narrowbandgap

Narrow-bandgap refers to semiconductors with a small energy difference between the valence-band maximum and the conduction-band minimum, known as the bandgap. Typically these energies are below about 1 to 1.5 eV, placing absorption in the near- to mid-infrared and resulting in higher intrinsic carrier concentrations at room temperature, which increases dark current and can require cooling or other thermal management in devices.

Common narrow-bandgap materials include indium gallium arsenide (InGaAs), indium arsenide (InAs), gallium antimonide (GaSb), germanium (Ge), and mercury cadmium telluride (HgCdTe). The bandgap of HgCdTe can be tuned by adjusting mercury content to cover a broad infrared range. Other systems, such as lead salts (PbSnTe) and various IV–VI compounds, as well as engineered structures like quantum wells and quantum dots, are also used to realize narrow gaps.

Applications are dominated by infrared detection and imaging, including uncooled and cooled infrared cameras, spectroscopy, and night-vision devices. Narrow-bandgap materials are also explored for infrared photovoltaics and laser diodes, with bandgap engineering through alloying, strain, or quantum confinement used to tailor absorption and operating wavelength.

Challenges include material quality and lattice matching in heterostructures, defects that increase nonradiative recombination, and intrinsic dark current at higher temperatures. Some narrow-bandgap materials raise environmental and handling concerns, such as toxicity in Hg-containing compounds. Research aims to improve crystal growth, interfaces, and compatibility with other semiconductor platforms.