Comparison of ZnO metal–oxide–semiconductor field effect transistor and metal–semiconductor field effect transistor structures grown on sapphire by pulsed laser deposition

ZnO thin film field effect transistors with 1.5–20μm 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 (∼0.8–0.9μm) 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 2.18×10−6Ωcm2 without annealing and the interdevice isolation currents were ∼10μA at 40V bias. The MOS structure with 50nm (Ce,Tb)MgAl11O19 gate dielectric showed a 1 order of magnitude lower gate leakage current than the MESFET, due to the relatively low barrier height of metals on n-type ZnO (0.6–0.8eV). Good drain–source current characteristics were obtained from MOS gate structures using P-doped ZnO channels, whereas the metal structures showed very poor modulation.