Reducing the cooling requirements of mid-wave IR detector arrays

The mid-wave IR (MWIR) band in the atmosphere is important for thermal sensing because it spans the optical wavelengths (3–5 m) at which all room-temperature objects emit significant quantities of electromagnetic radiation. Detectors that operate in this range are useful for security applications such as night vision cameras. High-performance photodiode focal plane array (FPA) MWIR detectors are usually made from the semiconductors mercury cadmium telluride (HgCdTe) or indium antimonide (InSb). However, a common limiting factor of detector performance is dark current, which arises from the thermal excitation of charge carriers across the semiconductor bandgap. Reducing dark current is key to advancing MWIR detector performance. A detector is described as operating in the diffusion limit if the dark current results from minority carriers—such as holes in an n-type semiconductor—that are excited in the photon-absorbing active layer and diffuse with Brownian-like motion to the collecting contact. A diffusion-limited current can be achieved in the best HgCdTe FPAs, which are typically grown on expensive cadmium zinc telluride substrates.1 In contrast, even the best InSb FPAs are generation-recombination (G-R) limited.2 In this limit, Shockley-Read-Hall traps3, 4—created by imperfections in the semiconductor crystal lattice—provide energy states that lie in the semiconductor bandgap. That is, they act as ’stepping stones’ for thermally excited electrons and holes to pass through. In the depletion region—a thin layer at the diode p-n junction—a built-in electric field exists, which separates the Figure 1. Indium antimonide integrated detector/cooler assembly. The gold shield (top) is the upper part of the Dewar assembly, which contains the detector. The large round engine (right) is part of the Stirling cooler.