Metal-Insulator-Semiconductor (MIS) capacitors were fabricated on In0.74Al0.26As/In0.74Ga0.26As/InxAl1-xAs heterostructure multilayer semiconductor materials. SiNx and SiNx/Al2O3 bilayer were applied as insulating layer to prepare MIS capacitors respectively. High-resolution transmission electron microscope (HRTEM) and X-ray photoelectron spectroscopy (XPS) measurements indicated that, compared with SiNx 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 In0.74Al0.26As. According to the capacitance-voltage (C-V) measurement result of MIS capacitors, the fast interface state density (Dit) of SiNx/Al2O3/In0.74Al0.26As is one order of magnitude lower than that of SiNx/In0.74Al0.26As. The above results are calculated using the high-low frequency method. Therefore, it can be concluded that Al2O3 deposited by ALD as a passivation film can effectively reduce the interface state density between Al2O3 and In0.74Al0.26As, thereby reducing the dark current of p-In0.74Al0.26As/i-In0.76Ga0.24As/n-InxAl1-xAs photodiodes.