Charge transport and device engineering for improvement of thin film transistor




Wang, Xiao, Ph. D.

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Semiconductors used in thin film transistors (TFTs) include a wide range of materials, such as semiconducting polymers, organic molecules, and amorphous metal oxides, etc. Although TFTs have already found applications in fields such as display technology and flexible electronics, there are still several technical and scientific challenges that remain in TFTs areas including understanding of charge transport and device physics in high mobility TFTs, and in developing new applications with better-performing short channel devices. In this dissertation, we start from describing charge transport in TFTs with the assistance of a proposed physical model, then build a device model based on the fundamentals of the charge transport to investigate the performances of TFTs, and finally, develop experimental techniques to overcome performance bottle necks in short channel length TFTs. An extended multiple trap and release (MTR) model is proposed as the basis to understand the physics of charge transport. The extended MTR model uses Boltzmann transport theory with multiple scattering mechanisms, combined with a phenomenological transport reduction factor, which originates from the statistical nature of the transport, and multiple trap and release process to describe the charge transport in high mobility TFTs. The extended MTR model can be applied to various types of TFTs and provides a deeper understanding of the charge transport in such TFTs. Modeling thin film device based on the framework of the extended MTR model is accomplished by implementing a self-consistent Poisson and current continuity solver. Physical quantities such as carrier velocity, lateral electric fields and carrier distributions in TFTs are studied. The effect of contact resistance is investigated and analyzed in short channel TFTs. It is clear from the results of device modeling together with experimental data that the contact resistance, which is mainly due to the formation of Schottky barrier in metal-semiconductor contact region, is the major bottle neck that prevents the TFTs from further scaling down channel lengths. Two techniques are proposed to solve this bottle neck. One is to use doped graphene as contacts for TFTs to reduce the Schottky contact barrier. Another is to enhance the field injection of the carriers by patterning the graphene contacts into arrays of nanospikes. Both techniques are demonstrated to substantially reduce the contact resistance and facilitate scaling down channel lengths in organic TFTs well below a micrometer.


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