In this work we present results of a study performed on metal-insulator-semiconductor (MIS) diodes with the following structure: substrate (glass)/Cr (2000 Å)/a-Si:H n+(400 Å)/a-Si:H i (5500 Å)/oxide (0-40 Å)/Au (100 Å) to determine the influence of the oxide passivation layer grown by different techniques on the electrical performance of MIS devices. The results achieved show that the diodes with oxides grown using hydrogen peroxide present higher rectification factor (2×106) and signal to noise (S/N) ratio (1×107 at -1 V) than the diodes with oxides obtained by the evaporation of SiO2, or by the chemical deposition of SiO2 by plasma of hexamethyldisiloxane. However, in the case of deposited oxides, the breakdown voltage is higher, 30 V instead of 3-10 V for grown oxides. The ideal oxide thickness, determined by spectroscopic ellipsometry, is dependent on the method used to grow the oxide layer and is in the range between 6 and 20 Å. The reason for this variation is related to the degree of compactation of the oxide produced, which is not relevant for applications of the diodes in the range of ±1 V, but is relevant when high breakdown voltages are required.