The performance of hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFTs) is influenced by the effective intrinsic a-Si:H thickness which is the remaining thickness of intrinsic a-Si:H after n+-Si etching for back- channel-etch type a-Si:H TFTs. As the thickness of the as- deposited a-Si:H increases, the on-current and linear mobility of the TFT decrease significantly, but the threshold voltage and saturation mobility only change slightly. It is caused by the series resistance and space- charge-limited current for the as-deposited a-Si:H film. On the other hand, when the thickness of the a-Si:H after n+-Si etching decreases from 200 nm to 150 nm, the Ion, (mu) s, (mu) lin, and Vth do not change greatly. However, if the effective thickness of a-Si:H reduces to 50 nm, the Ion, (mu) s, and (mu) lin decrease and Vth increase. It is the result of the Fermi level pinning at the back-channel surface and plasma-damage for thin a-Si after n+-Si etching.