what is diode | working, types

Diode

A diode is a specialized electronic component with two electrodes/terminal called the anode and cathode. Most diodes are made up with semiconductor materials such as silicon, germanium, or selenium.

Diode Symbol

The anode terminal of the diode is called as P-type.

The cathode terminal of the diode is called as N-type.

A diode is an electrical device which allows current to easily pass through it in one direction and prohibit the current from other direction.

Biasing:

Method of applying necessary voltages across P-N junction, is known as biasing.

There are two methods of Biasing:

• Forward Biasing
• Reverse Biasing

Forward Biasing:

• When the diode allows current to easily pass through it this condition is called as forward biased.
• In forward biased condition the anode of diode is connected with positive terminal of the battery.
• And the cathode of diode is connected with the negative terminal of the battery.

Reverse Biasing:

• In Reverse biased condition the anode of diode is connected with negative terminal of the battery.
• And the cathode of diode is connected with the positive terminal of the battery.
• A reverse biased diode does not allows current to easily pass through it.
  

How diode works?

Working in forward biased:

• A diode is made up of P- type semiconductor and N- type semiconductor.
• P-type has majority of holes and minority of electrons.
• N- type contains majority of electrons and minority of holes.

• When we connect supply to this the electrons of N region get repelled by the negative terminal of supply and jump into p region.
• Similarly the hole of P region gets repelled by the positive terminal of supply and jump into N region.

• And this way the diode acts as a closed switch and continuous current flows through a diode when it is forward biased.

Working in reverse biased:

• In this condition when the diode is in reverse biased condition the P region in connected with the negative terminal of the supply
• And the N region is connected with the positive terminal of the supply.

• And hence the holes of p region gets attracted towards the negative terminal of the supply means they move away from the junction.
• Similarly the electrons of N region gets attracted towards the positive terminal of the supply means they move away from the junction.
• And hence the diode acts as open switch and no current flow through the diode.

Reverse Biased diode:

• There is limitation to maximum forward & reverse voltage that can be applied to the diode.
• When large reverse voltage is applied across the diode, heavy current flows through it. This is called as break down of diode & the voltage at which diode breaks down is called as break down voltage.
• If reverse voltage is further increased ,diode may get damaged permanently.
• The maximum reverse voltage that can be applied to the diode without damaging is called peak inverse voltage (PIV).

Types of diode

• LED (Light Emitting Diode)
• LCD
• LDR (Light Dependent Resistor)
• Photo Diode
• Avalanche Photodiode(APD)
• Zener Diode
• Phototransistor
• Solar Cell
• Optocouplers
• Tunnel Diode
• PIN Diode
• Laser diode
• Varactor diode

 LED (Light Emitting Diode)

• It works on the principle of Electro-luminance.

• It is fabricated using direct bandgap material.

• It will emit light when properly excited.

• It emits light due to a large number of recombination at junctions.

• Color light depends on the concentration of dopants.

• It always operates under forward biased condition.

• With a forward current 20 mA, LED gives out the maximum intensity of light.

• In reverse bias, it works like a normal diode.

• Power dissipation in LED is of the order of mW.

• Response time is in Micro sec.

• Operating life is 100,000+ hrs.

• Cut in voltage 1.3V.

• It is used in Display Systems.

LCD ( Liquid Crystal Display)

• The operating principle is dynamic scattering of light.

• Power dissipation is of the order of Micro Watt.

• Response time in MS.

• Operating life is 50,000+ hrs.

• It is used as a display device.

• The material used in a liquid crystal that is Activated by electric current.

LDR (Light Dependent Resistor)

• Also called photoresistor or light-activated resistor.

• The principle of operation is photo resistive effect.

• The range of resistance is 5 ohms to 0.75 Mili ohms.

• The dark resistance of LDR is 0.75 Mili ohms.

• It is used in optocouplers.

• Materials used for fabrication are CdS (Cadmium sulfide), Se (Selenium).

Photo Diode

• Principle of Operation is photoconductive effect.

• Photosensitive material used is CdS, Se, Zn.

• It is also called a light operated switch.

• Ge- Diode responds to visible light.

• Si-Diode responds to infrared light.

• Photosensitive Coating is provided at the junction only.

• Compared to normal diode, Photodiode has a larger depletion width obtained from a lower level of doping.

• It is always operated under reverse bias condition.

• Compare to, normal diode it is 10 times faster, 100 times higher sensitive, but power handling capacity is low.

• The magnitude of photocurrent increases with an increase in the intensity of light falling at junction.

• Photocurrent flows from n to p.

• The photocurrent is a minority carrier current.

• It does not provide gain.

• The photocurrent is diffusion current.

• In forward bias works as a normal diode,

• It is used in optocouplers.

Avalanche Photodiode(APD)

• Basically a photodiode along with avalanche effect.

• APD can handle a large amount of power compared to Photo diode.

• Its response time is smaller than photo diode.

• APD widely used in fiber optic communication because of its high sensitivity.

Zener Diode

• Operates on the principle of the tunneling effect.

• Always operated under reverse bias.

• Mainly used in voltage regulation applications.

• Maintains constant voltage across a load if properly biased.

Phototransistor

• Its principle of operation is a photoconductive effect.

• Coating of photoconductive material is done at collector-base junction.

• It is basically a light operated switch.

• n-p-n phototransistor is faster than p-n-p phototransistor.

Solar Cell

• Its principle of operation is photovoltaic effect.

• The terminal voltage of solar cell can’t exceed the barrier potential of diode that’s why an array solar cells is used to an active higher voltage.

• We can measure the terminal voltage of solar cells using a voltmeter.

• Popularly used solar cells are Se Cells, Ni-cd Cells, PbS cell {sulfide (also spelled sulphide) is an inorganic compound with the formula PbS. Pbs, also known as galena, is the principal ore, and the most important compound of lead}.

• Ni-Cd Cells are rechargeable cells used in satellites.

• It is used in automatic traffic signal lightening.

• Generally “operated under open circuit condition”.

• It can be operated in forward biased condition and has a cut-in voltage equal to zero.

Optocouplers

• These are optically coupled but electrically isolated.

• These are faster than conventional devices.

• Widely used in industrial applications where very good dc isolation better than transformers is required.

Tunnel Diode

• It is the fastest switch.

• Its response time is of the order of Pico sec.

• Works on the principle of tunneling effect.

• It exhibits voltage-controlled negative resistance.

• It has very narrow depletion layer 100 A⁰ to 200 A⁰

• It is used in linear devices as well as a negative resistance device.

• The best material is GaAS having the highest swing.

• It is a p+ n+ diode having doping level of 1:103

• It is used in designing microwave oscillators, as a relaxation oscillator, in designing of pulse and switching circuits and as Parametric amplifier.

PIN Diode

• It is p⁺-I-n⁺ diode.

• I represent intrinsic material

• If I replaced by p-type material then it is called pπn diode and I is replaced by n-type material then it is called pyn diode.

• In PIN diode lightly doped intrinsic SC is sandwiched between highly doped p and n.

• It has low response time because of high resistivity of the I—Region.

• It is two terminal, three-layer, having a single-junction device.

• It is always operated in reverse bias condition

• When whole I— Region is covered with depletion layer then it is called swept out condition.

• In PIN diode if whole | region is not swept out then a signal loss will occur.

• It is used in handling microwave power, as microwave mixer, as a duplex-er, in designing of transmit-receive switch, in designing of an anti-transmit-receive switch.
  

Laser diode

• It stands for light amplification by stimulated emission of radiation.

• It is a source of coherent light.

• It is fabricated with direct bandgap material having a larger carrier lifetime.

• Emission in laser both spontaneous and stimulated.

• Population inversion occurs in the laser.

• Lasers are highly directional.

Varactor diode

• P-n junction diode that changes its capacitance as bias applied to the diode is varied.

• Operated under reverse bias.

• Used as low noise amplifier( Parametric amplifier).