Page 410 - Sensors and Control Systems in Manufacturing

364
Cha p te r
Se v e n

7.22.1 Types of APDs
Avalanche photodiodes are commercially available that span the
wavelength range from 300 nm to 1700 nm. Silicon APDs can be used
between 300 nm to 1100 nm, germanium between 800 nm and 1600 nm,
and InGaAs from 900 nm to 1700 nm. Although significantly more
expensive than germanium APDs, InGaAs APDs are typically avail-
able with much lower noise currents, exhibit an extended spectral
response to 1700 nm, and provide higher frequency bandwidth for a
given active area.
A germanium APD is recommended for applications in high elec-
tromagnetic interference (EMI) environments, where amplifier noise
is significantly higher than the noise from an InGaAs APD, or for
applications where cost is a primary consideration.

7.22.2 Understanding the Specifications Responsivity
and Gain
APD gain will vary as a function of applied reverse voltage, as shown
in Fig. 7.41. In addition, for many APDs, it is not possible, or practical,
to make an accurate measurement of the intrinsic responsivity: Ro(λ),
at a gain M = 1.
It is therefore inappropriate to state typical gain and diode sensi-
tivity at M = 1 as a method for specifying diode responsivity at a
given operating voltage.

1000

100

Gain 100

10

1
50 100 150 200 250 300 350 400
Bias voltage (V)

FIGURE 7.41 A typical gain-voltage curve for a silicon APD.