Organic Thin-Film Transistors with Low Voltage Operation and High Operational Stability
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Kim, Gunhee
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Abstract
Organic thin-film transistors (OTFTs) are electrical devices that has great potential for future display technologies including flexible and transparent screens, because they can be directly fabricated on top of flexible and transparent substrates. However, there are two technological hurdles still prevent for OTFTs from being commercialized: (i) poor operational stability and (ii) high operating voltage.
For the operational stability and high operating voltage, using a bottom bilayer gate dielectric, which consists of HfO2 and CYTOP layers, can be a solution. The thickness of the bilayer gate dielectric can be reduced down to 17 nm. The thin bottom bilayer gate dielectric can show an operating voltage as low as -1 V for the gate and drain voltages to achieve 1 µA of the IDS. Aging mechanisms (mechanisms for the positive IDS shift due to the HfO2 and the negative IDS shift due to the CYTOP/semiconductor interface) of the bilayer gate dielectric can provide only 0.1 V of threshold voltage shift after applying DC bias stress for 24 h.
Contact resistance is also one of the important problems to be solved for lowering the operating voltage. A quantitative model for the intrinsic contact resistance was designed using YFM and TLM for the second study. The model was applied to OTFTs which have the bottom bilayer gate dielectrics with a wide range of gate capacitance density (36.6 nF/cm2 to 231.7 nF/cm2). Applying the method and analyzing the intrinsic contact resistance, compared to the thick bilayer gate dielectric with 36.6 nF/cm2 of the capacitance density, bilayers with a high gate dielectric capacitance as high as 231.7 nF/cm2 could reduce the contact resistance values from 3.5 kΩ cm to values smaller than 1 kΩ cm. This result suggests that a higher gate capacitance density can provide lower contact resistance values.
As the third study, in-depth study for the operational stability during DC bias stress was conducted for better understanding of the characteristics of the operational stability, especially the positive IDS shift during the DC bias stress. In order to understand the mechanism, the charge injection characteristics from the gate electrode were modified by applying PEIE and MoO3 to make the top gate electrode with different work function values. By comparing the DC bias stability of the PEIE-coated devices and MoO3-coated devices, it can be shown that the charge injection from the gate electrode can make the large IDS increase at the early stage of the DC bias stress, and a higher concentration of charge carrier in the channel can slow down the decreasing of IDS.
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Date
2023-01-10
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Dissertation