A high performance, bias tunable,p-GaAs homojunction interfacial workfunction internal photoemission far-infrared detector is demonstrated. A responsivity of3.10±0.05 A/W,a quantum efficiency of 12.5&percent;, and a detectivityD*of5.9×1010 cmHz/Wwere obtained at 4.2 K for cutoff wavelengths from 80 to 100 &mgr;m. The bias dependences of the quantum efficiency, detectivity, and cutoff wavelength were measured and are well explained by the theoretical model. The effect of the layer number on detector performance and the uniformity of the detectors are discussed. A comparison with Ge:Ga photoconductive detectors suggests that similar or even better performance may be obtainable with a far-infrared detector. ©1997 American Institute of Physics.