SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

4.8. DIODE APPLICATIONS: AN OVERVIEW 185

RL

Variable current

source controlled

by light, voltage,

etc.

I

V

II I

III IV

PHOTODETECTOR SOLAR CELL

Figure 4.24: Simple circuit diagram (left) and current-voltage (I−V) plot showing regimes of
operation for ap−ndiode under illumination. When operated in quadrant III, the device acts
as a photodetector, whereas in quadrant IV it behaves as a solar cell

of the junction and be swept away. Minority electrons generated well beyond a lengthLnwill
recombine with holes resulting in the equilibrium concentration,np 0. Similarly holes generated
within,Lp, a diffusion length, of the depletion region edge will be swept into the depletion
region.
In the event that there is light shining on thep-njunction, as shown in figure 4.25a, then the
charge profile is perturbed in the following manner. Far in the bulk region, an excess minority
carrier concentration is generated, whereΔnp=GLτnandΔpn=GLτp. Thisisshownin
figure 4.25b. The new equation to be solved for reverse saturation current differs from the one
previously used in that a light generation term is added.

Dp

d^2 p

dx^2

+Gth−R+GL=0 (4.8.1)

or

Dp

d^2 p

dx^2

+

pn 0 −pn

τp

+GL=0 (4.8.2)

with boundary conditions similar to before.

pn(∞)=pn 0 +τpGL (4.8.3)

pn(Wn)=0 (4.8.4)

Solving these equations, we get

pn(x)=(pn 0 +τpGL)

[

1 −exp

(

−

x−Wn

Lp

)]

(4.8.5)