22.3 Photodiodes
609
Fig. 22.25 dc I –V
characteristic of an
InGaAs/InP MSM
photodetector
(InP:Fe/InGaAs:Fe/InP:Fe,
finger separation 1 µ m,
λ = 1.3 µ m) under
illumination for dark
environment (0 µ W) and
various illumination levels
as labeled. Adapted
from [1841]
Fig. 22.26 Simulation of
the time-dependent
response of an InGaAs:Fe
MSM photodetector to a
short light pulse. Adapted
from [1841]
the RC time constant. For 300 nm/300 nm fingers and a LT-GaAs a bandwidth of 510 GHz (pulsewidth
of 0.87 ps) was reported, which is faster than the intrinsic transit time (1.1 ps) and not limited by the RC
time constant (expected pulse width 0.52 ps), due to the recombination time (estimated to be 0.2 ps).
22.3.6 Avalanche Photodiodes
In an avalanche photodiode (APD) intrinsic amplification due to carrier multiplication (through impact
ionization) in a region with high electric field is used to increase the photocurrent. The field is generated
by a high reverse bias in the diode. In an ideal APD only one type of carrier is multiplied, resulting in
the lowest noise. If electrons are injected into the field region at x = 0 (Fig. 22.28a), the multiplication
factor for electrons is
M n = exp(α n w) ,
(22.49)
for α p = 0. Typically, both carrier types suffer multiplication. If the electron and hole impact ionization
coefficients are the same (α n = α p = α), the multiplication factor for electrons and holes M is given by
M =
1
1 − α w
.
(22.50)
The rms value of the current noise is the same as in the case of the pn-diode (22.25), only that now
the gain M is added