We describe a 9 &mgr;m AlGaAs/GaAs asymmetric quantum well infrared photodetector with voltage tunable spectral bandwidth. A very narrow spectral response of 9.2 meV (0.6 &mgr;m) full width half maximum is observed for an applied electric field of 28 kV/cm. The linewidth quadruples when the bias polarity is reversed, with very little shift in the peak detection wavelength. This structure is based on a conventional intersubband photodetector modified by using AlGaAs barriers that are graded in Al content and by adding a thin AlGaAs confinement layer on one side of the well. The asymmetry in the barriers is shown to give rise to the dependence of the spectral linewidth on applied bias. As well, a series of unusually well‐resolved and intense bound‐to‐continuum transitions are observed at low bias, that may indicate that the unique barrier shape also leads to enhanced electron interference effects at the well/barrier interfaces.