An infrared sensor unit has a thermal infrared sensor and an associated semiconductor device commonly developed on a semiconductor substrate. A dielectric top layer covers the substrate to conceal the semiconductor device formed in the top surface of the substrate. The thermal infrared sensor carried on a sensor mount which is supported above the semiconductor device by means of a thermal insulation support. The sensor mount and the support are made of a porous material which is superimposed on top of the dielectric top layer.