We have developed a high sensitivity far-infrared (FIR) photodetector by using the two-dimensional electron gas system in modulation-doped Al0.3Ga0.7As/GaAs heterojunctions. A FIR-sensitive channel was formed by applying a negative bias voltage to the split gates which were integrated with a log-periodic antenna. Although the area of sensitivity was small (850×850 μm2), the responsivity reached as high as 5.8×104 V/W. It is also demonstrated that the responsivity can be modulated by a factor of 10 by changing the split-gate voltages. The capability of ON/OFF switching of photoresponsivity suggests that this detector is suitable for realizing high sensitivity FIR imaging arrays. © 2002 American Institute of Physics.