Highly sensitive photodetectors for the mid infrared have been obtained by placing a photodiode inside a Fabry Perot cavity. These resonant cavity enhanced detectors (RCED) are sensitive at the resonances only, which depend on the distance between the two mirrors of the cavity. Displacing one of these mirrors allows changing the cavity length and thus selecting the detection wavelength. The design, simulation and fabrication of a MEMS mirror and its integration with the counter mirror and the photodiode grown by molecular beam epitaxy are presented. First results with external mirrors moved by piezoactuation are described, too.