In a previous paper we presented a new procedure to extract basic AIM-Spice model parameters of amorphous thin film transistor model in the above-threshold region, based on the integration of the experimental data current. In this paper we present a new procedure to determine subthreshold TFT parameters highly dependent on the fabrication technology as the density of deep states and characteristic temperature of deep states. These parameters are extracted in a simple and direct way from the experimental measurements, with no need of assigning predetermined values to any other model parameter. Other parameters required for device modeling in the subthreshold regime are also determined. The validity of the procedure is tested for a-Si:H TFTs with channel length down to 4 mum. We also show the good coincidence between calculated, using parameter values obtained with our new extraction procedure and experimental curves, in all working regions of the devices. Finally we discuss effects in the behavior of I-DS vs. V-DS curve related to the series resistance at the drain and source, which become evident during the device modeling. (C) 2003 Elsevier Science Ltd. All rights reserved.