Why upward band bending due to depletion of electrons?
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Answer:
When the diode is connected in forward bias, the electric field due to external voltage source and the electric field due to depletion region are in the opposite direction. This reduces the net electric field in the junction and the electrons can now pass from n-region to the p-region. As more electrons now flows into the depletion region, the number of positive ions is reduced. Same thing is also applied to holes. With the reduction in net electric field, the holes can now flow into the n-region. As the holes pass through the depletion region, the number of negative ions also decreases. Hence the thewidth of depletion region decreasesdue to reduction in the number of positive and negative ions.
Band bending in fact is the change in potential profile at interface due to charge transfer. The energy bands bend when there is an electric field as exists for example across a p-n junction or across the space-charge layer in an MOSFET or in a Schottky barrier (metal-semicoductor contact).