  Physics based numerical simulation has been carried out to probe the sub-gap
density of states (DOS) and underlying electron transport properties of
amorphous oxide based thin film transistors (TFTs). The DOS model of TFTs
consists of exponential band tails, Gaussian shallow donor levels and deep
acceptor states. Electrical transport and various TFT performance parameters
are found to be critically dependent on the sub-gap DOS. At low gate bias, when
Fermi level lies below the conduction band mobility edge, defect states
mediated trap limited conduction is found to be the dominant transport
mechanism. However, at relatively higher gate bias, percolation conduction
above the mobility edge becomes prevalent. Such possible crossover of electron
transport is well corroborated by gate bias dependent band bending and induced
free electron density in the channel layer. Such studies are important to
unravel the critical role of sub-gap DOS on the electrical performance and
charge transport processes of disordered amorphous oxide TFTs.
