Which of the following is a commonly used material for IR detectors?

The choice of mercury-doped germanium as a commonly used material for infrared (IR) detectors stems from its favorable properties for detecting IR radiation. This material is known for its sensitivity to infrared light, making it particularly suitable for applications that require the detection of heat or thermal radiation.

Mercury-doped germanium has a bandgap that allows it to operate effectively in the infrared spectrum, enabling better performance in thermal imaging and night vision technologies. The doping process enhances the semiconductor's ability to absorb IR photons, thereby improving the efficiency of detection.

While silicon is indeed used in some IR applications, its performance is typically limited to near-infrared wavelengths. Copper and glass do not possess the necessary properties for effective IR detection. Copper, being a metal, reflects infrared radiation rather than absorbs it, and glass lacks the semiconductor characteristics required for the detection of IR wavelengths. Hence, mercury-doped germanium stands out as the most appropriate choice for IR detector materials.