Background limited infrared hot-electron transistor at 77K

The dominant noise of a quantum well infrared photodetector (QWIP) is from the dark current density (JD) and the background photocurrent density (JB). The optimal detector is a background limited photodetector (BLIP, i.e. JB >> JD). For practical applications, the total current density (= JB+JD) must be compatible with the charge handling capacity of the read-out circuit.' The desirable operating temperature of the detector for thermal imaging is 77K. We report a BLIP infrared hot-electron transistor (IHET) with current density compatible with the read-out circuit at 77K. To reduce JD, we have designed a new GaAs/Al,Gal_,As QWIP, in which the aluminum molar ratio of the barriers increases in three steps. This barrier is able to suppress the dark current due to thermally assisted tunneling by providing a thicker effective barrier when the structure is under bias. The remaining J D is also largely eliminated by designing a band pass filter placed adjacent to the QWIP. The IHET is grown by MBE in the following sequence: Emitter -6000-A n' GaAs ( 1 . 2 ~ 1 0 ' ~ ~ m ~ ) ; QWIP -30 periods of 50-A nf-GaAs quantum well (1 .2x101' ~ m ~ ) , and three-step Al,Gal_,As barrier (x=0.28, 0.305, and 0.33; thickness: 161 A, 167 A, and 173 A); Base -300-A n+ Ino.ljGao.8;As and 200-A n+ GaAs ( 1 . 0 ~ 1 0 ' ~ ~ m ~ ) ; Energy band pass filter -40-A Alo.3Gao.~As, 50-A GaAs, 40-8, Alo.3Gao.~As, and 1999-A Alo.25Gao.85As; Collector -1 . l-pm nf GaAs ( 1 . 2 ~ 1 0 ' ~ ~ m ~ ) . At 77K, with VBE up to 2V, the dark current afetr filtering is a t least 10' times smaller than that of the typical QWIP. At VBE = 2V, JD is approximately equal to JB and is within the current limit of the read-out circuit, and %BLIP (= noise current due to JB / total noise current) is 70% under a high photon flux background. The detectivity, Dc* (after filtering) is measured to be 1.0~10'0 c m m / W . At lower bias, Dc* increases and at VBE = l V , Dc8 is