A long‐wavelength infrared detector is demonstrated using Si1−xGex/Si multiple quantum wells for the first time. A broad peak in the photoresponse is observed near 9 &mgr;m with a full width at half maximum of about 80 meV which provides the response in the 6–12 &mgr;m range. The position of the peak in the photoresponse is in good agreement with that observed in the absorption measurement using a waveguide geometry. A peak responsivity of about 0.3 A/W and detectivity ofD*=1×109cm &sqrt;Hz/W at 77 K are achieved. This suggests the possibility of monolithic integration of Si1−xGex/Si multiple quantum well detectors with signal processing electronics for potential focal plane array applications.