**6. Summary**

p+ BppN+

The measured and calculated dark current characteristics of the LWIR HgCdTe n+

bottom contact heterojunction.

84 Modeling and Simulation in Engineering Sciences

was implemented [24].

(b) under 0.5 V reverse bias.

connected to a bottom N+

**Figure 6.** Simulated band diagram for the LWIR HgCdTe n+

photodiode are presented in **Figure 5**. It is shown that the most effective current transport mechanism in the LWIR HgCdTe barrier photodiode are tunneling effects at the decisive heterojunctions (especially TAT). **Figure 6** presents the calculated bandgap diagrams of the simulated structures for unbiased and under –0.5 V bias. Bandgap diagrams under reverse bias clearly idicates that the tunneling mechanism occurs at the absorber and highly doped

Tunneling between trap centers and valence and conduction bands are main reasons of increasing SRH processes with the increasing reverse bias voltage. In contrast to our numerical program, the APSYS platform does not distinguish trap centers between metal site vacancies and dislocation-related centers. The best fit of experimental data with theoretical predictions has been obtained for the trap density (*NT*) assumed at the level of 1014 cm−3 with ionization energy (*ET*) counted from the conduction at 1/3*Eg*. In TAT simulation, the Hurkx et al. model

> p+ BppN+

Dominant SRH process override Auger supression due to the exclusion and extraction effect. For a low reverse biases, up to the threshold voltage (–60 mV), an increase in the dark current is observed. In this voltage region, the differential resistance increases and at the final stage becomes infinite. Above the threshold voltage, the dark current decreases (the current-voltage characteristics exhibits a negative differential resistance) reaching the minimum value.

Under reverse bias, the electrons are extracted from the absorber region by positive electrode

barrier layer. The energy barrier between the cap layer and absorber regions blocks the electron flow from the cap layer. As a consequence, the hole concentration also decreases. The exclusion effect is limited by the level of acceptor concentration (electrical carrier neutrality), as well as

by thermal generation that restores the thermal equilibrium state.


photodiode operated at 230 K: (a) unbiased and

The current-voltage characteristics of MWIR and LWIR HgCdTe barrier detectors operating with Peltier cooling were investigated by computer simulations confronted with experimental data. Two numerical programs—our original program developed at the Institute of Applied Physics, Military University of Technology (MUT) and the commercially available APSYS platform (Crosslight Inc.)—have been used for modeling. Both programs are based on numerical solution of commonly known Poisson's and electron/hole current continuity equations. The applied model incorporates HgCdTe electrical properties to estimate device performance taking into account Auger, SRH, as well as BTB and TAT tunneling mechanisms. Due to reabsorption of photons generated by carrier recombination, also called the photon recycling, the radiative recombination is not assumed to limit the performance of HgCdTe photodetectors and was omitted in the simulations.

Typically, reverse biased infrared detectors are characterized by diffusion and tunnel-like dark current. At low reverse biases, the diffusion dark current is mainly limited by the Auger and SRH processes. At higher voltages, field-enhanced TAT via traps located at dislocation cores as well as mercury vacancies seems to be the most important mechanism of dark current generation, especially in the LWIR device. This mechanism is significant at the p-N+ interface, characterized by a large electric field.

The architecture of the LWIR n+ p+ BppN+ photodiode needs to be optimized to reduce the tunneling mechanisms at the p-N+ heterojunction. Numerical analysis is a proper tool to indicate the fields of improving device performances. For example, one possibility to reduce TAT is tuning of composition at interfaces aimed to locate highly dislocated region at a wider gap part of the p-N+ transition region where they have little effect on the dark current.

Numerical calculations have been performed for the structures of HgCdTe; however, the models may also be generalized for other semiconductor materials.
