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performance is degraded by an “excess noise factor” (F) compared 363
to a PIN, equation 4. The total spectral noise current for an APD in
“dark” conditions is thus given by equation (1), ID = IDS + IDB ⋅ M:
where I is the surface leakage current.
DS
i = [2.q ⋅ I + I ⋅ M ⋅ F ⋅ B] 0.5 (2)
n DS DB
where q is the electron charge.
At higher signal light levels, the detector transitions to the photon
shot noise limited regime where sensitivity is limited by photon shot
noise on the current generated by the optical signal. Total noise from
the APD in “illuminated” conditions will therefore equal the qua-
dratic sum of the detector noise plus the signal shot noise. For a given
optical signal power, P , this is given by:
s
i = [2.q ⋅ I + I ⋅ M + R (λ) ⋅ M ⋅ P ⋅ F ⋅ B] 0.5 (3)
n DS DB s
In the absence of other noise sources, an APD therefore provides a
signal-to-noise ratio (SNR), which is worse than a PIN detector with
the same quantum efficiency. Noise equivalent power (NEP) cannot
be used as the only measure of a detector’s relative performance, but
rather detector signal-to-noise (SNR) at a specific wavelength and
bandwidth should be used to determine the optimum detector type
for a given application. The optimum signal-to-noise occurs at a gain
M, where total detector noise equals the input noise of the amplifier or
load resistor. The optimum gain depends in part on the excess noise
factor, F, of the APD, and ranges from M = 50 to 1000 for silicon APDs,
and is limited to M = 10 to 40 for germanium and InGaAs APDs.

7.22 Selecting an APD
APDs are generally recommended for high-bandwidth applications, or
where internal gain is needed to overcome high preamplifier noise.
The following is a simple guide that can be used to decide if an
APD is the most appropriate for one’s light detection requirements.

• Determine the wavelength range to be covered. (See the fol-
lowing section “Types of APDs,” to determine the specific
APD type useful for the wavelength range to be covered.)
• Determine the minimum size of the detector that can be used
in the optical system. Effective optics can often be more cost-
effective than the use of an overly large PIN or avalanche
photodetectors.
• Determine the required electrical frequency bandwidth of the
system; again, over-specifying bandwidth will degrade the
SNR of the system.