Direct Energy, 2018a

134 6.7 Photodetectors
6.7.2 Measures of Photodetectors
The frequency response is one of the most important measures of a photodetector.
Often it is plotted versus wavelength or photon energy instead of
frequency. Aphotodetector is only sensitive within a particular wavelength
range, and the frequency response is often not at.
As with all types of sensors, signal to noise ratio is another important
measure. While photodetectors have many sources of noise, one major
source is thermal noise due to the random motion of charges as they ow
through a solid [9, p. 220]. To mitigate thermal noise in photodetectors
used to detect very weak signals, the detectors are cooled with thermoelectric
devices or using liquid nitrogen. Ameasure related to signal to noise
ratio is the noise equivalent power. It is dened as the optical power in
watts that produces a signal to noise ratio of one [82].
Another measure of a photodetector is the detectivity, denoted D*, in
units cm·(Hz1/2 )
W
. It is a measure of the strength of the output assuming a
one watt optical input. By denition, it is equal to the square root of the
area of the sensor times the bandwidth under consideration divided by the
noise equivalent power [82] [83, p. 654].
√
Area · Bandwidth
D∗ =
Noise Equivalent Power
Figure 6.17 shows detectivity versus wavelength for optical detectors made
of various semiconductors.
Photodetectors are also characterized by their response times. Response
time is dened as the time needed for a photodetector to respond to a steplike
optical input [82]. Typical response times can range from picoseconds
to milliseconds [83, p. 656]. There may be a tradeo between response
time and sensitivity, so some detectors are designed for fast operation while
others are design for higher sensitivity [9, p. 220].