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Carrier transport and instability mechanisms in oxide semiconductor thin film transistors

Ghaffarzadeh, K.; (2011) Carrier transport and instability mechanisms in oxide semiconductor thin film transistors. Doctoral thesis, UCL (University College London).

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Abstract

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

Type:Thesis (Doctoral)
Title:Carrier transport and instability mechanisms in oxide semiconductor thin film transistors
Language:English
Additional information:Permission for digitisation not received
UCL classification:UCL > School of BEAMS > Faculty of Engineering Science > Electronic and Electrical Engineering

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