Threshold voltage dependence on channel length in amorphous-indium-gallium-zinc-oxide thin-film transistors

A threshold voltage (Vth) dependence on channel length (L) is reported for amorphous-indium-gallium-zinc-oxide thin-film transistors (TFTs). Vth is found to shift negatively with decreasing L and the negative shift is drastic in TFTs with L < 4 μm. Combined analysis of current-voltage (I-V) and capacitance-voltage (C-V) curves shows that the Fermi energy (EF) at flat band shifts towards the conduction band (EC) with decreasing L, hence the negative Vth shift. Using the same analysis, the flat band carrier density (nFB) is also found to increase with decreasing L, revealing unintentional doping of the channel by carrier diffusion from the n+ doped source and drain regions as the cause of the negative Vth shift.