What is Diode? What happen at a P-N junction?

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A diode is a semiconductor two terminal device that allow current to flow in one direction.
Diode have polarity, determined by an anode(positive terminal) & cathode (negative terminal)
When a diode allows current flow, It is forword biased. When a diode is reverse biased, It acts as an insulator & don't permit current to flow.
Formation of P-N junction-
In a P-N junction, the P-region has high concentration of holes & very few electrons. Where as the N-region has high concentration of electron & few holes.The electrons from N-region diffuse through the junction into P-region & hole from P-region diffuse into N-region.
Depletion region- Due to diffusion, neutrality of both N and P type semiconductor is distributed,a layer of negative charged ions apper near the junction in the P- crystal & a layer of positive ions appear near the junction in N-crystal. This layer is called depletion layer & region in called depletion region.
(1) Depletion is directly proportional to temperature.
(2) Width of depletion layer is inversely proportional to dopping.

Potential barrier- The potential difference created across the P-N junction due to the diffusion of electron & hole is called potential barrier.
For Germanium(Ge) potential barrier= 0.3V
&  For Silicon (Si) Potential barrier= 0.7V

Diffusion Current- Due to concentration difference holes/electrons try to diffuse from their side to other side. Only those holes/electrons crosses the junction, which have high kinetic energy. This diffusion results in an electric current from P-side to N-side known as diffusion current.
Drift Current- Electron ,hole pair (because of thermal collision) are continuously created in the depletion region. There is a regular flow of electrons the N-side & holes towards the P-side. This makes a current from the N-side to the P-side .This current is called the drift current.


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