pn junction - AC signal model

Let the applied voltage on a diode be:

Let an AC small signal be determined by:

Let the be:

Let the admittance be:

Thus, by imitating Ohm’s law, we define the dynamic resistance of the junction:

Under reverse bias: (), and we have no diffusion capacitance.

Under forward bias: the diffusion capacitance is superimposed on the junction capacitance. Hence: .

The equivalent circuits are, therefore:

Forward bias

Reverse bias

Ex. 11 p. 32

A p⁺n diode

Data:

a) What are I₁ and I₂?

- We address the forward bias model:

Then:

And:

Hence:

b) What are: ?

In forward bias:

Hence:

Taking two equations for two unknowns (namely, ):

We get:

With that knowledge, we substitute the value for C_j(0) in the equation for I₁:

c) What is ?

-is the minority carrier lifetime in the n –side of the p⁺n, and has the value of:

d) What is N_D?

We remember that: ,

and: .

Thus, we get:

From the data: . Thus we get: .

Now we can calculate N_D: .

e) What is V_bi?

We already calculated it: .

f) What is N_A?

Since: , we get: .

So, , justifies p⁺n!!