Channel mobility and contact resistance in scaled ZnO thin-film transistors

ZnO thin-film transistors (TFTs) with scaled channel lengths of 10 mu m, 5 mu m, 4 mu m, and 2 gm exhibit increasing intrinsic channel electron mobility at a gate bias of 10 V (15 V) from 0.782 cm(2)/Vs (0.83 cm(2)/Vs) in the 10 gm channel length TFT to 8.9 cm(2)/Vs (19.04 cm(2)/Vs) for the channel length scaled down to 2 mu m. Current-voltage measurements indicate an n-type channel enhancement mode transistor operation, with threshold voltages in the range of 8. 4 V to 5. 3 V, maximum drain currents of 41 mu A/mu m, 96 mu A/mu m, 193 mu A/mu m, and 214 mu A/mu m at a gate bias of 10 V, and breakdown voltages of 80 V, 70 V, 62 V, and 59 V with respect to channel lengths of 10 mu m, 5 mu m, 4 mu m, and 2 mu m. The channel electron mobility (excluding contact resistance) is extracted by the transmission line method (TLM) from the effective electron mobility (including contact resistance). The contact sheet resistance of4. 6 x 10(5) Omega/sq extracted from the measurements, which is 3.5 x larger than the contact sheet resistance of 1. 3 x 10(5) Omega/sq obtained from the DFT calculation and the 1D self-consistent Poisson-Shrodinger simulation, largely limits the drive current in the scaled ZnO TFTs.