Development of Charge Sensitive Infrared Phototransistors for the Far-Infrared Wavelength

Ultra-highly-sensitive far-infrared detectors are developed for potential application to astronomy. The detectors exploit a novel mechanism called Charge Sensitive Infrared Phototransistors (CSIPs), in which an upper quantum well (QW) in GaAs/AlGaAs double QW structures is positively charged up by photo-excitation via inter-subband transition. This causes the conductance of the lower QW channel to increase. The device is effectively a phototransistor, in which the upper QW serves as a photo-sensitive gate to the source-drain channel provided by the lower QW. Resultant extraordinary high photoconductive gain makes CSIPs so sensitive as to detect single photons. CSIPs are well established in the mid-infrared ( λ = 12–20 μ m), achieving noise equivalent power around 1.9 × 10 - 19 W/Hz 1 / 2 with a quantum efficiency of 7 %. CSIPs have been demonstrated to work in longer wavelengths up to 45 μ m, but the sensitivity was not as high as in the shorter wavelengths, probably due to lower quantum efficiency. Reported here is a remarkable improvement in the performance of longer wavelength CSIPs (45 μ m), achieved primarily by optimizing the doping concentration in the upper QW. This work indicates that longer wavelength CSIPs are promising detectors for the astronomical application.