Oxide semiconductors afford a promising alternative to organic semiconductors and amorphous silicon materials in applications requiring transparent thin film transistors (TFTs). We synthesized an aqueous inorganic precursor by a direct dissolution of zinc hydroxide in ammonium hydroxide solution from which a dense and uniform ZnO semiconducting layer is achieved. Solution-processed ZnO-TFTs prepared at 140 °C by microwave irradiation have shown enhanced device characteristics of ∼1.7 cm² V⁻¹s⁻¹ mobility and a ∼10⁷ on/off current ratio, with good air stability. Spectroscopic analyses confirmed that such a device improvement originates from accelerated dehydroxylation and better crystallization at low temperature by microwave irradiation. Our results suggest that solution-processable oxide semiconductors have potential for low-temperature and high-performance applications in transparent devices.