SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

172 CHAPTER 4. JUNCTIONS IN SEMICONDUCTORS:P-NDIODES

The prefactors to the recombination-generation current is

IGR^0 (0.5V)=1. 4 × 10 −^11 A

IGR^0 (0.6V)=1. 7 × 10 −^11 A

The current is now

I(0.5V) = 9. 83 × 10 −^16 exp

(

0. 5

0. 026

)

+1. 4 × 10 −^11 exp

(

0. 5

0. 052

)

=4. 33 × 10 −^7 A

and

I(0.6V) = 9. 83 × 10 −^16 exp

(

0. 6

0. 026

)

+1. 7 × 10 −^11 exp

(

0. 6

0. 052

)

=1. 21 × 10 −^5 A

We can write the diode current as

I∼=ISexp

(

eV

nkBT

)

Thus

I(V 2 )

I(V 1 )

∼=exp

(

e(V 2 −V 1 )

nkBT

)

Using this relation, we find that

n∼= 1. 15

4.5 ReverseBiasCharacteristics ............................

The case for reverse bias is very different. Here the application of bias increases barriers.
The only carriers that can flow are those that can diffuse to the depletion region and are swept
across by the field; these are minority carriers, holes in the n-region and electrons in the p-region
(figure 4.16).

4.5.1 FirstObservation..............................

Since we are only dealing with minority carrier currents we know that minority carrier drift can
be neglected, hence only minority carrier diffusion is relevant. To calculate diffusion currents we
need to know the charge profile. Charge profiles are obtained by solving the continuity equation
as shown in chapter 3. We assume that the large electric field in the reverse biasedp-njunction
sweeps minority carriers away from the edge of the junction. Using the Schockley Boundary
Conditions:

np(−Wp)=0 And pn(+Wn)=0