Thanks to its interesting mechanical and electrical properties, silicon represents the first candidate as a structural material in the Micro Electro, Mechanical Systems field. Doped polycrystalline silicon films are generally used, particularly when electrostatically movable mechanical structures are needed. Here, we investigate the effects of the doping type as well as the postdeposition thermal treatments on the mechanical behaviour of in situ doped polycrystalline silicon films deposited by low pressure chemical vapour deposition from a mixture of silane SiH4 and phosphine or diborane. Stress measurements, performed using micro-Raman spectroscopy, are related to the behaviour of micro fixed-fixed beams as determined from optical and scanning electron microscopy observations. The films, regardless of their doping type, are found tensely stressed when the amorphous deposited films are solid phase crystallised at 600 degreesC. The tensile stress is reduced becoming compressive when the crystallization temperature is increased. An optimum tensile stress value, corresponding to the maximum of the beam free length, is determined. Finally, air-gap thin film transistors (TFTs) using these doped fixed-fixed beams are realised. Electrical parameters of these TFTs (field effect mobility, threshold voltage, and subthreshold slope) may be considered as good. Particularly the low value of the threshold voltage, 2.5 V, is very interesting for handling devices where the power consumption saving is crucial.