Model for determination of mid-gap states in amorphous metal oxides from thin film transistors

The electronic density of states in metal oxide semiconductors like amorphous zinc oxide (a-ZnO) and its ternary and quaternary oxide alloys with indium, gallium, tin, or aluminum are different from amorphous silicon, or disordered materials such as pentacene, or P3HT. Many ZnO based semiconductors exhibit a steep decaying density of acceptor tail states (trap DOS) and a Fermi level (E-F) close to the conduction band energy (E-C). Considering thin film transistor (TFT) operation in accumulation mode, the quasi Fermi level for electrons (E-q) moves even closer to E-C. Classic analytic TFT simulations use the simplification E-C - E-F > 'several' kT and cannot reproduce exponential tail states with a characteristic energy smaller than 1/2 kT. We demonstrate an analytic model for tail and deep acceptor states, valid for all amorphous metal oxides and include the effect of trap assisted hopping instead of simpler percolation or mobility edge models, to account for the observed field dependent mobility. (C) 2013 AIP Publishing LLC.