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

•Our study includes the impact of a scaled channel length and two gate voltages (10 V and 15 V) on significant device parameters such as threshold voltage VTh, drain induced barrier lowering (DIBL), subthreshold voltage, subthreshold swing (SS), on-current IOn, leakage current IOff, and on/off ratio.•The ZnO TFTs are fabricated using atomic layer deposition (ALD) to improve a quality of the surface passivation which has an impact on electron mobility in the TFTs.•The physically based simulations to show a theoretical limit of the contact resistance by a new approach which combines density functional theory (DFT) calculations with a quantum-mechanical transport through the interface between metal and semiconductor contact.•Our study demonstrates a good performance of the on/off ratio. The on/off ratio is 1.5×104, 4.2×104, 5.3×104 and 8.3×104 for channel lengths of Lch=10μm, 5 μm, 4 μm, and 2 μm, respectively. ZnO thin-film transistors (TFTs) with scaled channel lengths of 10 μm, 5 μm, 4 μm, and 2 μm exhibit increasing intrinsic channel electron mobility at a gate bias of 10 V (15 V) from 0.782 cm2/Vs (0.83 cm2/Vs) in the 10 μm channel length TFT to 8.9 cm2/Vs (19.04 cm2/Vs) for the channel length scaled down to 2 μm. 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 μA/μm, 96 μA/μm, 193 μA/μm, and 214 μA/μm 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 μm, 5 μm, 4 μm, and 2 μm. 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 of4.6×105Ω/sq extracted from the measurements, which is 3.5× larger than the contact sheet resistance of 1.3×105Ω/sq obtained from the DFT calculation and the 1D self-consistent Poisson-Shrödinger simulation, largely limits the drive current in the scaled ZnO TFTs.