We have designed and demonstrated a narrow bandpass filter, which is composed of a double‐barrier structure, for an infrared hot‐electron transistor with a cutoff wavelength at 14 &mgr;m. The filter is capable of suppressing the low energy thermally assisted tunneling current as well as the high energy thermionic emission current that are not degenerate with the photoelectrons. As a result, the detector can be operated at 77 K with a 45% BLIP level and an estimatedNE&Dgr;Tof 38 mK. ©1995 American Institute of Physics.