We present the fabrication and characterization of a novel uncooled infrared sensor for room-temperature infrared imaging. The sensitive element of the sensor is a freestanding amorphous silicon thin film transistor (a-Si TFT)with the temperature coefficient of the drain current (TCC) of 0.015-0.08/K. The TCC value is sensitive to the ambient temperature and can be controlled by the gate voltage of the a-Si TFT. The complete procedures based on the porous silicon micromachining technique for fabricating thermally isolated air bridges are described. The isolation structures have a thermal conductance of 5 × 10-6 W/K and a thermal capacitance of 4.9 × 10-8 J/K.The effects of the gate voltage on the performance figures such as responsivity, noise voltage and detectivity are described and analysed in detail. The maximum detectivity reaches 4.33 × 108 cmHz1/2W-1 at a chopping frequency of 27Hz and a gate voltage of-15 V.