Reverse bias current
Band diagram. No-bias vs. reverse bias. Basic exponential current relationship.
Forward bias current
Band diagram. No-bias vs. forward bias. Basic exponential current relationship.
Junction capacitance
Junction capacitance in a PN junction diode consists of 2 components :
Diffusion capacitance
Due to variation of stored minority charge in the quasi-neutral regions. Quasi-neutral regions are the area outside the depletion region where there is no electric field.
$$C_D=\cfrac{dQ}{dV_A}$$
$$\implies{}C_D=\tau{}_p\cfrac{dI}{dV_A}=\tau{}_p\cfrac{I_{DC}}{kT/q}$$
This diffusion capacitance affects the turn-on/off transient of the diode.
Depletion capacitance
In the depletion region, we can observe that the opposite charges are separated by some distance. This leads to capacitance.
$$C_j=\cfrac{dQ_{dep}}{dV_A}$$
$$\implies{}C_j=\cfrac{A\epsilon{}_s}{W}$$
How to reduce Cj?
- Lower the dopant concentrations (NA,ND). It increases the W.
- Higher reverse bias (increases the W).
- Reduce the junction area (A).
What is Avalanche breakdown ?
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