Image sensors perform better when cooled because the low temperature reduces the dark current, lowering the image sensor's noise.
The challenge of operating at a higher temperature 230 to 290 K (-40 to 25˚C) is to maintain a low dark current which can be achieved by maximising the minority carrier lifetime. To this end, SWIRup is developing two image sensor technologies.
InGaAs/GaAsSb type-II superlattice
A superlattice is a periodic structure of alternating layers of semiconductor materials. The project will use superlattice made up of materials (In0.53Ga0.47As and GaAs0.51Sb0.49) that are lattice-matched to the InP substrates.
Maintaining lattice-matching to the substrates is important in the image sensors targeted at space applications. This will avoid having excessive level of defects in the semiconductor materials, which are known to increase dark current and noise in the form of Random Telegraph Noise.
Depending on its exact composition, HgCdTe has a bandgap from 0.7 to 25 µm, thus covering the upper band of SWIR. It has large optical absorption coefficient and favourable recombination mechanisms for low dark currents.
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