**4. HgCdTe barrier detectors—principle of operation for higher operating temperatures**

The introduction of unipolar barrier in various photovoltaic configurations causes a drastic change in architecture and the principle of operation of IR detectors. The idea of unipolar barrier infrared detector (BIRD) implies that barriers can block one carrier type (electron or hole) but allow the unimpeded flow of the other. Assuming that Simple BIRD nBnn detector can be concluded as a hybrid of a photoconductor and a photodiode. The nBnn detector looks like a photodiode in a part except that the junction (space charge region) is replaced by a unipolar barrier (Bn: n-type doped barrier layer) blocking the electrons, whereas p-contact is replaced by the n-contact. The nBnn detector is nearly lacking the depletion region in an active layer, which leads to the reduction of SRH contribution to the net dark current. In low temperatures (below the crossover temperature), the nBnn detector should exhibit a higher signal-to-noise ratio in comparison with a conventional *p*-*n* photodiode operating at the same temperature and should operate at a higher temperature with the same dark current.

The idea of the nBnn detector was proposed for bulk III-V materials; however, its introduction to the second type of superlattices has allowed the implementation of the concept of nBnn with a greater control of arrangement of optimal band structure. Contrary to III-V materials, uniformly n-type-doped HgCdTe does not exhibit valence band offset (VBO) *≈* 0 eV between the absorber and the barrier (e.g., MWIR HgCdTe − VBO < 200 meV depending on both the absorber/barrier composition and doping, *T* = 200 K), which is a key limiting detector per‐ formance [26]. Depending on the wavelength of operation, a relatively high bias (so-called "turn on" voltage) is required to be applied to the device to collect the photogenerated carriers. This leads to strong BTB and TAT effects due to a high electric field at the barrier-absorber heterojunction.

Proper p-type doping at the cap barrier and barrier absorber heterojunctions should lower VBO in HgCdTe. However, p-type doping is the technological challenge posed by dopant activation after molecular beam epitaxy (MBE) growth. Metalorganic chemical vapor deposi‐ tion (MOCVD) technology is considered more favorable, which allows both *in situ* donor and acceptor doping. It seems to be more attractive in terms of the growth of pBpn and pBpp (Bp: p-type barrier) HgCdTe barrier structures. Barrier structures with p-type-doped constituent layers grown by MOCVD were presented by Kopytko et al. [27, 28].
