though transistor is reverse biased , it produces a large amount of current  inspite of a diode which when reverse biased do not allow the current to flow.why?

Asked by vasturushi | 19th Nov, 2017, 11:13: AM

Expert Answer:

DIODE

Depletion Region:
- The thin layer region on both sides of a p-n junction which has immobile ions and is devoid of any charge carrier is called depletion region or depletion layer.
 
 
Reverse Bias:
- A p-n juction is said to be reverse biased if the positive terminal of the battery is connected to n-side and the negative terminal to the p-side of the p-n junction.
- In reverse biasing the applied voltage of battery mostly drops across the depletion region of the p-n junction and its direction of voltage is same as that of the potential barrier.Due to it, the reverse bias voltage supports the potential barrier. 
i..e during reverse biasing the applied dc voltage aids the fictitious battery developed across the junction.
- Due to this the potential drop across the junction increases and as a result the diffusion of holes and electrons across the junction decreases. It makes the depletion layer thick and the juction diode offers high resistance during reverse bias. 

Transistor
It is nothing but a switch where the base current controls the flow of current from collector to emitter.
-  In NPN transistor when operating in active region the base to collector junction is reverse biased (positive to negative , negative to positive)
-  A large number of electrons in emitter layer is repelled by negative terminal and they flow towards b-e junction.
- They cross the junction and enter into small base layer.- In this region, some electrons combine with holes (in the base) and are attracted by positive terminal and remaining maximum number of electrons flow into collector layer, crossing the second junction i.e. collector-base junction.
- The electrons in the collector region are repelled by these newly arrived electrons and thus, then all the electrons are present in collector layer are attracted by positive terminal.
- Thus, all these electrons complete their journey back into emitter layer and produce conventional currents.

Answered by Abhijeet Mishra | 8th Dec, 2017, 04:32: PM

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