IR imaging based on uncooled sensors

Since the early 1990s, infrared imaging has undergone a veritable revolution, with the development and availability of uncooled infrared detection arrays for the civilian market. Until then, the use of infrared imaging was reserved for the high-end military, space and civilian sectors. Indeed, the only detectors available were based on quantum-type detection, which requires a semiconductor material whose bandgap is adapted to the wavelength to be detected, as is the case with silicon for detection in the visible range. In the infrared, the energies to be detected are lower (around 0.1 eV for the spectral band centered at a wavelength of 10 µm), so the bandgap of the semiconductor material must be 0.1 eV or 100 meV. This low energy means that, at room temperature, the photonic current generated in the detection structure (photodiode) is completely masked by the thermal current. The only solution is to cool the photodetector to reduce the thermal current to a value that allows the photonic current to be read. These constraints (low bandgap semiconductor material and cooling machine) mean that quantum infrared detectors are expensive to develop and produce, and expensive to operate.

On the other hand, thermal detection, based on measuring the temperature rise of a material absorbing the incident infrared flux, no longer requires cooling to operate. The challenge is to integrate the functions required for this type of detection into pixels small enough to produce reasonably-sized two-dimensional retinas suitable for imaging applications. Developments in silicon-based microelectronics technologies have overcome these difficulties and enabled this revolution in infrared imaging.