Diffusion Capacitance

Diffusion Capacitance

AU : May-03, 06, 07, 16, 17, Dec.-03, 07

• During forward biased condition, an another capacitance comes into existence called diffusion capacitance or storage capacitance, denoted as CD.

• In forward biased condition, the width of the depletion region decreases and holes from p side get diffused in n side while electrons from n side move into the p side. As the applied voltage increases, concentration of injected charged particles increases. This rate of change of the injected charge with applied voltage is defined as a capacitance called diffusion capacitance.

C_D = dQ / dV        ...(1.12.1)

• The diffusion capacitance can be determined by the expression,

C_D  = τI / η V_T ...(1.12.2)

where τ = Mean life time for holes.

• So diffusion capacitance is proportional to the current. For forward biased condition, the value of diffusion capacitance is of the order of nano farads to micro farads while transition capacitance is of the order of pico farads. So C_D is much larger than C_T.

• However in forward biased condition, C_D appears in parallel with the forward resistance which is very very small. Hence the time constant which is function of product of the forward resistance and C_D is also very small for ordinary signals.

Key Point : Hence for normal signals C_D has no practical significance but for fast signals C_D must be considered.

• The graph of C_D against the applied forward voltage is shown in the Fig. 1.12.1.

1. Derivation of Expression for Diffusion Capacitance

• In a p-n junction the total current at the junction (x = 0) is given by,

I = I_pn (0) + I_np (0) A

where I_pn(0) = Current due to holes diffusing from p to n side

I_np(0) = Current due to holes diffusion from n to p side

• Let p side is heavily doped with respect to other hence Inp(0) is negligible compared to Ipn(0).

• I = I_pn (0)

• The diffusion current density is given by,

• The excess minority Q xists only on n side and given by,

CD = τ I / ηV_T = Diffusion capacitance

Review Questions

1. Explain the origin of diffusion capacitance and discuss its importance.

AU : May-03, 06. 07, 16, Dec.-03, 07, Marks 8

2. Derive the expression for diffusion capacitance of PN junction diode.