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Historically, the first barrier detector was proposed by A. M. White in 1983 as a high-impedance photoconductor. This detector postulates an n-type heterostructure with a narrow-bandgap absorber region coupled to a thin wide-bandgap layer followed by a narrow-bandgap contact region. A. M. White in his prescient patent also proposed a bias-selectable two-color detector realized and exploited currently in HgCdTe and in T2SL material systems. The barrier detector concept assumes almost zero one-band offset approximation throughout the heterostructure, allowing flow of only minority carriers in a photoconductor. Little or no valence band offset (VBO) was difficult to realize using standard infrared detector materials such as InSb and HgCdTe. The situation changed dramatically in the middle of first decade of the 21st century after the introduction of the 6.1-A III-V material detector family, and when the first high-performance detectors and FPAs were demonstrated. Introduction of unipolar barriers in various designs based on T2SLs drastically changed the architecture of infrared detectors.4 In general, unipolar barriers are used to implement the barrier detector architecture for increasing the collection efficiency of photogenerated carriers and reducing dark current generation without inhibiting photocurrent flow. The ability to tune the positions of the conduction and valence band edges independently in a broken-gap T2SL is especially helpful in the design of unipolar barriers.
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