ZnO-based metal-semiconductor field-effect transistors with Ag-, Pt-, Pd-, and Au-Schottky gates

Metal-semiconductor field-effect transistors (MESFETs) were fabricated by reactive dc sputtering of either Ag, Pt, Pd, and Au as Schottky gate contacts on ZnO thin films grown by pulsed-laser deposition on a-plane sapphire substrates. The individual properties and influences of the four gate metals on the performance of the MESFETs have been investigated. Pt- and Ag-gate MESFETs show excellent electric properties with on/off-ratios of 4.5 × 106 and 1.1 × 108, respectively, and low off-currents in the picoampere range. The leakage currents for Pd- and Au-MESFETs are 2 and 4 orders of magnitude higher than for Ag. Maximum off-voltages of − 1.4 V have to be applied at the gate in order to fully close the n-type (normally-on) channels. Channel mobilities of 6.3, 11.4, 12.8, and 24 cm2/Vs were observed for Ag, Pt, Pd, and Au, respectively. Studies of the device performance at elevated temperatures in the range between 25 °C and 150 °C revealed that the MESFETs are stable at least until 75 °C. An annealing effect, which improved the MESFET's electric properties, could be observed for Ag, Pt, and Au.