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 1.5-20 μm 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 (∼0.8-0.9 μm) 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 TiAlPtAu showed specific contact resistances of 2.18× 10-6 Ω cm2 without annealing and the interdevice isolation currents were ∼10 μA at 40 V bias. The MOS structure with 50 nm (Ce,Tb) Mg Al11 O19 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 (0.6-0.8 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.