Infrared photodetectors are critical components in military, astronomical, and industrial applications due to their ability to detect electromagnetic radiation in the infrared spectrum. Among the most prominent technologies are mercury cadmium telluride (HgCdTe) detectors, indium antimonide (InSb) detectors, and quantum well infrared photodetectors (QWIPs). Each of these technologies offers distinct advantages in terms of spectral range, cooling requirements, and performance characteristics, making them suitable for specific use cases.

HgCdTe detectors are among the most widely used infrared photodetectors due to their tunable bandgap, which allows optimization for different infrared wavelengths. By adjusting the composition of mercury and cadmium in the alloy, the cutoff wavelength can be tailored from near-infrared (1–3 µm) to very long-wavelength infrared (VLWIR, beyond 14 µm). This flexibility makes HgCdTe suitable for a broad range of applications, including thermal imaging, missile guidance systems, and astronomical observations. However, HgCdTe detectors typically require cooling to reduce dark current and improve signal-to-noise ratio. For mid-wave infrared (MWIR, 3–5 µm) and long-wave infrared (LWIR, 8–14 µm) operation, cooling to 77 K using liquid nitrogen or mechanical coolers is common. In VLWIR applications, even lower temperatures may be necessary. Despite their high performance, HgCdTe detectors are expensive to produce due to material challenges such as uniformity and defect control.

InSb photodetectors are another key technology, primarily operating in the MWIR range (3–5 µm). InSb offers high quantum efficiency, fast response times, and excellent uniformity, making it ideal for high-performance imaging systems. These detectors are widely used in military applications, including target tracking and surveillance, as well as in astronomy for observing thermal emissions from celestial objects. Like HgCdTe, InSb detectors require cryogenic cooling, typically to around 77 K, to minimize thermal noise. While InSb excels in MWIR detection, its spectral range is narrower compared to HgCdTe, limiting its use in LWIR applications. Nevertheless, its well-established fabrication processes and reliable performance make it a preferred choice for many MWIR systems.

Quantum well infrared photodetectors (QWIPs) represent a different approach, leveraging intersubband transitions in semiconductor quantum wells to detect infrared radiation. QWIPs are typically based on gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs) heterostructures, which offer advantages in terms of material uniformity and large-area fabrication. Unlike HgCdTe and InSb, QWIPs are primarily sensitive to LWIR radiation (8–12 µm), making them suitable for thermal imaging applications. One of the key benefits of QWIPs is their compatibility with standard semiconductor manufacturing processes, which reduces production costs compared to HgCdTe. However, QWIPs have lower quantum efficiency than HgCdTe detectors and require cooling to similar temperatures (around 70 K) for optimal performance. Additionally, QWIPs are inherently polarization-sensitive due to the selection rules governing intersubband transitions, which can be a limitation in some applications.

Cooling requirements are a critical consideration for all three detector technologies. The need for cryogenic cooling arises from the thermal generation of charge carriers, which increases noise and reduces sensitivity at higher temperatures. Stirling coolers, Joule-Thomson coolers, and liquid nitrogen dewars are commonly used to maintain the necessary operating temperatures. The choice of cooling system depends on factors such as power consumption, size constraints, and mission duration. For space-based applications, where power and weight are at a premium, passive radiative cooling or advanced mechanical coolers are often employed.

Military applications of infrared photodetectors are extensive, encompassing surveillance, night vision, missile seeker heads, and threat detection. HgCdTe detectors are frequently used in high-performance thermal imaging systems due to their broad spectral coverage and high sensitivity. InSb detectors are favored for MWIR applications where fast response and high resolution are critical. QWIPs, while less common in military systems compared to HgCdTe, are used in large-format focal plane arrays for persistent surveillance and reconnaissance.

In astronomy, infrared detectors enable the study of cold cosmic objects, dust-obscured star-forming regions, and exoplanets. HgCdTe detectors are widely used in ground-based and space telescopes due to their sensitivity across multiple infrared bands. Instruments such as the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space Telescope have employed HgCdTe arrays. InSb detectors are used in specialized instruments for MWIR observations, while QWIPs have been explored for large-format LWIR imaging in next-generation telescopes.

Industrial applications include non-destructive testing, gas sensing, and process monitoring. HgCdTe detectors are used in Fourier-transform infrared (FTIR) spectrometers for chemical analysis, while InSb detectors find use in industrial thermal imaging for equipment inspection. QWIPs are less common in industrial settings but have potential in low-cost thermal cameras for preventive maintenance.

The choice between HgCdTe, InSb, and QWIPs depends on the specific requirements of the application, including spectral range, cooling constraints, and cost considerations. HgCdTe offers unparalleled versatility and sensitivity but at a higher cost. InSb provides excellent MWIR performance with established reliability. QWIPs present a cost-effective solution for LWIR detection but with trade-offs in efficiency and polarization sensitivity. Advances in material growth, device architecture, and cooling technologies continue to push the boundaries of infrared detection, enabling new capabilities in defense, astronomy, and beyond.