An enhanced original computer program is applied to explain in detail the current-voltage characteristics of n-on-p long wavelength infrared HgCdTe photodiodes. The computer program solves the system of nonlinear continuity equations for carriers and Poisson equations. In the model ideal diode diffusion, generation-recombination, band-to-band tunneling, trap-assisted tunneling, and impact ionization are included as potential limiting mechanisms in the photodiodes. It is a clearly explained influence of Hg-vacancy doping and extrinsic doping of an active device region on dark current-voltage characteristics and an R0A product of HgCdTe photodiodes in a wide region of temperature and wavelengths. Special attention is directed to the dependence of tunneling probability on the shape of potential barrier with in an depletion region. The theoretical predictions are compared with experimental data of high quantity photodiodes published in the available literature.

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The effect of dislocation on the 1/f noise current in long-wavelength infrared (LWIR) reverse biased HgCdTe photodiodes working at liquid nitrogen (LN) temperature was analyzed theoretically by using a phenomenological model of dislocations as an additional Shockley-Read-Hall (SRH) generation-recombination (G-R) channel in heterostructure. Numerical analysis was involved to solve the set of transport equations in order to find a steady state values of physical parameters of the heterostructure. Next, the set of transport equations for fluctuations (TEFF) was formulated and solved to obtain the spectral densities (SD) of the fluctuations of electrical potential, quasi-Fermi levels, and temperature. The SD of mobility fluctuations, shot G-R noise, and thermal noise were also taken into account in TEFF. Additional expressions for SD of 1/f fluctuations of the G-R processes were derived. Numerical values of the SD of noise current were compared with the experimental results of Johnson et al. Theoretical analysis has shown that the dislocations increase the G-R processes and this way cause the growth of G-R dark current. Despite the fact that dislocations increase both shot G-R noise and 1/f G-R noise, the main cause of 1/f current noise in LN cooled LWIR photodiodes are fluctuations of the carriers mobility determined by 1/f fluctuations of relaxation times. As the noise current is proportional to the total diode current, growth of G-R dark current caused by dislocations leads to the growth of noise current.

The dark current of near-room-temperature long-wavelength heterojunction photodiodes was studied. The dark current of the devices is much greater than that calculated from the Auger generation mechanisms. A model of trapassisted tunneling via traps located at dislocation cores is proposed as the mechanism of enhanced thermal generation of charge carriers in reversebiased diodes. Field-induced reduction of trap activation energies can increase thermal generation and create conditions for tunneling currents. The model qualitatively explains experimental currentÀvoltage characteristics of the diodes assuming a dislocation density of approximately 10 8 cm À2 at the graded gap interface between absorber and contact regions of the photodiode.

The effect of an abrupt CdTe∕HgCdTe passivation heterointerface on generation recombination and dark currents in n-on-p midwave infrared photodiodes with 5.2-μm cut-off wavelength has been investigated. Experimentally, it was observed that the zero-bias-dynamic resistance, R0, at low temperatures scales with the perimeter of the n-on-p junction, rather than with the junction area, suggesting that surface effects are dominant. The diode current–voltage characteristics at low temperatures indicate significant contributions from tunneling effects, which is the dominant leakage current mechanism for reverse bias greater than approximately 30mV. These two observations suggest that the region where the junction terminates at the CdTe∕HgCdTe abrupt interface is responsible for the above effects. A two-dimensional model has been developed to investigate the dark current mechanisms in the vicinity of the junction termination at CdTe∕HgCdTe interface, which also takes into account the effect of dislocations on generation-recombination processes. Calculated profiles of the energy bands and electric field along different cross sections of the photodiode indicate that the electric field achieves a maximum value of the order of mid −105V∕cm in the region where the junction terminates at the CdTe∕HgCdTe interface. The presence of such a high localized electric field in this area decreases the ionization energy of trap levels in the band gap and, hence, increases efficiency of the Shockley–Read–Hall generation-recombination processes. In addition to diffusion, generation recombination, and trap-assisted tunneling mechanisms, the calculations of dynamic resistance include Poole–Frenkel and phonon-trap-assisted tunneling effects. The best fit to the zero bias dynamic resistance versus temperature results has been obtained using an aerial dislocation density in the bulk of the HgCdTe layer equal to 106cm−2. Although the direct band-to-band tunneling and impact ionization processes were also considered, their contributions are shown to be insignificant.