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In a conventional photodiode, the responsivity and diffusion length are closely coupled and an increase in the absorber thickness beyond the diffusion length may not result in the desired improvement in the signal to noise ratio (SNR). This effect is particularly pronounced at high temperatures, where diffusion lengths are typically reduced. Only charge carriers that are photogenerated at a distance shorter than the diffusion length from a junction can be collected. In HOT detectors the absorption depth of LWIR radiation is longer than the diffusion length. Therefore, only a limited fraction of the photogenerated charge contributes to the quantum efficiency. To avoid the limitation imposed by the reduced diffusion length and to effectively increase the absorption efficiency, innovative detector designs based on multistage detection and currently termed as cascade infrared detectors (CIDs) have been introduced in the last decade. CIDs contain multiple discrete absorbers, where each one is shorter or narrower than the diffusion length. In this discrete CID absorber architecture, the individual absorbers are sandwiched between engineered electron and hole barriers to form a series of cascade stages. The photogenerated carriers travel over only one cascade stage before they recombine in the next stage, and every individual cascade stage can be significantly shorter than the diffusion length, while the total thickness of all of the absorbers can be comparable or even longer than the diffusion length. In this case, the SNR and the detectivity will continue to increase with an increasing number of discrete absorbers, resulting in improved device performance at elevated temperatures compared to a conventional p-n photodiode. In addition, the flexibility to vary the number and thicknesses of the discrete absorbers results in the ability to tailor the CID designs for optimized performance in meeting specific applications.
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