258
6 Photodetection Devices
Fig. 6.7 Schematic representation of a reverse-biased pin photodiode
that are produced outside of the depletion layer in the bulk of the semiconductor (i.e.,
in the n and p regions) and diffuse into the reverse-biased junction. The drift current
density can be found from Eq. (6.4):
J dr =
i p
A
= q 0
1 − e
−α s w
(6.23)
where A is the photodiode area and 0 is the incident photon flux per unit area given
by
0 =
P in
(1 − R f
Ahν
(6.24)
The surface p layer of a pin photodiode is normally very thin. The diffusion current
is thus principally determined by hole diffusion from the bulk n region. The diffusion
of holes in this material can be determined by the one-dimensional diffusion equation
[10]
D p
∂
2 p n
∂ x 2 −
p n − p n0
τ p
+ G(x) = 0
(6.25)
where D p is the hole diffusion coefficient, p n is the hole concentration in the n-type
material, τ p is the excess hole lifetime, p n0 is the equilibrium hole density, and G(x)
is the electron–hole generation rate given by
G(x) = 0 α s e
−α s x
(6.26)
From Eq. (6.25), the diffusion current density J diff is found to be