Page 16 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Photodetection Basics

Photodetection Basics 9

TABLE 1.1 Approximate Energy Bandgaps and Equivalent Wavelengths of
Some Common Semiconductors
Material Bandgap energy (eV at 300K) Equivalent wavelength l g(mm)

C (diamond) 5.5 0.23
GaN 3.5 0.35
SiC 3.0 0.41
GaP 2.24 0.55
GaAs 1.43 0.87
InP 1.29 0.96
Si 1.1 1.11
In x Ga 1-x As 0.48–0.73 1.70–2.60
GaSb 0.67 1.85
Ge 0.66 1.88
PbS 0.41 3.02
PbTe 0.32 3.88

available for many years. They show reasonable responsivity out to almost
2mm and have the big advantage of detection down to 0.6mm. This allows exper-
iments to be set up and their throughput optimized more conveniently with red
light, before switching to infrared light beyond 1mm. Although germanium
covers the 1.3- to 1.6-mm region so important to fiber optic communications,
this application is often better handled by another material, the ternary semi-
conductor indium gallium arsenide (InGaAs). Photodiodes formed of this mate-
rial can have higher responsivity than germanium, and much lower electrical
leakage currents. Recently a large choice of photodiodes formed in In xGa 1-xAs
has become available, driven by the fiber optic communications market. By
varying the proportion x in the semiconductor alloy, the sensitive range of these
devices can be tailored. In standard devices with x = 0.53 and bandgap E g =
0.73eV the response limits are 0.9mm and 1.7mm. By increasing x to 0.83 and
changing E g = 0.48eV the response can be shifted to 1.2 to 2.6mm. Figure 1.5
shows examples of both these responses. The advantage and disadvantage of
InGaAs detectors for free space beam experiments are their lack of significant
sensitivity in the visible, making visible source setups more difficult but cutting
down interference from ambient light. Some help can come from the use of near
infrared LED sources emitting at 0.94mm, which are detected both by silicon
and InGaAs devices.
Photodetectors are also available in several other materials. Gallium phos-
phide (GaP) offers a better match to the human eye response, especially the low
illumination level scotopic response. We can even avoid the use of the correc-
tion filters which must be used with silicon detectors for photometric mea-
surements. Gallium arsenide phosphide (GaAsP) is available both as diffused
and as metal-semiconductor (Schottky) diodes and is insensitive above 0.8mm.
Hamamatsu has a range of both these materials (e.g., G1962, G1126). Opto
Diode Corp. offers detectors of gallium aluminum arsenide (GaAlAs, e.g., ODD-
45W/95W), which show a response strongly peaked at 0.88mm, almost an

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