We have successfully demonstrated the first CMOS-compatible monolithic epitaxial integration of GaN-based infrared (IR) detectors on Si. The device is a GaN/AlGaN quantum cascade detector (QCD) grown selectively in windows on a 4-in Si (111) substrate using plasma-assisted molecular beam epitaxy. The CMOS compatibility of the QCD growth method was verified by applying it to Si circuitry fabricated on (100) surface of a double hetero-oriented silicon on insulator (SOI) substrate. The photo signal, centered at a wavelength of 4.6 <inline-formula> <tex-math notation="LaTeX">\mu \text{m} </tex-math></inline-formula>, was measured at 110 K. The zero-bias responsivity of 81 <inline-formula> <tex-math notation="LaTeX">\mu \text{A} </tex-math></inline-formula>/W at 18 K and detectivity of <inline-formula> <tex-math notation="LaTeX">1.2\times 10^{{7}} </tex-math></inline-formula> Jones were recorded.