A method for calculating values of parameters of a TFT includes calculating a set of simulated current-voltage (I-V) values using state-density-functions over an entire energy band in a band gap of an amorphous semiconductor of the TFT. The method further includes comparing the set of simulated I-V values with a set of measured I-V values of the TFT to determine a value of a parameter of the TFT. The method may further include calculating values of an acceptor state-density-function gA using a set of electrostatic capacity-voltage (C-V) values of the TFT measured according to a frequency. The method may further include determining values of a donor state-density-function gD and values of an interface state-density-function Dit over the entire energy band in the band gap.