**5.3. LWIR HgCdTe barrier detectors**

**5.2. MWIR HgCdTe barrier detectors**

82 Modeling and Simulation in Engineering Sciences

ionization energies and capture coefficients (**Table 1**).

**Figure 3.** Measured current-voltage characteristics for MWIR HgCdTe (a) p+

**Figure 4.** Simulated band diagram for MWIR HgCdTe (a) p+

unbiased, and under 0.5 V reverse bias.

Auger processes.

Our original numerical program incorporates HgCdTe electrical properties to estimate MWIR device performance taking into account Auger, SRH, as well as BTB and TAT tunneling mechanisms. Two types of SRH centers were included in the model: metal site vacancies and dislocation related centers [29, 30]. The two types of centers are characterized by different

operated at 230 K. The experimental data are compared to the theoretical prediction considering overall GR effect and

BpnN+

**Figure 3** presents an example of simulated fitting characteristics of MWIR HgCdTe barrier detectors at a temperature of 230 K. The calculations were made taking into account all

and (b) p+

BpnN+ and (b) p+

BppN+

BppN+ photodetectors operated at 230 K,

barrier detectors

The commercially available APSYS platform (Crosslight Inc.) was implemented for LWIR device simulation procedures. The applied model incorporates HgCdTe electrical properties to estimate device performance taking into account Auger, SRH, as well as BTB and TAT tunneling mechanisms.

**Figure 5.** Measured current-voltage characteristics for the LWIR HgCdTe n+ p+ BppN+ barrier photodiode operated at 230 K. The experimental data are compared to the theoretical prediction considering Auger, SRH, and BTB/TAT mech‐ anisms.

The measured and calculated dark current characteristics of the LWIR HgCdTe n+ p+ BppN+ 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 bottom contact heterojunction.

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 was implemented [24].

**Figure 6.** Simulated band diagram for the LWIR HgCdTe n+ p+ BppN+ photodiode operated at 230 K: (a) unbiased and (b) under 0.5 V reverse bias.

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 connected to a bottom N+ -layer. The electrons are also excluded from the absorber near the 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.
