The Capacitances of a Semiconductor Diode

The depletion layer has a high resistance and acts as a dielectric on each side of which there are two unlike space charges, + Q _R and — Q _R, produced by the ionized atoms of the donor and acceptor impurities. Therefore, a p-n junction has a capacitance similar to that of a two-plate flat capacitor. It is called the junction capacitance. In the case of a. c. voltage, it is given by:

C_j = ∆Q_R /∆U_R = , (3.2)

where C_j0 – the equilibrant value of junction capacitance and 0.3< n ≤0.5.

When the reverse voltage is raised, the depletion layer broadens, and C_j decreases. The manner in which C_j varies as a function of U_R is shown by the plot in Fig. 3.2.

Fig. 3.2. Junction capacitance as a function of reverse bias voltage

When forward-biased, a p-n junction has what is known as the diffusion capacitance, C_dif, in addition to the junction capacitance. For each value of for­ward voltage there is a certain value for the two equal but unlike space charges, + Q_dif and – Q_dif, stored in the n- and p -regions owing to the diffusion of carriers across the junction. In the case of a. c. voltage, C_dif is the ratio of charge to potential difference:

C_dif = ∆Q_dif / ∆U_F = , (3.3)

where τ – a transition time for minority carrier from base.

The diffusion capacitance of a p-n junction is appreciably greater than its barrier capacitance, but there is no way of putting it to any use because it is shunted by the low forward resis­tance of the diode itself.

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