Trap-Limited and Percolation Conduction Mechanisms in Amorphous Oxide Semiconductor Thin Film Transistors (TFTs)

The amorphous oxide semiconductor thin film transistor (TFT) is a highly promising candidate for large area displays in terms of its optical transparency and high electron mobility even when fabricated at room temperature. Despite sharing similar attributes as other TFT technologies, this class of materials has unique carrier transport mechanisms. Unlike hydrogenated amorphous silicon (a-Si:H), its conduction band edge (Em) is composed of spherical overlapping orbitals leading to a reduced density of band tail states in the amorphous phase, thus eliminating Fermi level (EF) pinning in TFTs.

In this work, LCN researchers, Sungsik Lee, Khashayar Ghaffarzadeh and Professor Arokia Nathan investigate carrier transport in bilayer In-Zn-O/Ga-In-Zn-O TFTs in collaboration with Cambridge University and Samsung Advanced Institute of Technology (SAIT).

The study reveals that the electron conduction mechanism in the above-threshold regime in oxide semiconductor thin film transistors is controlled by both trap-limited conduction and percolation, depending on gate voltage. The band tail state slope controls the field-effect mobility in the limit of lower gate voltage, while the average spatial coherence length and potential fluctuation control percolation conduction in the limit of higher gate voltage. In these limits, the field-effect mobility is found to follow a power law, from which a universal mobility versus carrier concentration dependence is extracted.

This work has been published in Applied Physics Letters (Sungsik Lee et al., Applied Physics Letters, 98, 203508, 2011).

Journal link:

Fig. 1 Schematic band diagram of an oxide semiconductor TFT showing the presence of band tail states and potential fluctuations above Em, giving rise to trap-limited conduction and percolation.

Fig. 2 Carrier densities (nfree and ntrap) as a function of Fermi-level energy (EF) from the mobility edge (Em); inset shows schematic band diagram at the SiOx/channel (In-Zn-O/In-Ga-Zn-O) interface.