Junction Capacitance :

i) Transition Capacitance (CT) or Space-charge Capacitance

When a P-N junction is reverse-biased, the depletion region acts like an insulator or as a dielectric material essential for making a capacitor. The P- and N-type regions on either side have low resistance and act as the plates. We, therefore, have all the components necessary for making a parallel-plate capacitor. This junction capacitance is called transition or space charge capacitance (Cpn or CT). It may be calculated by the usual formula C = ∈A/d. Its typical value is 40 pF. Since thickness of depletion (or transition) layer depends on the amount of reverse bias, capacitance CT can be controlled with the help of applied bias. This property of variable capacitance possessed by a reverse biased P-N junction is used in the construction of a device known as varicap or varactor. This capacitance is voltage dependent as given by the relation

CT =K/(Vk+Vr)n

where

Vk = knee voltage ;

Vr = applied reverse voltage

K = constant depending on semiconductor material

n = 1/2 – for alloy junction and

= 1/3 –for diffused junction

Diffusion Or Storage Capacitance (CD)

This capacitive effect is present when the junction is forward-biased. It is called diffusion capacitance to account for the time delay in moving charges across the junction by diffusion process. Due to this fact, this capacitance cannot be identified in terms of a dielectric and plates. It varies directly with the magnitude of forward current as explained below in more details.

Consider a forward-biased junction which is carrying a forward current IF. Suppose the applied voltage is suddenly reversed, then increases suddenly but leaves lot of majority charge carriers in the depletion region. These charge carriers must get out of the region which, to their bad luck, becomes wider under the reverse bias. Hence, it is seen that when a forward-biased P-N junction is suddenly reverse-biased, a reverse current flows which is large initially but gradually decreases to the level of saturation current I0. This effect can be likened to the discharging current of a capacitor and is, therefore, rightly represented by a capacitance called diffusion capacitance CD. Since the number of charge carriers left in depletion layer is proportional to forward current, CD is directly proportional to IF. Its typical value is 0.02 µF which is 5000 times CT. The capacitance assumes great significance in the operation of devices which are required to switch rapidly from forward to reverse bias. If CD is large, this switchover cannot be rapid. It will delay both the switch-on and the switch-off. This effect of CD is variously known as recovery time or carrier storage.

CD=dQ/dV

= τI / ηVT