We report the study of a fabrication process and characterization of a thermal IR sensor based on silicon–germanium–boron alloys (a-SixGeyBz:H) deposited by plasma at low temperature. The sensor is an un-cooled micro-bolometer fabricated with surface micromachining techniques and is fully compatible with the CMOS technology. The temperature dependence of conductivity δ(T) was measured in the sensor in order to calculate the activation energy, Ea, the temperature coefficient of resistance, TCR and the room temperature conductivity, δRT. Current–voltage characteristics, I(U), in darkness and under IR radiation were measured in the device in order to calculate its current responsivity, RI. Spectral noise density was measured and the micro-bolometer detectivity, D* was calculated. The thermal time constant of the micro-bolometer was also measured.