Page 15 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Photodetection Basics

8 Chapter One

are simply not absorbed and hence pass through the crystal without being use-
fully detected. They serve only to warm up the back contact. The cutoff wave-
length in microns is given approximately by l g = hc/E g ª 1.24/E g (V), where
E g(V) is the bandgap energy measured in electron-volts. Indeed, with a halogen
lamp, which emits both visible and near infrared light, and an infrared viewer
or camera sensitive to 1.3mm you can see through a silicon wafer as if it were
glass.
Variations in doping can make minor changes in the bandgap and hence in
sensitivity at long wavelengths, either to increase it or to decrease it. In appli-
cations using the important neodymium laser sources emitting at around
1.06mm, even minor increases to cutoff wavelengths in silicon photodiodes can
be useful in increasing detection sensitivity. The peak sensitivity in conven-
tional silicon diodes occurs at around 0.96mm.
A reduction in long wavelength sensitivity is also sometimes useful, helping
to suppress detection of interfering infrared light, when low level signals at
visible and ultraviolet wavelengths are the target of interest. As we will discuss
in later chapters, the signals we want to see can often be swamped by signals
at other wavelengths. Tailoring the spectral sensitivity curve can bring great
advantages. For the high-volume applications of 0.88mm and 0.95mm LEDs,
used in handheld remote controls and IRDA short-range communications
systems, silicon photodiodes embedded in black filtering plastic are available.
The material is transparent in the near infrared but cuts out much of the visible
light below 0.8mm, which greatly reduces disturbance from ambient light
sources.
The quantum photodetection process suggests that even short-wavelength
ultraviolet and x-ray photons should generate charge carriers in a silicon
photodiode. This is the case, but the component’s detection process functions
only if the charge carriers are generated in or close to the depletion region. At
the short wavelength end of Fig. 1.5 the silicon is becoming too absorbing;
photons are being absorbed too close to the surface in a region where charge
carriers will not be swept away to contribute to the photocurrent. Again they
end up as heat. Improvements in ultraviolet (UV) sensitivity can be made
through careful control of detector doping, contact doping, and doping thick-
nesses to give a depletion region lying very close to the surface.

1.6 Other Detector Materials
Although important, silicon is not the only detector material that can be used
to fabricate photodetectors. The wavelengths you want to detect, and those you
would rather not detect, should give you an idea what energy gap is appro-
priate and hence what material is best. A few semiconductor energy bandgaps
are shown in Table 1.1.
As we have seen, to be detected the incident light must have a photon energy
greater than E g (or a wavelength l < l g). At wavelengths beyond 1.1mm, where
silicon is almost transparent, germanium diodes are widely used and have been

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