This is a basic semiconductor device. PN junction diode is made up of silicon. Silicon is extracted from sand which is widely available. Once silicon get extracted it undergoes technological process called doping. In process of doping P type material which is group 13 elements of periodic table and N type material which is group 15 elements of periodic table are diffused in silicon.

Due to presence of p-type and n-type semiconductor there is change in concentration at the junction which create depletion region and there exist bulit in potential. 

Biasing of diode:-

Forward bias- in forward bias diode p side is connected to positive terminal of battery and N side is connected to negative terminal of battery.In forward bias diode act as closed switch and hence act as short circuit ie passes input to the output.

Reverse bias:- 

In reverse bias diode p side is connected to negative terminal of battery and N side of diode is connected to positive terminal of battery and hence it act as open switch. In reverse bias it offer high impedence. 

APPLICATION 

1• Used as switch in electronic            circuit 

2• Used in your adapter for rectification purpose to convert AC to DC value.

3• Used in Clamper circuit to provide DC level or we can say that to shift AC signal below or above reference line.

4• In clipper to clip some portion of signal. ie - in communication system to reduce noise 

Diffusion of majority charge carrier in forward bias is main reason for flow of current in diode.

In diode depletion region shift toward lightly doped region.

Types of diode 

LED - light emitting diode, lights emitting diode is made up of direct semiconductor. Direct bandgap semiconductors are compounded semiconductor like indium phosphide, gallium arsenide etc. 

Photo didode : - photo diode is used to detect light. It is used in reverse bias condition. It is used on receiver side of communication system to receive optical signal.

Solar cell:- it is also a PN junction diode which convert solar energy int electrical energy.It work on principle of photovoltaic effect. 

Poly crystalline silicon is used to make solar cell. Solar cell has low efficiency only about 25 percent. 

Diode is base of all electronic devices from BJT to MOSFET, OP AMP.

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