Hongki Kang

High‐Performance Printed Transistors Realized Using Femtoliter Gravure‐Printed Sub‐10 μm Metallic Nanoparticle Patterns and Highly Uniform Polymer Dielectric and Semiconductor Layers

Printed electronics has received a great deal of attention as a means of realizing a wide range of low-cost printed electronic systems. High-speed roll-based printing is particularly attractive due to its potential for very high throughput and low cost of ownership. As a result, there have been several attempts by numerous groups including our own to use various types of rollto-roll printing, such as direct gravure printing, off-set printing, and fl exographic printing to fabricate printed transistors. [ 1–6 ]

Transparent high-performance thin film transistors from solution-processed SnO2 /ZrO2 gel-like precursors.

This work employs novel SnO(2) gel-like precursors in conjunction with sol-gel deposited ZrO(2) gate dielectrics to realize high-performance transparent transistors. Representative devices show excellent performance and transparency, and deliver mobility of 103 cm(2) V(-1) s(-1) in saturation at operation voltages as low as 2 V, a sub-threshold swing of only 0.3 V/decade, and /(on) //(off) of 10(4) ~10(5) .

Hydrostatic optimization of inkjet-printed films.

In this work, we study the optimization of the geometry of inkjet-printed polymer films and develop a simple analytic framework to understand our results and establish limitations on inkjet-printed patterns. We show how drop spacing and ink concentration affect the thickness of a printed film and how hydrostatic conditions with contact angle hysteresis have to be considered to print optimized rectangular features. If advancing and receding contact angle are not taken into account, printed features will either bulge or break up into smaller beads. Thus, we provide a comprehensive analysis of the limits of film formation using regular assemblies of droplets.

Methodology for Inkjet Printing of Partially Wetting Films

Inkjet printing of precisely defined structures is critical for the realization of a range of printed electronics applications. We develop and demonstrate a methodology to optimize the inkjet printing of two-dimensional, partially wetting films. When printed inks have a positive retreating contact angle, we show that any fixed spacing is ineffective for printing two-dimensional features. With fixed spacing, the bead contact angle begins large, leading to a bulging overflow of its intended footprint. Each additional line reduces the bead contact angle, eventually leading to separation of the bead. We propose a printing scheme that adjusts the line-to-line spacing to maintain a beads contact angle between its advancing and retreating values as it is printed. Implementing this approach requires an understanding of the two-dimensional bead surface and compensation for evaporation during the print. We derive an analytic equation for the beads surface with pinned contact lines and use an empirical fit for mass loss due to evaporation. Finally, we demonstrate that enhanced contact angle hysteresis, achieved by preprinting a features border, leads to better corner definition.

Analysis of flicker noise in two-dimensional multilayer MoS2 transistors

Using low-frequency noise (LFN) analysis, we examined the quality of the semiconductor, oxide, and oxide–semiconductor interface of back-gated multilayer MoS2 transistors. We also investigated the mechanism of the LFN and extracted γ exponents from the LFN behavior, 1/fγ; the value of γ was >1 at negative gate bias because of active slow traps. As VG increased, the slow traps were filled and thus γ decreased, stabilizing at ≈0.95. Various other parameters extracted from the LFN indicated that the carrier number fluctuation (Δn) model was the dominant origin of the LFN. The multilayer MoS2 structure had better noise immunity than a single-layer case in air.

Analytical Threshold Voltage Model for Double-Gate MOSFETs With Localized Charges

An analytical threshold voltage model for double-gate MOSFETs with localized charges is developed. From the 2-D Poissons equation with parabolic potential approximation, a compact threshold voltage model is derived. The proposed model is then verified with a 2-D device simulator. The model can be used to investigate hot-carrier-induced device degradation for various device dimensions and various charge distributions.

High-Speed Printing of Transistors: From Inks to Devices

The realization of a high-speed printing technique with high resolution and pattern fidelity is critical to making printed electronics a viable technology for electronics manufacturing. The printing requirements of printed electronics are substantially different that those of graphic arts. To make printed electronics a reality, it is necessary to deliver high resolution, good reproducibility, excellent pattern fidelity, high process throughput, and compatibility with the requisite semiconductor, dielectric, and conductor inks. In this paper, we review the physics of pattern formation from pixelated primitives, such as those that exist during inkjet and gravure printing, and will show how control of drop merging and drying can be used to produce high-fidelity shapes, including lines, squares, and intersections. We additionally discuss the physical underpinnings of gravure printing and inkjet printing, and show how these techniques can be scaled to produce high-fidelity highly scaled patterns, including sub-2 micron features at printing speeds of ~1 m/s. Finally, in conjunction with high-performance materials, we describe our realization of high-performance fully printed transistors on plastic, offering high-switching speed, excellent process throughput, and good fidelity over large areas.

Measurement, analysis, and modeling of 1/f noise in pentacene thin film transistors

In order to facilitate accurate noise modeling of organic thin-film-transistors (OTFTs), we provide comprehensive experimental results and analysis of unique low frequency noise characteristics in OTFTs. We conduct drain current noise measurements for pentacene-based thin-film-transistors (TFTs) having different grain size and operating region and use the resulting data to provide detailed mechanistic understanding of the underlying noise-generation phenomena that exist in OTFTs. The results show carrier trapping by traps within the semiconductor is the dominant source of low frequency noise and can be used in conjunction with a conventional unified noise model to accurately describe the noise behavior of pentacene TFTs.

Measurement and analysis of 1/f noise under switched bias in organic thin film transistors

An understanding of 1/f noise in organic thin film transistors (OTFTs) is critical to their deployment in a range of analog and mixed signal applications. In particular, an understanding of 1/f noise behavior during switching is vital and has not been reported to date. Here, we conduct drain current noise measurements for polymer based OTFTs while the OTFT switch between accumulation mode and depletion mode. The results show that capture and emission of the carriers by/from traps within the semiconductor is the dominant mechanism of the 1/f noise in OTFTs, and the 1/f noise in OTFTs decreases when the switching signal is applied to the gate terminal.

Inkjet-Printed Biofunctional Thermo-Plasmonic Interfaces for Patterned Neuromodulation

Localized heat generation by the thermo-plasmonic effect of metal nanoparticles has great potential in biomedical engineering research. Precise patterning of the nanoparticles using inkjet printing can enable the application of the thermo-plasmonic effect in a well-controlled way (shape and intensity). However, a universally applicable inkjet printing process that allows good control in patterning and assembly of nanoparticles with good biocompatibility is missing. Here we developed inkjet-printing-based biofunctional thermo-plasmonic interfaces that can modulate biological activities. We found that inkjet printing of plasmonic nanoparticles on a polyelectrolyte layer-by-layer substrate coating enables high-quality, biocompatible thermo-plasmonic interfaces across various substrates (rigid/flexible, hydrophobic/hydrophilic) by induced contact line pinning and electrostatically assisted nanoparticle assembly. We experimentally confirmed that the generated heat from the inkjet-printed thermo-plasmonic patterns can be applied in micrometer resolution over a large area. Lastly, we demonstrated that the patterned thermo-plasmonic effect from the inkjet-printed gold nanorods can selectively modulate neuronal network activities. This inkjet printing process therefore can be a universal method for biofunctional thermo-plasmonic interfaces in various bioengineering applications.