Thin film transistors and arrays having controlled threshold voltage and improved ION/IOFF ratio are provided in this invention. In one embodiment, a thin film transistor having a first gate insulator of high breakdown field with positive fixed charges and a second gate insulator with negative fixed charges is provided; said negative fixed charges substantially compensate said positive fixed charges in order to reduce threshold voltage and OFF state threshold voltage of said transistor. In another embodiment, a thin film transistor having a first passivation layer with negative fixed charges is provided, the negative charges reduce substantially unwanted negative charges in the adjacent active channel and hence reduce the OFF state current and increase ION/IOFF ratio, which in turn reduce the threshold voltage of the transistor.