We report a metal-insulator-semiconductor (MIS) diode with an alpha-Ga2O3 thin-film insulator layer grown by liquid-phase deposition (LPD). alpha-Ga2O3 exhibits a high energy bandgap of 4.9-5.3 eV, which can effectively reduce the leakage current density and improve the breakdown voltage of the diode. The alpha-Ga2O3 thin films are synthesized from GaOOH with LPD. The alpha-GaOOH crystal is simply obtained by the dissociation of the Ga(OH)(3) precursor solution. GaOOH can be transformed into alpha-Ga2O3 crystal and form a uniform thin film following post-growth annealing. When the alpha-Ga2O3 thin film is inserted in between Ni and Si to form a Ni/alpha-Ga2O3/Si MIS diode, the barrier height of the diode increases by 0.4 eV and the on/off ratio by 100-fold from those of the Ni/Si Schottky diode. The Ni/alpha-Ga2O3/Si MIS diode exhibits a leakage current density of 1.07 x 10(-5) A/cm(2) under - 2V bias. The breakdown voltage of the diode reaches - 166 V without the guard ring and other insulation structures. Our results demonstrate that LPD-grown alpha-Ga2O3 thin films can obtain uniform and dense structure under short deposition time and at an annealing temperature of 400 degrees C. The uniform insulating layer of alpha-Ga2O3 has a high potential in enhancing the electrical characteristic of diodes and other power electronic devices.