Question

Explain why the width of a pn junction can be manipulated by an external volatge. Determine the polarity of the external voltage if one wants to increase the width of the pn-junction.

Answer 1

When we apply forward bias to a P-N junction it energises the valance electrons of the depletion layer and these electrons start breaking their bond hence the depletion layer starts collapsing and breaks immediately as it gets a voltage beyond its threshold voltage. A forward bias decreases the potential drop across the
junction. As a result, the magnitude of the electric field
decreases and the width of the depletion region narrows.

When we apply reverse bias the same valance electrons increases due to combining of electron and holes available at either sides of depletion layer in the P and N region which results a widen depletion layer. Reverse bias increase the voltage drop across the junction. As voltage drop increases , it increases the depletion layer width.

Reverse bias should be applied to pn junction to increase the depletion region width. Positive voltage at n-side and negative terminal at p-side gives reverse bias .