What is Diode :- Diode is a two terminal unidirectional, non linear solid state  device which passes current in one direction and blocks in other direction.

It has two junction one P type and another N type that is why called as diode i.e. two electrodes. The terminal connected to P type is called Anode and  connected to N type layer is called Cathode. When we connect  positive terminal of battery to Anode and negative terminal of battery to Cathode the diode conducts electric current and called as diode is in forward bias. Where as when we connect positive terminal of battery to cathode and negative terminal of battery to Anode the diode does not conduct electric current and called as reverse bias.

Fig 1.1 PN Diode in Forward Bias

Fig 1.2 PN diode in Reverse Bias

It happens beacuse in forward bias there are a lots of charge carriers are available and as we supply voltage more than its barrier potential it starts to conduct, the voltage above  which a diode starts to conduct is called knee voltage. The value of knee voltage is 0.7 V for a silicon diode and 0.3 V for a germanium diode.
In reverse bias as we increase the voltage there are only minority carriers so few  micro Ampere current flows which is called as leakage curent but as we increase voltage more  than a limit a point reach where the barrier between PN junction collapse and current increases abruptly. This voltage at which the barrier collapse is called reverse breakdown voltage of diode which can vary from 5V to 8 V depending upon type and construction of diode. In reverse bias the diode works on Zener and Avalanche effects, in Zener effect barrier breaks because of elecctric field across the PN junction where as in case of Avalanche effect barrier breaks beacuse of carriers multiplication.
Note:-
1.  Temperature coefficient is negative for breakdown voltage less than 5V (Zenor effect = - 1.4 mV/°C) i.e. if Temperature increases by 1°C the breakdown voltage decreases by 1.4 mV.
2.  Temperature coefficient is positive for breakdown voltage greater than 6V(Avalanche  effect = 2 mV/°C) i.e. if temperature increases by 1°C the breakdown voltage increases by 2 mV.

Fig 1.3 PN diode VI characteristics

Uses of diode:- Diodes are used in switching, rectification, filtration, indicators, teleswiching, wireless signal transmission etc.

Types of Diode:-

1. Light Emittig Diode:- LED is a special purpose diode which emits light when forward biased. It is made of Gallium,Arsenic and phosphorus and produces non coherent light.

2. Photo Diode:-  When light falls on the PN junction of photo diode it dislodges the valence electrons from their orbit producing free electrons and holes in the process. The more the light striking the junction, the larger the reverse current in a diode.
Note:- Optocouoler combines a LED and a Photo Diode. It couples input signal to output circuit without electrical connection.

3. Laser Diode:- LASER is acronim for Light Amplification by Stimulated Emissiom of Radiation. It provides coherent light (Light waves are in same phase to each other) and used in communication, medical, industries and defence etc.

4. The Schottky Diode:- It is a special purpose diode with almost zero reverse recovery time and used where short switchig times are needed. Reverse recovery time is the time a diode takes to shut off after it is suddenly switched from forward to reverse bias. This type of diodes are used in digital devices above 300 MHz (SMPS). It has metal such as Gold, silver or platinum on one side of junction and doped silicon ( N type) on the other side and it has no depletion layer so no stored charges at junction.

5. The Varactor Diode:- The width of depletion layer increases with the reverse voltage. This is why the capacitance of a varactor diode can be controlled by the reverse voltage. Varactor diodes are used in Television, and radio receivers for tuning, tuning range of diode depends on the doping level.

6. Tunnel Diode ( Esaki Diode ) :- Tunnel diodes exhibit negative resistance phenomenon i.e. Increase in forward voltage produces decrease in forward current. When diode is forward biased acts as a voltage controlled device, the negative resistance of tunnel diodes is useful in high frequency circuits called microwave oscillators.

7. PIN Diode:- Positive Negative Intinsic Diode  has intrinsic layer sandwiched between p type n type layer and operate as a variable resistor at RF and microwave frequencies. When forward biased it acts like a current controlled device and when reverse biased acts as a fixed capacitor. It is used in microwave communication, RF switches, attenuators, photodetectors, and phase shifters.

8. Gunn Diode:- A Gunn Diode (Transferred Elecron  Device) is a form of diode with negative resistance used in high frequencies between 1 and 100 GHz. It id used in electronic oscillators to generate microwaves example are RADAR speed guns and microwave relays.

9. Impatt Diode:- Impatt diode uses avalanche transit time delays to provide a negative resistance region. It operates at frequencies between about 3 and 100 GHz or more. It has high power capability and high level of phase noise and used in microwave amplifier & oscillators.

10. Baritt Diode:- It uses thermoionic emission to produce negative resistance. Baritt diode is similar to Impatt diode but has lower phase noise and used in microwave signal generators like Burglar Alarm etc.

11. Zener Diode:- A special purpose diode designed to operate in breakdown region and used as a voltage regulator. To check whether the diode is operating in breakdown region or not, just find Vth across load If Vth is grater than Vz then Zener diode will act in breakdown region otherwise not.

12. Shockley Diode:- It is a four layer PNPN type diode which is used for high power applications in power electronics and belongs to thyristor family. It is mainly used in controlled converters as it can withstand very high current and it is sometimes also called as two transistor diode. It operates in two states On or Off and mainly used as switching device.

Applications of Diodes:-
1. Converters:- Diodes in different combinations are used in inverter, rectifier, chopper and cyclo Converters.
2. Clippers and clampers
3. LED are used as indicators in different type of Electrical and electronics instruments.
4. Infrared LEDs are used for wireless controlling of electrical and electronics systems like TV, Speakers etc.
5. Laser are used in medical for operations and in defence for precise attacks.
6. Optocouplers are used in automatic doors and robotics.
7. Varactor diodes are used for tuning in TV and Radio receivers.
8. Some special purpose diodes are used in microwave frequencies oscillators and amplifier.

Author:- Amarjeet Singh Jamwal authored articles on Basic Electronics EngineeringElectric Traction, and Electronics Practical for INFO4EEE Website