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InP-photodiodes from different manufacturers have got rather low noise level, a good response
uniformity over the sensitive surface and a wide dynamic range. Therefore they are good
devices to built radiometers in the NIR spectral region. As in any photodiode, the spectral shortcircuit
responsivity is determined by the wavelength and the photodiode's reflectance and
internal quantum efficiency. Then if these quantities were known, the photodiode's responsivity
would be known without being compared to another standard radiometer; i. e. the photodiode
would be an absolute radiometer for optical radiation measurements.
This idea was firstly developed for silicon photodiodes in the eighties, once the technology was
able to produce low defects photodiodes. Following this reference, the reflectance could be
approached from a superimposed thin layers model. By knowing the thicknesses of the layers
and the optical constants of the materials, it is possible to determine the device reflectance.
However, this information is not completely available for InP photodiodes: the actual thickness
of the layers is not known and optical constants of materials are only approximately known for
bulk. Nevertheless it's possible to measure reflectance at some wavelengths and to fit the
thicknesses of a layer model that would reproduce those experimental values. The internal
quantum efficiency cannot be determined as in [1], since InP photodiodes are hetero-junctions
rather than homo-junctions as silicon photodiodes are. In the other hand, since the internal
structure is not accurately known, it is not possible to model the internal quantum efficiency
without having experimental values for it.
Therefore the attainable scope at present is just to obtain a model to be able to calculate spectral
responsivity values at any wavelength. To get this, a model has been developed to calculate
reflectance values from experimental ones at some wavelengths and another model has been
developed to interpolate spectral internal quantum efficiency values from some values got from
reflectance and responsivity measurements at some wavelengths. Both models will be presented
in this communication.
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