A new approach to the design of a Si‐based infrared detector is demonstrated, based on internal photoemission over a Si1−xGex/Si heterojunction barrier. The heterojunction internal photoemission device structure is grown by molecular beam epitaxy (MBE). The detector requires a degenerately dopedp+‐Si1−xGexlayer for strong infrared absorption and photoresponse. Doping concentrations to 1020cm−3are achieved using boron from a HBO2source during MBE growth of the Si1−xGexlayers. The photoresponse of this device is tailorable, and most significantly, can be extended into the long‐wavelength infrared regime by varying the Ge ratioxin the Si1−xGexlayers. Results are obtained withx=0.2, 0.28, 0.3, and 0.4 on patterned Si (100) substrates. Photoresponse at wavelengths ranging from 2 to 10 &mgr;m is obtained with quantum efficiencies above ∼1% in these nonoptimized structures.