UBC Theses and Dissertations
Organic metal-semiconductor field-effect transistor (OMESFET) Takshi, Arash
Organic electronics offers the possibility of producing ultra-low-cost and large-area electronics using printing methods. Two challenges limiting the utility of printed electronic circuits are the high operating voltage and the relatively poor performance of printed transistors. It is shown that voltages can be reduced by replacing the capacitive gate used in Organic Field-Effect Transistors (OFETs) with a Schottky contact, creating a thin-film Organic Metal-Semiconductor Field-Effect Transistor (OMESFET). This geometry solves the voltage issue, and promises to be useful in situations where low voltage operation is important, but good performance is not essential. In cases where high voltage is acceptable or required, it is shown that OFET performance can be greatly improved by employing a Schottky contact as a second gate. The relatively thick insulating layer between the gate and the semiconductor in OFETs makes it necessary to employ a large change of gate voltage (~40 V) to control the drain current. In order to reduce the voltage to less than 5 V a very thin (<10 nm) insulating layer and/or high-k dielectric materials can be used, but these solutions are not compatible with current printing technology. Simulations and implementations of OMESFET devices demonstrate low voltage operation (<5 V) and improved sub-threshold swing compared to the OFET. However, these benefits are achieved at the expense of mobility. In order to achieve good performance in an OFET, including threshold voltage, current ratio and output resistance, the semiconductor thickness has to be less than 50 nm, whereas the thickness of a printed semiconductor is typically larger than 200 nm. The addition of a top Schottky contact on the OFET creates a depletion region thereby reducing the effective thickness of the semiconductor, and resulting in enhanced transistor performance. Simulations and experimental results show improvements in the threshold voltage, the current ratio, and the output resistance of a dual gate transistor, when compared to those in an OFET of the same thickness. The transistors introduced in this work demonstrate means of improving the performance of thick-film OFETs and of achieving substantially lower operation voltage in organic transistors.
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