We present transistors and inverters based on the MESFET principle. The channel consists of thin ZnO:Mg thin films on sapphire, deposited with pulsed laser deposition. The ohmic source and drain contacts are formed with sputtered gold. The Schottky gate electrode is formed by metal oxides providing high barrier height and a reliable contact. The voltage swing of our inverters is superior to any other reported oxide devices. Annealing studies show that our devices withstand temperatures up to 150°C, partly improving during annealing.