Long-wavelength HgCdTe photodiodes: n+-on-p versus p-on-n structures

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Abstract
The performance of long‐wavelength n+‐on‐p and p‐on‐n HgCdTe photodiodes is reexamined theoretically. It is assumed that the performance of photodiodes is due to thermal generation governed by the Auger mechanism. The influence of junction position on the R0A product, photoelectrical gain, and noise for both types of HgCdTe photodiodes operated at 77 K, with 0.1 eV base material, is considered. Especially, the R0A product as a function of cutoff wavelength and temperatures is analyzed in detail for both type of structures. For assumed doping concentrations in the base region of homojunctions (Na=5×1015 cm−3 for the n+‐on‐p structure and Nd=5×1014 cm−3 for the p‐on‐n structure), the influence of a p‐type cap layer on the effective R0A product is more serious for p‐on‐n structures in comparison with influence of an n+‐type layer on the R0A product for n+‐on‐p junctions. Therefore, to suppress the deleterious influence of cap layers (especially in the case of p‐on‐n junctions), a wider band‐gap cap layer is necessary. For a given cutoff wavelength, the theoretical values of the R0A product for p‐on‐n photodiodes in the temperature range below 77 K are a little greater than for n+‐on‐p photodiodes, which is due to lower doping. In the higher temperature range for very long‐wavelength photodiodes, p‐type base devices are more advantageous. Results of calculations are compared with experimental data reported by other authors.

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