Role of <formula formulatype="inline"><tex Notation="TeX">${\\hbox{HfO}} _{2} $</tex></formula>/<formula formulatype="inline"><tex Notation="TeX">${\\hbox{SiO}}_{2}$</tex> </formula> Gate Dielectric on the Reduction of Low-Frequent Noise and the Enhancement of a-IGZO TFT Electrical Performance