Diodes are electronic components functions as a one-way valve it means it allow current to flow in one direction. These diodes are manufactured by the semiconductor materials germanium, silicon and selenium. Operation of diode can be classified in two ways, if it allows the current then it is forward biased otherwise it is reverse biased.

For silicon diodes the forward voltage is 0.7v and for germanium it is 0.3v.

There are many types of diodes and some of these are listed as follows.

  1. Small signal Diode
  2. Large Signal Diode
  3. Zener Diode
  4. Light Emitting Diode (LED)
  5. Laser Diode
  6. Consatnt Current Diodes
  7. Schottky Diode
  8. Shockley Diode
  9. Step Recovery Diodes
  10. Tunnel Diode
  11. Varactor Diode
  12. Crystal Diode
  13. Avalanche Diode
  14. Silicon Controlled Rectifier
  15. Vacuum Diodes
  16. PIN Diode
  17. Gunn Diode

1. Small Signal Diode

It is a small device with disproportional characteristics and whose applications are mainly involved at high frequency and very low currents devices such as radios and televisions etc. To protect the diode from contamination it is enveloped with a glass so it is also named as Glass Passivated Diode which is extensively used as 1N4148.

The appearance of signal diode is very small when compared with the power diode. To indicate the cathode terminal one edge is marked with black or red in color. For the applications at high frequencies the performance of the small signal diode is very effective.


1 Small signal diodes are often used in high frequency applications or in switching and clipping applications which deal with short-duration pulse waveforms.
2 Small signal diodes are also found as a switch in rectifiers, snubbers, limiters, and wave-shaping circuits.
3 Yet another application for small signal diodes is to create a regulated voltage supply where diodes are connected together in series in order to provide a constant DC voltage across the diode combination.

2 Large Signal Diode

These diodes have large PN junction layer. Thus the transformation of AC to DC voltages is unbounded. This also increases the current forward capacity and reverse blocking voltage. These large signals will disrupt the functional point also. Due to this it is not suitable for high frequency applications.


  • The main applications of these diodes are in battery charging devices like inverters.
  • In these diodes the range of forward resistance is in Ohms and the reverse blocking resistance is in mega Ohms.

3. Zener Diode

It is a passive element works under the principle of zener breakdown. It also allows current in reverse direction when the applied voltage reaches the breakdown voltage. It is designed to prevent the other semiconductor devices from momentary voltage pulses. It acts as voltage regulator.

Zener Break Down

The Zener Breakdown is observed in the Zener diodes having Vz less than 5V or between 5 to 8 volts. When  a reverse voltage is applied  to a Zener diode, it causes a very intense electric field to appear across a narrow depletion region. Such an intense electric field is strong enough to pull some of the valence electrons into the conduction band by breaking their covalent bonds .these electrons then become free electrons which are available for conduction.  A large number of such free electrons will constitute a large reverse current through the Zener diode and breakdown is said to have occurred due to the Zener effect.

Avalanche breakdown

The avalanche breakdown is observed in the Zener Diodes having Vz having than 8 V. In the reverse biased condition, the conduction will take place only due to the minority carriers. As we increase the reverse voltage applied to the Zener diode, these minority carriers tend to accelerated. Therefore, the kinetic energy associated with them increases. While travelling, these accelerated minority carriers will collide with the stationary atoms and impart some of the kinetic energy to the valence electrons present in the covalent bonds.


  • Zener diodes are used in Voltage stabilizers (or) shunt regulators
  • Used in Surge suppression circuitry for device protection
  • Used in Over voltage protection circuits.
  • Zener diodes are used in clipping and clamping circuits especially peak clippers .
  • They are used as Reference elements.
  • Used in switching applications.

4 Light Emitting Diode (LED)

These diodes convert the electrical energy in to light energy. First production started in 1968. It undergoes electroluminescence process in which holes and electrons are recombined to produce energy in the form of light in forward bias condition.

Earlier they used in inductor lamps but now in recent applications they are using in environmental and task handling. Mostly used in applications like aviation lighting, traffic signals, camera flashes.


  • In motor vehicles and bicycle lights.
  • In traffic light Indicators, signs and signals.
  • In data displaying boards.
  • In medical applications and toys
  • Non visual applications.
  • In light bulbs and many more.

5 Laser Diode

Similar to LED in which active region is formed by p-n junction. Electrically laser diode is p-i-n diode in which the active region is in intrinsic region. Used in fiber optic communications, barcodereaders, laser pointers, CD/DVD/Blu-ray reading and recording, Laser printing.

Laser Diode Types

  1. Double Heterostructure Laser: Free electrons and holes available simultaneously in the region.
  2. Quantum Well Lasers: lasers having more than one quantum well are called multi quantum well lasers.
  3. Quantum Cascade Lasers: These are heterojunction lasers which enables laser action at relatively long wavelengths.
  4. Separate Confinement Heterostructure Lasers: To compensate the thin layer problem in quantum lasers we go for separate confinement heterostructure lasers.
  5. Distributed Bragg Reflector Lasers: It can be edge emitting lasers or VCSELS.


  • CD and DVD players
  • Barcode scanners
  • Cable and High Definition (HD) TV transmission
  • Medical applications including surgical instruments and to heal retina and brain.
  • Intrusion detection systems
  • Remote control applications

6 Constant Current Diodes

It is also known as current-regulating diode or constant current diode or current-limiting diode or diode-connected transistor. The function of the diode is regulating the voltage at a particular current.


  • It functions as a two terminal current limiter.

7 Schottky Diode

In this type of diode the junction is formed by contacting the semiconductor material with metal. Due to this the forward voltage drop is decreased to min. The semiconductor material is N-type silicon which acts as an anode and the metal acts as a cathode whose materials are chromium, platinum, tungsten etc.

Due to the metal junction these diodes have high current conducting capability thus the switching time reduces. So, Schottky has greater use in switching applications.



  • Schottky diodes are used as general-purpose rectifiers.
  • Schottky diodes are used in radio frequency (RF) applications.
  • Widely used in power supplies.
  • Used to detect signals.
  • Used in logic circuits.

8 Shockley Diode

It was the invention of first semiconductor devices it has four layers. It is also called as PNPN diode. It is equal to a thyristor without a gate terminal which means the gate terminal is disconnected. As there is no trigger inputs the only way the diode can conduct is by providing forward voltage.

It stays on one’s it turned “ON” and stays off one’s it turned “OFF”. The diode has two operating states conducting and non-conducting. In non-conducting state the diode conducts with less voltage.


  • Trigger switches for SCR.
  • Acts as relaxation oscillator.

9 Step Recovery Diodes

It is also called as snap-off diode or charge-storage diode. These are the special type of diodes which stores the charge from positive pulse and uses in the negative pulse of the sinusoidal signals. The rise time of the current pulse is equal to the snap time. Due to this phenomenon it has speed recovery pulses.


  • In higher order multipliers and
  • In pulse shaper circuits.

10 Tunnel Diode

It is used as high speed switch, of order nano-seconds. Due to tunneling effect it has very fast operation in microwave frequency region. It is a two terminal device in which concentration of dopants is too high.

The transient response is being limited by junction capacitance plus stray wiring capacitance. Mostly used in microwave oscillators and amplifiers. It acts as most negative conductance device. Tunnel diodes can be tuned in both mechanically and electrically.


  • Oscillatory circuits.
  • Microwave circuits.
  • Resistant to nuclear radiation.

11 Varactor Diode

These are also known as Varicap diodes. It acts like the variable capacitor. Operations are performed mainly at reverse bias state only. These diodes are very famous due to its capability of changing the capacitance ranges within the circuit in the presence of constant voltage flow.

They can able to vary capacitance up to high values. In varactor diode by changing the reverse bias voltage we can decrease or increase the depletion layer.


  • Voltage-controlled capacitors.
  • Voltage-controlled oscillators.
  • Parametric amplifiers.
  • Frequency multipliers.

12 Crystal Diode

This is also known as Cat’s whisker which is a type of point contact diode. Its operation depends on the pressure of contact between semiconductor crystal and point.

In this a metal wire is present which is pressed against the semiconductor crystal. In this the semiconductor crystal acts as cathode and metal wire acts as anode. These diodes are obsolete in nature.


  • Mainly used in microwave receivers and detectors.
  • Used in many applications like crystal radio receiver.

13 Avalanche Diode

This is passive element works under principle of avalanche breakdown. It works in reverse bias condition. It results large currents due to the ionisation produced by p-n junction during reverse bias condition.


  • The Avalanche diode is used to protect the circuit. When the reverse bias voltage begins to enhance, the diode purposely starts an avalanche effect at a fixed voltage.
  • Designers employ the diode more for protecting the circuit against unwanted voltages.
  • These diodes are used as white noise generators.

14 Silicon Controlled Rectifier

It consists of three terminals they are anode, cathode and a gate. It is nearly equal to the Shockley diode. As its name indicates it is mainly used for the control purpose when small voltages are applied in the circuit.


  • Power switching circuits (for both AC and DC)
  • Zero-voltage switching circuits
  • Over voltage protection circuits
  • Controlled Rectifiers
  • Inverters
  • AC Power Control (including lights, motors, etc.)
  • Pulse Circuits
  • Battery Charging Regulator
  • Latching Relays
  • Computer Logic Circuits
  • Remote Switching Units

15 Vacuum Diodes

Vacuum diodes consist of two electrodes which will acts as an anode and the cathode. Cathode is made up of tungsten which emits the electrons in the direction of anode. Always electron flow will be from cathode to anode only. So, it acts like a switch.

If the cathode is coated with oxide material then the electrons emission capability is high. Anode is a bit long in size and in some cases their surface is rough to reduce the temperatures developing in the diode. The diode will conduct in only one case that is when the anode is positive regarding to cathode terminal.


  • Atomic Clocks
  • Audio Systems
  • Car Dashboards
  • Cellular Telephone Satellites
  • Computer Monitors
  • DVD Players and Recorders
  • Electromagnetic Testing
  • Electron Microscopes
  • Guitar Amplifiers

16 PIN Diode

The improved version of the normal P-N junction diode gives the PIN diode. In PIN diode doping is not necessary. The intrinsic material means the material which has no charge carriers is inserted between the P and N regions which increase the area of depletion layer.

When we apply forward bias voltage the holes and electrons will pushed into the intrinsic layer. At some point due to this high injection level the electric field will conduct through the intrinsic material also. This field made the carriers to flow from two regions.

  • Used as a high voltage rectifier.
  • The PIN diode is used as an ideal radio frequency switch.
  • The PIN diode is used as a photo detector to convert the light into the current which takes place in the depletion layer of a photo diode

17 Gunn Diode

Gunn diode is fabricated with n-type semiconductor material only. The depletion region of two N-type materials is very thin. When voltage increases in the circuit the current also increases. After certain level of voltage the current will exponentially decrease thus this exhibits the negative differential resistance.

  • Used as Gunn oscillators to generate frequencies ranging from 100mW 5GHz to 1W 35GHz outputs.
  • Used as sensors for detecting trespassers, to avoid derailment of trains.
  • Used as efficient microwave generators with a frequency range of up to hundreds of GHz.
  • Used for remote vibration detectors and rotational speed measuring tachometers.


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