Charge-sensitive infrared phototransistors (CSIPs) with a 16×4 μm2 active area, which are fabricated in a GaAs/AlGaAs double-quantum-well structure, are studied with an all-cryogenic spectrometer operated at 4.2 K. Extremely low level of background radiation makes reliable determination of detector characteristics at 4.2 K possible: The detection band is found to be centered at the wavelength λ=14.7 μm with a bandwidth (full width at maximum) Δλ=1 μm. The quantum efficiency (η), the current responsivity (R), the noise equivalent power (NEP), and the specific detectivity (D∗) are derived to be η=(2±0.5)%, R=4×104–4×106 A/W, NEP≅6.8×10−19 W/Hz1/2, and D∗≅1.2×1015 cm Hz1/2/W. The dynamic range of detection is demonstrated to exceed 106 (approximately attowatts to picowatts), but the upper limit of the radiation power is limited by the radiation source intensity. The intrinsic dynamic range of the detector is suggested to reach 1013 (approximately attowatts to microwatts). The detection speed is suggested to be around 3 ns (300 MHz). The sensitivity of CSIPS is so high that single-photon signals are discerned in the photocurrent as stepwise increases in given amplitude. The value of D∗ is by a few orders of magnitude higher than that of the state-of-the-art multi-quantum-well infrared photodetectors. The extremely high sensitivity will open up the possibility of developing ultrahigh-speed imaging and/or ultrahigh-resolution passive microscopy system in the long wavelength infrared region.

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The performance of charge-sensitive infrared phototransistors (λ∼14.7 μm) is studied at temperatures of up to 30 K. The devices, with a 16×4 μm2 photoactive area, are fabricated in GaAs/AlGaAs double-quantum-well structure. An excellent specific detectivity D∗=9.6×1014 cm Hz1/2/W is derived in a T range of up to T=23 K. Experimental results are theoretically studied based on WKB approximation, in which photogenerated holes in the floating gate (FG) are recombined with thermal emission or thermally assisted tunneling from the outside of FG through the barriers. The model well reproduces the experimental results, including the vanishing of photosignal at 30 K under 280 fW incident radiation. The model is used to predict a temperature-dependent specific detectivity D∗ in ideal devices free from 1/f noise.

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