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Cha p te r
Se v e n

Ionization Typical Excess Noise (Factor
Detector Type Ratio X-Factor Gain at Typical Gain)
(k) — (M) (F)
Silicon (reach- 0.02 0.3 150 4.9
through structure)
Silicon Epitaxial 0.06 0.45 100 7.9
APDs
Silicon (SLiK 0.002 0.17 500 3.0
low-k structure)
Germanium 0.9 0.95 10 9.2
InGaAs 0.45 0.7–0.75 10 5.5

TABLE 7.3 Typical Values of k, X, and F for Si, Ge, and InGaAs APDs

7.21 Critical Performance Parameters
An APD differs from a PIN photodiode by providing internal photo-
electronic signal gain. Therefore, output signal current, I , from an
s
APD equals I = M.Ro (λ) × P , where Ro(λ) is the intrinsic responsiv-
s s
ity of the APD at a gain M = 1 and wavelength l, M is the gain of the
APD, and P is the incident optical power. The gain is a function of the
s
APD’s reverse voltage, V , and will vary with applied bias. A typical
R
gain-voltage curve for a silicon APD is shown in Fig. 7.41.
The gain as a function of the bias voltage varies with the structure
of the APD.
One of the key parameters to consider when selecting an APD is
the detector’s spectral noise. Like other detectors, an APD will nor-
mally be operating in one of two noise-limited detection regimes;
either detector noise limited at low power levels, or photon shot noise
limited at higher powers. Since an APD is designed to be operated
under a reverse bias, sensitivity at low light levels will be limited by
the shot noise and the APD’s leakage current. Shot noise derives from
the random statistical Poissonian fluctuations of the dark current, I
D
(or signal current). Dark current shot noise (IN - SHOT) is normally
½
given by I = (2.q.B.I ) , for a PIN detector, where B is the system
N(SHOT) D
bandwidth. This differs for an APD, however, as bulk leakage cur-
(4)
rent, I , is multiplied by the gain, M, of the APD . Total leakage
DB
current I is therefore equal to:
D
I = I + I ⋅ M (1)
D DS DB
where I is the surface leakage current.
DS
In addition, the avalanche process statistics generate current fluc-
tuations, and APD