We investigated the behavior of the dark current (Id) in quantum well infrared photodetectors (QWIPs) in which the barrier layers were selectively doped instead of the well layers. Because the selective doping bends the conduction band (CB) edge in the portion of the barrier near the interface, the mechanism by which carriers in the wells can be emitted over the barriers, i.e. thermal emission and tunneling through this portion of the barrier, could be emphasized. We first confirmed that selectively doping the barrier layers clearly affects the Id-V characteristics. Then, by evaluating the activation energy obtained from the temperature dependence of Id, we found that the Poole-Frenkel emission (PFE) mechanism and the thermal-assisted tunneling (TAT)-like mechanism are dominant in the lower bias and higher bias regions, respectively.
We have investigated the reverse current-voltage characteristics of both 3 - 5 micrometer and 8 - 10 micrometer band HgCdTe photodiodes under background illumination in the temperature range of 40 K to 120 K. The experimental results show that the differential resistance of reverse biased photodiodes decreases with an increase in the incident photon flux density. In the higher reverse biased illuminated photodiodes, breakdown occurs. The reverse differential resistance is limited by this breakdown mechanism. In order to determine the causal mechanism, we calculated the photocurrent multiplication resulting from the electron impact ionization in the photodiode depletion region. For the calculation, we applied Shockley's 'lucky electron' equation assuming that only electrons multiply the photocurrent. The calculated reverse differential resistance-voltage characteristics agrees well with the measured results. The calculated differential resistance at a low reverse bias voltage as a function of incident photon flux density corresponds to the measured results in the temperature range of 40 K to 100 K. This indicates that the photocurrent multiplication by electron impact ionization occurs in the photodiode depletion region. In the low temperature region, the measured differential resistance increases with the decrease in temperature. This is caused by the acceptor freeze-out effects. We concluded that the differential resistance-voltage characteristics of HgCdTe photodiodes are limited by the effect of photocurrent multiplication, and the photomultiplication effect is limited by the carrier freeze-out in the low temperature region.