GaAs

Why does this shift occur? The physical explanation for this phenomenon is found in the variation that occurs into the semiconductor’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, the gap gets narrower –which means that less additional energy is required 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 does not carry 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’s temperature increases, less energy will be required to jump across the gap, and photons with less and less energy (longer and longer wavelengths) will be absorbed “by the band”, as they say.
Consequently, measuring the position of the absorption shift gives a measure of the semiconductor’s temperature. It is important to note that this technology is wavelength dependent instead of intensity dependent.