to infinite resistance or zero current in other direction. In the case of real diodes, infinite resistance does not exist. We can say it as high resistance.

Here, we will discuss the effect of forward and reverse bias on the diode and various types of the diode.

But, from the diodes list, the Zener diode is the only diode that can conduct current in both forward and backward directions. We will discuss Zener diodes later in the topic.

Effect of Bias on the Diode

The bias signifies the forward and reverse bias in the diode. Let's discuss this in detail.

  • Forward bias
    The forward bias means the voltage that easily allows the current to flow across diode terminals. The depletion region in the case of forward bias is narrow. It easily allows the flow of electrons from p-side to n-side.
    The polarity of a diode is shown below:
    The above image clearly shows the polarity of a diode when it is forward biased. It means that the voltage source's positive terminal is connected to the anode, and the negative terminal is connected to the cathode.
    The current increases with the increase of voltage when the diode operates in forward bias. The current flow depends on the majority carriers.
  • Reverse bias
    The reverse means the voltage that allows the current to flow in the reverse direction. The depletion region in the case of reverse bias is wider.
    The polarity of a diode is shown below:
    The above image clearly shows the polarity of a diode when it is reverse biased. It means that the voltage source's positive terminal is connected to the cathode, and the negative terminal is connected to the anode. The current flow depends on the minority carriers.
    The diode in the case of reverse bias generally behaves like an open switch.

Types of Diodes

There are various types of diodes with different uses.

The diodes are mainly categorized as vacuum diodes, signal diode, and semiconductor diodes. Let's discuss the types of a diode in detail. Here, we will also discuss their applications.

Vacuum diode

Vacuum diodes are considered the simplest production to control the free electrons. The two electrodes of the vacuum diode are cathode and anode. A filament in the diode heats the cathode element.

The uses of such diodes were reduced after the invention of semiconductor diodes. But, vacuum diodes are still used today in few high-power applications. It is due to the robustness and voltage tolerance ability over other semiconductor diodes.


The applications of vacuum diodes are listed below:

  • Amplifiers
  • CRT (Cathode Ray Tube)
  • Vacuum tubes

Signal diodes

The signal diodes are further categorized as:

  • Small signal diode
  • Large signal diode

Let's discuss a short description of the two types of the signal diode.

Small signal diode

The small-signal diodes come with low current and power ratings. It can perform better for the high-frequency applications. The pulse amplitude of such diodes ranges to few volts.

Large signal diode

The p-n junction of the large signal diode is large. But, it is not suitable for high-frequency applications like small-signal diodes.


The common applications of small signal diodes are listed below:

  • Waveform clamping
  • Waveform clipping

The common application of the large signal diode is as follows:

  • Inverters

Semiconductor diodes

Semiconductor diodes are the common type of diodes. It was the first to develop among semiconductor devices.

The different types of semiconductor diodes are listed below:

  • Junction diodes
  • Optoelectronic diode devices
  • Gunn Diode
  • Laser diodes
  • PIN diodes
  • Tunnel Diode
  • Impatt Diodes
  • Crystal Diodes

Junction diodes

Here, we will discuss the common types of junction diodes.

Junction diodes are characterized as:

  • P-N Junction Diodes
  • Varactor Diodes
  • Zener Diodes
  • Switching Diodes
  • Schottky Diodes

Let's discuss the above type of junction diodes in detail.

P-N Junction Diodes

The combination of the p and n side results in the p-n junction diode. The electron flows from the n-side to the p-side contributing to the flow of current in the circuit.

The impurities are added on the p-side and n-side to increase the charge carriers' concentration in the diode. The n-side comprises of majority carriers electrons and positively charged ions known as donors. The p-side comprises of majority carriers holes and negatively charged acceptor ions known as immobile ions.

The p-n junction diode is shown below:


Here, Xp and Xn are the charged ions.

If one side of the p-n junction is highly doped compared to the other, the diode is known as the abrupt p-n junction diode. An abrupt p-n junction (either heavily doped on one side or both sides) offers dynamical resistance for a longer range of applied potential difference.


The applications of p-n junction diodes are listed below:

  • Rectification
  • Voltage Multipliers

Varactor Diodes

The varactor diode is a p-n junction diode used for varying capacitance across the junction. The depletion region of a p-n junction diode containing donors and acceptors ions plays a role like a variable capacitor under biasing.

The varactor diodes are used in tuning circuits. It also provides usable dc voltage to tune a circuit, for example, simple remote control or automatic tuning function device.


The application of Varactor diode is as follows:

  • Variable tuning capacitor

Zener Diodes

Zener diode is the heavily doped p-n junction diode. The p-n junction of the Zener diode is heavily doped to get a narrow depletion region. The narrow depletion region allows the ions to gain energy from the external source quickly. In the reverse bias, the junction potential of the Zener diode is increased. It results in an increased current through the Zener diode.

The symbol of the Zener diode is shown below:


The Zener diode's advantages include voltage protection, low cost, voltage shift, better performance, and operation at high voltages. Hence, the Zener diode is preferred for applications, such as waveform clippers, voltage regulators, etc.

The structure of the circuit comprising of the Zener diode is shown below:


The resistor in series is connected with the diode to limit the excess current that may damage the diode.


The applications of the Zener diode are listed below:

  • Voltage regulators
  • Clipping circuits
  • Surge suppressors

Switching Diodes

A p-n junction diode can be used as a switch between conducting and non-conducting state if the depletion region's space charge is controlled along with the carrier lifetime. The switching characteristics of a p-n junction diode should store a small charge. It should have a short carrier lifetime for the hole.

When the switching diode operators in the reverse bias conduction, little time is required to remove the neutral region's stored charge.


The application of switching diodes is as follows:

  • Switching small signal

Schottky Diodes

The working concepts of Schottky diodes are different from the ordinary p-n junction diodes. The difference lies in the rectification that occurs due to the difference in the work function values of metal and semiconductor.

The other differences between the Schottky diodes and ordinary p-n junction diodes are non-uniform doping and conduction controlled by majority carriers' saturated emissions. It means that the recombination of minority carriers cannot govern the switching speed of the Schottky diode. Thus, the switching rate is a majority carrier phenomenon.

The common advantages of the Schottky diode include less noise, better performance, and fast recovery time.


The applications of Schottky diodes are listed below:

  • Rectifiers

Optoelectronic diode devices

The optoelectronic devices are the devices based on the interaction between photon and materials. There are three possible ways in which a photon can interact with the material and undergo absorption, spontaneous emissions and stimulated emissions.

The energy of the incident photon in such diodes should be greater than the bandgap energy to excite an electron from the valence and to the conduction band.

The optoelectronic diode devices are characterized as:

  • P-n junction Photodiode
  • P-i-n junction Photodiode
  • Avalanche Photodiodes
  • Light Emitting Diodes

Let's discuss the above types of diodes in detail.

P-n junction Photodiode

A p-n junction photodiode is an ordinary photodiode. The reverse saturation current in the photodiode flows due to the minority carriers in the reverse bias. The depletion width in the case of reverse bias is large. A photon with energy higher than the bandgap energy is absorbed in the material. It transfers some of its energy to other electrons present in the upper part of the valence band. Such electrons that receive energy are excited to the conduction band. Hence, an electron-hole pair is generated in the depletion region. The applied depletion field separates these electron-hole pairs to execute the reverse current.

Hence, the magnitude of the produced photocurrent depends on the responsivity and quantum efficiency.

The circuit of the p-n junction photodiode is shown below:



The applications of p-n junction photodiode are listed below:

  • Smoke detectors
  • Medical devices

P-i-n junction Photodiode

The lightly doped layer of intrinsic material is sandwiched between the heavily doped p and n layers. The combination of the three layers results in the increase of depletion region width. The enlarged depletion width is utilized for the incidence of the photons. The photo carriers generated in the depletion width are collected much faster by the net bias voltage across the region. The collection process is much faster than the diffusion process.

The circuit of the p-i-n junction photodiode is shown below:


The excitation of the electrons results in the creation of electron-hole pair. The high electric field separates the generated electron-hole pair in the depletion region due to reverse bias. It further gives rise to the flow of current known as photocurrent.


The applications of the p-i-n junction photodiode are listed below:

  • RF switches
  • Microwave switches
  • Photovoltaic cell

Avalanche Photodiodes

The avalanche photodiode is used in the detection of low-intensity optical signals. It is due to the impact ionization process, which is also responsible for avalanche breakdown. The circuit of avalanche photodiode is shown below:


It comprises the lightly doped p-slab and the combination of intrinsic semiconductor layers sandwiched between the heavily doped p and n layers. Such combination in an avalanche diode provides extra length to the depletion region of the diode. It further results in an increased number of atoms and acceptors. A large number of charge carriers is present at the boundary of the depletion region.


The applications of avalanche photodiodes are listed below:

  • Relief valves (used to control the pressure of the system).

Light Emitting Diodes

The LED or Light Emitting Diodes are a p-n junction diode with an energy gap in the range required for visible light production. The forward voltage applied to the p-n junction diode allows the majority carriers to move quickly between the neutral regions. The electromagnetic radiation produced in the diodes is like a particle occupying a high energy state to low energy. It is due to the recombination of a majority carrier electron with the majority carrier hole.

The LED symbol is shown below:


The structure of Light Emitting diode is shown below:



Some of the common applications of the Light Emitting Diodes are listed below:

  • Automotive headlights
  • Traffic signals
  • Flashlights
  • Camera flash

Gunn Diode

The Gunn diode is an n-type GaAs (Gallium Arsenic) diode. It works with different conduction band energy levels. The transfer of electrons occurs from the lower conduction band energy level to the higher conduction band energy level.

The transferred electron at the conduction band's upper energy level is free than the electron at the lower energy level.


The applications of Gunn diode are listed below:

  • Radar speed guns
  • Automatic door openers

Laser diodes

An optical cavity or a resonant cavity comprising of the LED with parallel polishes faces is known as lasers. Such diodes are generally preferred for barcode readers, optical communications, laser scanning, etc. It is also known as an injection laser diode. The laser diodes are responsible for converting the electrical energy in the form of light.


Some of the common applications of the laser diodes are listed below:

  • Laser printing
  • Laser scanning
  • Barcode readers
  • Disc recording

Impatt diodes

IMPATT or Impact Ionization Avalanche Transit Time Diode is the n-p-i-p junction diode. The structure of the IMPATT diode is shown below:


The two corner p and n layers of the diode are heavily doped. It signifies the intrinsic layer in the diode. The mechanism of the devices is based on two regions:

  1. Lightly doped p-region
  2. I-region

The lightly doped p-region is present at the boundary where avalanche multiplication takes place. Such a region is also called an avalanche region or high field region. The i-region is the intrinsic region, which is also known as the drift region. The depletion region obtained between the n-p and i-p layer is called a space-charge region.


The applications of Impatt diode are listed below:

  • Amplification with negative resistance
  • Microwave oscillators

PIN diodes

The PIN diode signifies a diode with a combination of a semiconductor layer of p-type, intrinsic, and n-type. The intrinsic semiconductor layer between the p and n-type is the undoped (no doping present) layer. In comparison, the other two semiconductor layers are highly doped (p-type and n-type).

The reason for the two reasons being heavily doped is for the ohmic contacts, which are considered a non-rectifying electrical junction.


The applications of the PIN diodes are listed below:

  • RF switches
  • Photodetectors

Note: The PIN diodes are different from the p-i-n photodiodes. The role of PIN diodes is limited. While the functions of p-i-n photodiodes are low sensitivity, high-intensity electric field region, etc.
P-i-n diode is a normal junction diode, while PIN photodiode is a type of photodiode.

Tunnel Diode

The negative resistance region of the tunnel diode was found during the measurements of I-V (Current-Voltage) characteristics.

The tunnel diode can operate in the negative resistance region. Hence, it can be used as an amplifier or an oscillator in a wide range of frequencies and applications. The tunneling action of the diodes occurs very fast, which results in no transit time effect. Due to this, no discontinuity is observed in signals. Another advantage of fast tunneling action is the use of diode in high-frequency applications.

The other advantages of tunnel diode are low cost, high speed, and low power. The charge leakage and low output voltage fluctuations are also their disadvantages.


The applications of tunnel diode are listed below:

  • Logic memory storage device
  • Oscillator
  • Amplifier

Four-layer diode

The four-layer diode is called as P-N-P-N Diode.

P-N-P-N Diode

The name of the diode depicts that it is a four-layer diode. The structure of the P-N-P-N diode is shown below:


The diode's p-side is in contact with the anode terminal, and the n-side of the diode is in contact with the cathode terminal. There are three depletion regions between the four layers of the diode. When the diode is in reverse bias, junctions J1 and J3 are reverse biased, while junction J2 is forward biased. Hence, we can say that junction J2 has no contribution until an avalanche breakdown occurs.

Similarly, in the case of forward bias, junction J1 and J3 are forward biased, and junction J2 is reverse biased.


The applications of the P-N-P-N diode are listed below:

  • SCS (Silicon Control Switches)
  • SCR (Silicon Control Rectifier)

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