This paper presents a fast and accurate method for extraction of the above-threshold physical parameters (such as threshold voltage, power parameter, effective mobility, and contact resistance) from measurement data in the linear and saturation regions of hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) fabricated on glass and plastic substrates. The method of extraction is different from techniques that are currently used by virtue of the departure from the square law dependence of the current-voltage characteristics. In addition, a broader range of process-induced variation in material properties is expected, which is accentuated by the effects of different substrates, leading to wide-ranging device parameters. In particular, non-ideal parameters such as contact resistance may vary by orders of magnitude due to process variations, thus strongly influencing the extracted values of TFT parameters if its effect is not considered in the extraction method. In this paper, the effect of contact resistance and other non-ideal parameters is systematically identified and eliminated using TFTs with different channel length. The extracted values for TFTs on glass and plastic substrates clearly highlights the differences in material properties stemming from the different process conditions and substrate properties, and provide insight that is invaluable for subsequent device/process optimization.