A near-infrared image sensor with monolithically integrated Ge photodiodes is demonstrated. The technology for the integration of the Ge photodiodes into the CMOS process is outlined, and the measurement results of test-diodes and the full imager are discussed in detail. The heterojunction-photodiodes show a quantum efficiency of about 30% up to a wavelength of 1500 nm. A tensile strain of 0.17% was measured in the epitaxial Ge layer, which is in good agreement with the optically measured direct bandgap absorption edge of 1580 nm. The image sensor can be operated at room temperature or with moderate cooling. (C) 2011 American Institute of Physics. [doi:10.1063/1.3608245]