narrowgap

Narrow-gap semiconductors are materials whose electronic band gap is small, typically less than about 1 eV. Because of the small gap, they strongly absorb infrared light and can conduct at moderate temperatures, enabling specialized optoelectronic devices and detectors.

Prominent examples include mercury cadmium telluride (HgCdTe), indium antimonide (InSb), indium arsenide (InAs), indium gallium arsenide (InGaAs), and the lead chalcogenides lead sulfide (PbS) and lead selenide (PbSe). The band gap of HgCdTe is tunable by adjusting mercury content and temperature, allowing access to mid- and long-wavelength infrared ranges; InSb and InAs have fixed narrow gaps suitable for infrared sensing and communication.

Properties and challenges: narrow-gap materials often exhibit high carrier mobility and strong infrared absorption, but they also suffer from high intrinsic carrier concentrations at room temperature, leading to high dark currents. This frequently requires cooling and careful material quality control; growth is complicated by lattice mismatch with substrates, requiring careful epitaxial techniques and buffer layers.

Applications: infrared imaging and thermal cameras, spectroscopy, gas sensing, free-space and fiber-optic communications in the near- to mid-infrared, and thermophotovoltaics. They are widely used in military, space, astronomy, and industrial sensing due to their sensitivity to infrared radiation.

Manufacturing: common growth methods include molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MOCVD), and liquid-phase epitaxy, often with buffer layers to manage lattice strain. Ongoing research aims to improve material quality, uniformity, and scalability for large-area detector arrays.