Charge-sensitive infrared phototransistors: Characterization by an all-cryogenic spectrometer

Charge-sensitive infrared phototransistors (CSIPs) with a 16x4 mu m(2) 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 lambda=14.7 mu m with a bandwidth (full width at maximum) Delta lambda=1 mu m. The quantum efficiency (eta), the current responsivity (R), the noise equivalent power (NEP), and the specific detectivity (D(*)) are derived to be eta=(2 +/- 0.5)%, R=4x10(4)-4x10(6) A/W, NEP congruent to 6.8x10(-19) W/Hz(1/2), and D(*)congruent to 1.2x10(15) cm Hz(1/2)/W. The dynamic range of detection is demonstrated to exceed 10(6) (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 10(13) (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. (C) 2008 American Institute of Physics.