Transition and diffusion capacitance

Posted by AJ × 23:41

• The figure shows the capacitance v/s applied voltage across the diode.

• Shunt capacitive effects that can be ignored at very lower frequencies since Xc=1/2πfc

is very large (open circuit)

• However this can not be neglected in very high frequencies since it introduces a low

reactance (shorting) path.

• Two types of capacitive effects to be considered in FB and RB condition.

• In RB region transition or depletion region capacitance CT in FB diffusion

capacitance CD or storage capacitance.

• W.k.t C=εA/d.

• ε is the permittivity of dielectric between tow plates of area A separated by distance d.

• In RB, depletion region which is free of carriers that behaves essentially like an

insulator between the layers of opposite charges. This depletion region width increase

with increase in RB potential.

• Since d is increasing, capacitance effect is more in FB.

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