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Industrial Sensors and Contr ol
In order to characterize APD response, one must specify APD 365
responsivity (in amps/watt) at a given operating voltage. However,
because of diode-to-diode variations in the exact gain voltage curve
of each APD, the specific operating voltage for a given responsivity
will vary from one APD to another.
Manufacturers should therefore specify a voltage range within
which a specific responsivity will be achieved.
An example of a typically correct specification for diode respon-
sivity, in this case for an InGaAs APD, is as follows:

R (1300 nm) = 9.0 A/W,
MIN
V = 50 V to 90 V,
OP
M = 10

7.23 Dark Current and Noise Current
As can be seen from the noise equation (Equation 2), the total APD
dark current (and the corresponding spectral noise current) is only
meaningful when specified at a given operating gain.
Dark current at M = 1 is dominated by surface current, and may
be significantly less than

I × M
DB
Since APD dark and spectral noise currents are a strong function
of APD gain, these should be specified at a stated responsivity level.
An example of a typically correct specification for diode dark
current and noise current—in this case, for an InGaAs APD—is as
follows:

I (R = 9.0A/W) = 10 nA (max),
D
M = 10
i (R = 6.0 A/W, 1 MHz, 1 Hz BW) = 0.8 pA/√Hz (max),
N
M > 5

7.23.1 Excess Noise Factor
All avalanche photodiodes generate excess noise due to the statistical
nature of the avalanche process. This excess noise factor is generally
denoted as F.
As shown in the noise equation (Equation 2), √F is the factor by
which the statistical noise on the APD current (equal to the sum of the
multiplied photocurrent plus the multiplied APD bulk dark current)