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EasyGrid User Guide
105
14 Gallium Arsenide Principle
14.1 Gallium Arsenide Theory
14.1.1 The Principle
FISO
’s fiber
-optic temperature sensors,
for high voltage temperature measurement,
are based on white
light absorption/transmission by a GaAs (gallium arsenide) semiconductor. The effects of temperature
variations on this semiconductor are well known and predictable. As the temperature of the
sem
iconductor increases, the semiconductor’s transmission spectrum (i.e. the light that is not absorbed)
shifts to higher wavelengths. At any given temperature, transmission jumps from essentially 0% to 100%
at a specific wavelength. This jump is called the absorption shift, and the relationship between the
specific wavelength where the absorption shift takes place and temperature is predictable.
Why does this shift occur? The
physical explanation for this
phenomenon is found in the
variation in the semicond
uctor’s
energy band gap.
This “gap” refers to
the energy required to bump the
electrons in the material into an
excited state (as opposed to the
relaxed, steady state). As more
energy enters the semiconductor, in
the form of heat as its temperature rises, this gap gets narrower
—
less additional energy is needed to
excite an electron. The photons (particles of light) entering the semiconductor are what actually excite
the electrons. If a photon is carrying enough energy to get an electron across the gap, it will be absorbed.
If it is not carrying enough energy, then it will be transmitted.
The shorter a photon’s wavelength is, the
more energy it carries. Since the band gap narrows as the semiconductor temperature increases and less
energy is needed o jump across the gap, photons with less and less energy (longer and longer
wavelengths) will be absorbed “by the band”, as they say. The effect is to move the absorption shift
to
longer wavelengths.
Consequently, measuring the position of the absorption shift gives a measure of the semiconductor’s
temperature. It is important to note that technology isn’t intensity dependent but wavelength
dependent.