A novel 48 micrometers cutoff wavelength ((lambda) c) Si far-IR (FIR) detector is demonstrated. Internal photoemission over a Si interfacial work-function of a homojunction consisting of molecular beam epitaxy grown multilayers is employed. The detector shows high responsivity over a wide wavelength range with a peak responsivity of 12.3 +/- 0.1 A/W at 27.5 micrometers and detectivity D* of 6.6 X 1010 cm (root) Hz/W at 4.2 K. The (lambda) c and bias dependent quantum efficiency agree well with theory. Based on the experimental results and the model, Si FIR detectors with high performance and tailorable (lambda) cs can be realized using higher emitter layer doping concentrations.