598
22 Light-to-Electricity Conversion
Fig. 22.12 a Structure of
BIB photodetectors. Highly
doped contact layers
(black), doped
semiconductor (white) and
blocking (intrinsic) layer
(grey). b Band diagram
under small forward bias.
Shaded area represents the
donor impurity band.
c Electric field in the
structure
n
+
n
+
-w
i
h
n
0 b
E C
E V
(a)
(b)
0
(c)
i
From the Poisson equation the electric field is given by
E(x) = −
e
s
N A (w + x) , −w ≤ x ≤ 0
(22.18a)
E(x) = −
e
s
N A w = E i , 0 ≤ x ≤ b .
(22.18b)
The voltage drops across the blocking layer V b and the doped semiconductor V s fulfill
V = V b + V s .
(22.19)
Integration of the fields yields
V s =
e
s
N A
w
2
2
(22.20a)
V b =
e
s
N A w b .
(22.20b)
Substituting (22.20a,b) into (22.19) results in the width of the ‘depletion layer’
w =
2 s V
e N A
+ b 2 − b .
(22.21)
The high dopant concentration allows for much thinner absorption layers than in a conventional photoconductivity detector, making it less susceptible to background high-energy cosmic radiation. The
recombination in the depletion layer is negligible. Detector performance is modeled in [1836].