Transient response of Diode

The diode will not respond to the reverse voltage until excess minority carriers in neutral n and p regions have been withdrawn.

Model of diode

If we apply reverse bias suddenly then the diode passes a reverse current higher than reverse saturation current for some t. The current then falls as the stored minority carriers are withdrawn, eventually reaching .

One can use charge storage model to understand this transient behaviour.

Consider a junction-depletion region on the n side then the essential minority carrier is p only.

Fig 10.7 depicts the hole distribution in neutral n-region.

Multiplying continuity equation by qAdx and integrating it over the entire neutral n region from x n to w 1 , we get

which can be written as

where is the excess minority carrier stored in a n region at any time. is hole current at x = w 1 and is related to the transit time and neutral n region of width w 1

A time constant is defined by the relation,

is related to the passage time through the neutral n-region of width W n , so we can write

where

the switching trajectory is shown in Fig. 10.12

Now in storage phase almost constant so the equation

has the solution

where is a constant and is determined by the condition that

at t = 0 ,

hence

.

Now we assume a triangular hole distribution where the charge remains constant in the n-region at , as depicted in Fig. 10.13

and

where is the time till the diode remains forward biased

Thus

The stored charge is equal to the area of the triangle with base , so

writing and solving for we get

Solving

More accurate derivation of involves solution of time-dependent continuity equation with appropriate boundary conditions and is given as