Carrier transport and instability mechanisms in oxide semiconductor thin film transistors

The growing demand for amorphous oxide semiconductor thin film transistors (TFT) necessitates \nthe development of a physics-based field-effect mobility model that links the terminal characteristics \nof TFTs to their material properties. This need is particularly acute since existing approaches fail to \nexplicitly account for the unique carrier transport properties of oxide semiconductors. \nThe first part of this thesis specifically addresses this challenge. Here, it is shown that the electron \nconduction mechanism in the above-threshold regime of amorphous oxide semiconductor TFTs is \ncontrolled by both percolation and trap-limited conduction. \nIn the limit where trap-limited conduction prevails, the characteristic temperature of tail states \ncontrols the field-effect mobility; whereas in the limit where percolation prevails, the properties of \nconduction’s band potential fluctuation govern it. Irrespective of the operation regime, the fieldeffect \nmobility is found to follow a power law form. \nThe value of the trap density plays a critical role in determining the transition voltage. This is \nbecause a high value leads to Fermi level pinning, and thus trap-limited conduction dominates; \nwhereas a low value results in percolation since the Fermi level becomes able to cross the \nconduction band edge for trap density of <1020 cm-3eV-1. \nThe threshold voltage (VT) stability of TFTs is a critical figure-of-merit that largely determines the \nlifetime of active matrix backplanes. This, coupled with the growing demand on the TFTs to \nperform analogue functions, necessitates the study of the instability mechanisms in oxide \nsemiconductor TFTs. \nThe second part of this thesis specifically addresses this challenge. Here, electrical and/or optical \nstress/recovery measurements reveal the presence of charge trapping and persistent \nphotoconductivity (PPC) in passivated HfInZnO/SiOx and GaInZnO/InZnO/SiOx TFT systems. \nIn the absence of light, charge trapping is found dominant; whereas in the presence of light, PPC is. \nThe PPC keeps the active channel in a state of high conductivity long after the removal of the light \nsource, causing the TFT to act as though n-doped. This results in a negative VT shift, irrespective of \nbias magnitude and polarity. The origin of the PPC is attributed to the ionisation of oxygen vacancy sites, mainly based on the \nobservations of its temperature- and wavelength-dependence. Interestingly, the gate voltage is found \nto control the decay of the PPC, giving rise to a memory action. This is explained in terms of the \ndependence of the rate of the PPC recovery reaction, Vo \n+++2e /rightarrow Vo, on carrier concentration.