The detectivity characteristics of interband cascade infrared type-II superlattice detectors for long-wave infrared detection (λcut-off = 8 μm at room temperature) are discussed. We present comparison of two superlattices: InAs/GaSb and InAs/InAsSb, assuming the characteristic parameters—absorption coefficients α and carrier lifetimes τ published in literature. Dependence of the Johnson-noise limited detectivity on the absorber thickness for a different number of stages is reported. Higher detectivity D* value can be achieved by increasing the carrier lifetime. However, for detectors based on type-II InAs/GaSb superlattice increasing the carrier lifetime up to 25 ns leads to a situation in which one stage is preferred, i.e. for detector with a single absorber, we obtain the highest detectivity value. In the case of InAs/InAsSb material, the situation is similar for τ ≥ 80 ns. We have shown that the optimal absorber thickness at which the highest detectivity values are obtained depends not only on the absorption coefficient α and the number of stages NS, but also on the carrier diffusion length L. According to a calculations, cascade detectors based on Ga-free material should have much higher optimal absorber thicknesses than materials based on InAs/GaSb.
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