22.3 Photodiodes
615
Fig. 22.36 Ideal
MIS-diode (with p-type
semiconductor) as
photodetector (principle of
a CCD pixel). a Without
bias (see Fig. 21.30b). b
Immediately after an
external (reversely poled)
voltage V > 0 has been
applied, the surface
potential is s = V and no
charges have moved yet. c
Strong depletion (still not
in thermodynamic
equilibrium) with signal
charge and reduced surface
potential s < V . d The
semiconductor in
equilibrium (E F is
constant) with depletion
and inversion layer (see
Fig. 21.34). For all
diagrams, V = V − i + s
E V
E F
E i
E C
e B
V>0
e S
neutral
region
depletion
region
inversion
region
V i
(d)
E V
E F
E i
E C
e B
V>0
e S
V i
E V
E F
E i
E C
e B
V>0
e S
E V
E F
e B
E i
E C
(a)
(b)
(c)
semiconductor
metal insulator
semiconductor is not in thermodynamic equilibrium (as it is in Fig. 22.36d) when the quasi-Fermi level
is constant throughout the semiconductor. The inversion charge has yet to build up.
There are three mechanisms to generate the inversion charge. (a) generation-recombination, (b)
diffusion from the depletion-layer boundary and (c) carrier generation by light absorption. Mechanisms
(a) and (b) represent dark currents for the photodetector. The conductivity due to these two processes is
shown in Fig. 22.37 and slowly builds up the inversion charge. Two temperature regimes are obvious;
at low temperatures the generation dominates (∝ n i ∝ exp(−E g /2kT )), at high temperatures the