Q1Analog Electronics

Question

(b) Describe Transition and Diffusion capacitance in PN junction diode.

Answer 1

Transition capacitance (also called depletion or junction capacitance) exists in a reverse-biased PN junction. The depletion region acts as a dielectric and the P and N regions act as plates of a parallel-plate capacitor. It is given by:

C_{T} = \frac{ε A}{W}

where $ε$ is permittivity, $A$ is junction area, and $W$ is the depletion width. As reverse voltage increases, $W$ increases, so $C_{T}$ decreases.

Diffusion capacitance exists in a forward-biased PN junction. Minority carriers injected across the junction store charge. This stored charge changes with applied voltage, giving rise to diffusion capacitance:

C_{D} = \frac{τ \cdot I _{F}}{η V _{T}}

where $τ$ is minority carrier lifetime, $I_{F}$ is forward current, and $V_{T}$ is thermal voltage (â‰ˆ26 mV). $C_{D} ≫ C_{T}$ under forward bias, making diffusion capacitance the dominant effect.

Q1Analog Electronics

Question

Answer