We assessed the effects of ultra dry-air plasma surface treatments on the properties of Al2O3-GPTMS-PMMA hybrid dielectric layers for applications to high-performance amorphous Indium Gallium Zinc Oxide (a-IGZO) thin film transistors (TFTs). The hybrid layers were deposited by an easy dip coating sol-gel process at low temperature and then treated with dry-air plasma at 1, 2 and 3 consecutive cycles. Their properties were analyzed as a function of the number of plasma cycles and contrasted with those of the untreated ones. The dielectric characteristics of the hybrid layers were determined from I-V and C-f measurements performed on metal-insulator-metal and metal-insulator-semiconductor devices. The results show that the plasma treatments increase the surface energy and wettability of the hybrid films. There is also a reduction of the OH groups and oxygen vacancies in the hybrid network improving the dielectric properties. The incorporation of nitrogen into the hybrid films surface is also observed. The plasma-treated hybrid dielectric layers were applied as dielectric gate in the fabrication of a-IGZO TFTs. The best electrical performance of the fabricated TFTs was achieved with the 3 cycles plasma-treated hybrid dielectric gate, showing high mobility, 29.3 cm2 V-1 s-1, low threshold voltage, 2.9 V, high I ON/OFF current ratio, 106, and low subthreshold swing of 0.42 V dec-1.