ZnO thin film field effect transistors with 1.5-20 mu m gate width were fabricated using either a metal gate [metal-semiconductor field effect transistor (MESFET)] or a metal-oxide-semiconductor (MOS) gate. In both cases we found that use of a thick (similar to 0.8-0.9 mu m) ZnO buffer was necessary on the sapphire or glass substrate prior to growing the active layers in order to reduce gate leakage current. Source/drain contacts of e-beam deposited Ti/Al/Pt/Au showed specific contact resistances of 2.18 x 10(-6) Omega cm(2) without annealing and the interdevice isolation currents were similar to 10 mu A at 40 V bias. The MOS structure with 50 nm (Ce,Tb)MgAl11O19 gate dielectric showed a I order of magnitude lower gate leakage current than the MESFET, due to the relatively low barrier height of metals on n-type ZnO (0.6-0.8 eV). Good drain-source current characteristics were obtained from MOS gate structures using P-doped ZnO channels, whereas the metal structures showed very poor modulation. (c) 2005 American Vacuum Society.