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GDB of PHY-301 open date 28th July 2016 at 12:00am and close date 29th July 2016 at 11:59pm

The topic is:

 “By increasing the reverse biase value to a P-N junction, breakdown occurs. On which factor of P-N junction breakdown depends. What phenomenon will occur in case of heavily doped and lightly doped junction, by increasing reverse voltage".

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Replies to This Discussion

Dear students discuss here about this gdb ...

empty reply :O

anybody did gdb or not ??

Kiya yaad rakho gay yaar :P :P  lou 

nhi hm nhi yad rakhy gay

1. breakdown par 2 qism kay effects hongay, zener and avalanche

2.  zener for highly doped and avalanche is phenomenon is for lightly doped

Regards

Adnan

thanks

Electrical break down of any material (say metal, conductor, semiconductor or even insulator) can occur due to two different phenomena.
Those two phenomena are 1) Zener breakdown and 2) Avalanche breakdown.
Zener Breakdown
When we increase the reverse voltage across the pn junction diode, what really happens is that the electric field across the diode junction increases (both internal & external).
This results in a force of attraction on the negatively charged electrons at junction.
This force frees electrons from its covalent bond and moves those free electrons to conduction band. When the electric field increases (with applied voltage), more and more electrons are freed from its covalent bonds.
This results in drifting of electrons across the junction and electron hole recombination occurs. So a net current is developed and it increases rapidly with increase in electric field.
Zener breakdown phenomena occurs in a pn junction diode with heavy doping & thin junction (means depletion layer width is very small).
Zener breakdown does not result in damage of diode. Since current is only due to drifting of electrons, there is a limit to the increase in current as well.

Avalanche breakdown occurs in a pn junction diode which is moderately doped and has a thick junction (means its depletion layer width is high).
Avalanche breakdown usually occurs when we apply a high reverse voltage across the diode (obviously higher than the zener breakdown voltage,say Vz).
So as we increase the applied reverse voltage, the electric field across junction will keep increasing.

If applied reverse voltage is Va and the depletion layer width is d;

then the generated electric field can be calculated as Ea =Va/d

This generated electric field exerts a force on the electrons at junction and it frees them from covalent bonds.
These free electrons will gain acceleration and it will start moving across the junction with high velocity. This results in collision with other neighboring atoms.
These collisions in high velocity will generate further free electrons. These electrons will start drifting and electron-hole pair recombination occurs across the junction.
This results in net current that rapidly increases.


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