SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

4.4. REAL DIODES:DEFECTS AND CARRIER GENERATION 163

–Wp 0

Ohmic

contact

Ohmic

contact

Injected charge

δnp(x)

–Wp Wn Wn

δpn(x)

–Wp –Wp 0 Wn Wn

p-side n-side

(a)

(b)

Linear decay of

minority charge

Figure 4.9: (a) A schematic of the narrowp-ndiode with ohmic contacts at the boundaries. (b)
The injected charge distribution.

4.4 REAL DIODES: CONSEQUENCES OF DEFECTS AND

CARRIER GENERATION

In the discussion so far we have assumed the diode we are dealing with is ideal, i.e., there are
no defects and associated bandgap states that may lead to trapping, recombination, or generation
terms. In section 3.7 we discussed the effects of bandgap states produced by defects. In a real
diode, a number of sources may lead to bandgap states. The states may arise if the material
quality is not very pure so that there are chemical impurities present. Let us assume that the
density of such deep level states isNt. We will assume that the deep level is at the center of the
bandgap.
We learned in chapter 3 in the SRH analysis that under the approximation of:

1.σ=σn=σpand

2.Et=Ei,and

3.en,ep,σn,σpare unperturbed in non-equilibrium