Ridley, B. "Printed Inorganic Transistors"

Abstract

Forty years of exponential growth of semiconductor technology have been predicated on the miniaturization of the transistors that comprise integrated circuits. While complexity has greatly increased within a given area of processed silicon, the cost per area has not decreased. Current fabrication methods are further hindered by high facility costs and environmentally unfriendly processing. Moving to a new means of semiconductor fabrication may drastically reduce both financial and environmental costs. One such approach is based on the extension of printing techniques to the fabrication of electronic devices. Such printed electronics are envisioned to enable applications in flexible displays and electronic paper, personal fabrication, wearable computing, and disposable medical diagnostics.

This dissertation focuses on the development of printable materials, specifically inorganic semiconductor inks. At the outset of this research, organic semiconductors were the only materials known and pursued as printable semiconductors. The ability to process organic semiconductors in common organic solvents makes them amenable to a wide range of printing technologies, but their electrical performance is fundamentally limited and their utility is confined to applications in which only low speeds are required.

The goal of this thesis was to demonstrate the feasibility of printing inorganic materials, the same materials that are used to fabricate high quality semiconductor devices. Cadmium selenide was studied as a model inorganic semiconductor and silicon was studied because of its commercial dominance. The insolubility and high processing temperatures of inorganic semiconductors, both of which can prevent their use in printed electronics, were overcome through the use of nanoparticle inks. At very small sizes, nanoparticles can be highly soluble in organic solvents and can have a pronounced melting point depression. Leveraging these size-dependent properties, the first semiconductor nanoparticle inks were developed using cadmium selenide and the first all-printed inorganic thin film transistors were demonstrated. Printed active layers in thin film transistors attained a semiconductor mobility of 1 cm2V-1s-1 and an ON/OFF ratio in excess of 104. Further development of inorganic nanoparticle inks and efforts to extend this approach to silicon are described, addressing silicon nanoparticle synthesis, purification, and ink formulation.