Metal-Insulator-Semiconductor （MIS） capacitors were fabricated on In_0.74Al_0.26As/In_0.74Ga_0.26As/In_xAl_1-xAs heterostructure multilayer semiconductor materials. SiN_x and SiN_x/Al₂O₃ bilayer were applied as insulating layer to prepare MIS capacitors respectively. High-resolution transmission electron microscopy （HRTEM） and X-ray photoelectron spectroscopy （XPS） measurements indicated that， compared with SiN_x deposited by inductively coupled plasma chemical vapor deposition （ICPCVD）， Al₂O₃ deposited by atomic layer deposition （ALD） can effectively suppresses In₂O₃ at the interface between Al₂O₃ and In_0.74Al_0.26As. According to the capacitance-voltage （C-V） measurement result of MIS capacitors， the fast interface state density （D_it） of SiN_x/Al₂O₃/In_0.74Al_0.26As was one order of magnitude lower than that of SiN_x/In_0.74Al_0.26As. Therefore， it can be concluded that Al₂O₃ deposited by ALD as a passivation film can effectively reduce the interface state density between Al₂O₃ and In_0.74Al_0.26As， thereby reducing the dark current of p-In_0.74Al_0.26As/i-In_0.76Ga_0.24As/n-In_xAl_1-xAs photodiodes.