Junction Capacitance Of Pn Junction Diode

In the previous Posts we discussed about the Pn Junction Diode And VI Characteristics and Diode Parameters And Applications And today we going to discuss about the Junction Capacitance Of Diode

U can Also Read Our Posts Regarding PN Junction Diode

Zener Breakdown And Avalanche Breakdown

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

ZENER BREAKDOWN

When the P and N regions are heavily dopped, direct rupture of the covalent bonds takes place because of the electric field. At the junction of the PN junction diode the new electron -hole pairs will be created increase the reverse current takes place at a constant reverse bias voltage typically 6v for heavily dopped diodes . As a result of hevy dopping the diplition region is reduced then the field across the diplition is very large 10^7 this is called as the zener breakdown

What is PN junction

Lets Know about Pn Junction

PN Junction

It is possible to manufacture a single piece of a semiconductor
material half of which is doped by P-type impurity and the other half
by N-type impurity ,The plane dividing the
two zones is called junction. Theoretically, junction plane is assumed
to lie where the density of donors and acceptors is equal. The P-N
junction is fundamental to the operation of diodes, transistors and
other solid-state devices.
Let us see if anything unusual happens at the junction. It is found
that following three phenomena take place :
1. A thin depletion layer or region (also called space-charge
region or transition region) is established on both sides of the junction and is so called
because it is depleted of free charge carriers. Its thickness is about 10−6m.
2. A barrier potential or junction potential is developed across the junction.
3. The presence of depletion layer gives rise to junction and diffusion capacitances