  We describe a novel GaAs/AlGaAs double-quantum-well device for the infrared
photon detection, called Charge-Sensitive Infrared Phototransistor (CSIP). The
principle of CSIP detector is the photo-excitation of an intersubband
transition in a QW as an charge integrating gate and the signal amplification
by another QW as a channel with very high gain, which provides us with
extremely high responsivity (10^4 -- 10^6 A/W). It has been demonstrated that
the CSIP designed for the mid-infrared wavelength (14.7 um) has an excellent
sensitivity; the noise equivalent power (NEP) of 7x10^-19 W/rHz with the
quantum efficiency of ~2%. Advantages of the CSIP against the other highly
sensitive detectors are, huge dynamic range of >10^6, low output impedance of
10^3 -- 10^4 Ohms, and relatively high operation temperature (>2K). We discuss
possible applications of the CSIP to FIR photon detection covering 35 -- 60 um
waveband, which is a gap uncovered with presently available photoconductors.
