The materials of extended wavelength InxGa1-xAs photodetectors (50% cut-off wavelength of 2.4μm at room temperature) with three different structures were grown by using gas source molecular beam epitaxy (GSMBE) and were processed into mesa type photodetectors. Surface morphology, x-ray diffraction rocking curve and photolnminescence measurements show that the quality of materials is obviously improved by using digital graded superlattice at the InAlAs/InGaAs heterointerfaces . The dark current at reverse bias of 10mV for the 300μm-diameter mesa type photodetectors without digital graded superlattice is 0.521μA at room temperature, however it is reduced to 0.480μA for photodetectors with digital graded superlattice. Besides, the growth of an InP buffer layer between InP substrate and InxAl1-xAs linear graded buffer layer is also beneficial to the material quality and device performance.