Si, GaAs and Ge materials:
At 300 K, the bandgaps of the semiconductor materials silicon (Si) and gallium-arsenide (GaAs) are 1.12 eV and 1.42 eV, respectively. For Si, this means that photons, having a wavelength greater than 900 nm, are not absorbed in the material. For GaAs, only photons with a wavelength greater than 700 nm are not absorbed in the material. Therefore, these materials, opaque in visible, are transparent for SWIR InGaAs wavelengths (900 - 1700 nm).
The bandgap of germanium (Ge), a semiconductor material often used for space solar cells, is 0.only 66 eV. Only photons having a wavelength greater than approximately 1600 nm are not absorbed. Therefore, a SWIR camera based on MCT (XEVA 2.5 320; wavelength response up to 2500 nm) has to be used for inspection of this wafer material.
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