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FIGURE 7.39 Avalanche photodiode detectors (APD)

and photon correlation studies. This section of the chapter discusses
APD structures, critical performance parameters, and excess noise
factors. For low-light detection in the 200- to 1150-nm range, the
designer has three basic detector choices: the silicon PIN detector,
the silicon avalanche photodiode (APD), and the photomultiplier tube
(PMT).
APDs are widely used in instrumentation and aerospace appli-
cations, offering a combination of high speed and high sensitivity
that is unmatched by PIN detectors, and quantum efficiencies at
>400 nm that is unmatched by PMTs. Providing a guideline to
defining and selecting avalanche photodiodes should offer the
user of spectroscopy sensing devices the fundamental tools needed
to apply minimum signal-to-noise spectra and would include the
following:

• Explanation of an avalanche photodiode
• How to select an APD
• Discussion of excess noise factors
• Explanation of Geiger Mode
• Possible applications

7.20 Structures of Avalanche Photodiodes—APD
Structures
In order to understand why more than one APD structure exists, it is
important to appreciate the design tradeoffs that must be accommo-
dated by the APD designer. The ideal APD would have zero dark
noise, no excess noise, a broad spectral and frequency response, a
gain range from 1 to 106 or more, and a low cost. More simply, an
ideal APD would be a good PIN photodiode with gain! In reality,
however, this is difficult to achieve because of the need to trade off