Hyun-Joong Chung

Electronic transport properties of amorphous indium-gallium-zinc oxide semiconductor upon exposure to water

The effect of water exposure on amorphous indium-gallium-zinc oxide (a-IGZO) semiconductors was reported. It was found that water can diffuse in and out of the a-IGZO film, reversibly affecting the transistor properties. Two competing mechanisms depending on the thickness of the active channel were clarified. The electron donation effect caused by water adsorption dominated for the thicker a-IGZO films (⩾100nm), which was manifested in the large negative shift (>14V) of the threshold voltage. However, in the case of the thinner a-IGZO films (⩽70nm), the dominance of the water-induced acceptorlike trap behavior was observed. The direct evidence for this behavior was that the subthreshold swing was greatly deteriorated from 0.18V/decade (before water exposure) to 4.4V/decade (after water exposure) for the thinnest a-IGZO films (30nm). These results can be well explained by the screening effect of the intrinsic bulk traps of the a-IGZO semiconductor.

3.1: Distinguished Paper: 12.1-Inch WXGA AMOLED Display Driven by Indium-Gallium-Zinc Oxide TFTs Array

The full color 12.1-inch WXGA active-matrix organic light emitting diode (AMOLED) display was, for the first time, demonstrated using indium-gallium-zinc oxide (IGZO) thin-film transistors (TFTs) as an active-matrix back plane. It was found that the fabricated AMOLED display did not suffer from the well-known pixel non-uniformity of luminance, even though the simple structure consisting of 2 transistors and 1 capacitor was adopted as a unit pixel circuit, which was attributed to the amorphous nature of IGZO semiconductor. The n-channel a-IGZO TFTs exhibited the field-effect mobility of 8.2 cm2/Vs, threshold voltage of 1.1 V, on/off ratio of > 108, and subthreshold gate swing of 0.58 V/decade. The AMOLED display with a-IGZO TFT array would be promising for large size applications such as note PC and HDTV because a-IGZO semiconductor can be deposited on large glass substrate (> Gen. 7) using conventional sputtering system.

3D multifunctional integumentary membranes for spatiotemporal cardiac measurements and stimulation across the entire epicardium

Means for high-density multiparametric physiological mapping and stimulation are critically important in both basic and clinical cardiology. Current conformal electronic systems are essentially 2D sheets, which cannot cover the full epicardial surface or maintain reliable contact for chronic use without sutures or adhesives. Here we create 3D elastic membranes shaped precisely to match the epicardium of the heart via the use of 3D printing, as a platform for deformable arrays of multifunctional sensors, electronic and optoelectronic components. Such integumentary devices completely envelop the heart, in a form-fitting manner, and possess inherent elasticity, providing a mechanically stable biotic/abiotic interface during normal cardiac cycles. Component examples range from actuators for electrical, thermal and optical stimulation, to sensors for pH, temperature and mechanical strain. The semiconductor materials include silicon, gallium arsenide and gallium nitride, co-integrated with metals, metal oxides and polymers, to provide these and other operational capabilities. Ex vivo physiological experiments demonstrate various functions and methodological possibilities for cardiac research and therapy.

Materials for bioresorbable radio frequency electronics.

Materials, device designs and manufacturing approaches are presented for classes of RF electronic components that are capable of complete dissolution in water or biofluids. All individual passive/active components as well as system-level examples such as wireless RF energy harvesting circuits exploit active materials that are biocompatible. The results provide diverse building blocks for physically transient forms of electronics, of particular potential value in bioresorbable medical implants with wireless power transmission and communication capabilities.

Flexible electronics under strain: a review of mechanical characterization and durability enhancement strategies

Flexible electronics incorporate all the functional attributes of conventional rigid electronics in formats that have been altered to survive mechanical deformations. Understanding the evolution of device performance during bending, stretching, or other mechanical cycling is, therefore, fundamental to research efforts in this area. Here, we review the various classes of flexible electronic devices (including power sources, sensors, circuits and individual components) and describe the basic principles of device mechanics. We then review techniques to characterize the deformation tolerance and durability of these flexible devices, and we catalogue and geometric designs that are intended to optimize electronic systems for maximum flexibility.

12.1‐in. WXGA AMOLED display driven by InGaZnO thin‐film transistors

— A full-color 12.1-in.WXGA active-matrix organic-light-emitting-diode (AMOLED) display was, for the first time, demonstrated using indium-gallium-zinc oxide (IGZO) thin-film transistors (TFTs) as an active-matrix backplane. It was found that the fabricated AMOLED display did not suffer from the well-known pixel non-uniformity in luminance, even though the simple structure consisting of two transistors and one capacitor was adopted as the unit pixel circuit, which was attributed to the amorphous nature of IGZO semiconductors. The n-channel a-IGZO TFTs exhibited a field-effect mobility of 17 cm2/V-sec, threshold voltage of 1.1 V, on/off ratio >109, and subthreshold gate swing of 0.28 V/dec. The AMOLED display with a-IGZO TFT array is promising for large-sized applications such as notebook PCs and HDTVs because the a-IGZO semiconductor can be deposited on large glass substrates (larger than Gen 7) using the conventional sputtering system.

Stretchable, multiplexed pH sensors with demonstrations on rabbit and human hearts undergoing ischemia

Stable pH is an established biomarker of health, relevant to all tissues of the body, including the heart. Clinical monitoring of pH in a practical manner, with high spatiotemporal resolution, is particularly difficult in organs such as the heart due to its soft mechanics, curvilinear geometry, heterogeneous surfaces, and continuous, complex rhythmic motion. The results presented here illustrate that advanced strategies in materials assembly and electrochemical growth can yield interconnected arrays of miniaturized IrOx pH sensors encapsulated in thin, low-modulus elastomers to yield conformal monitoring systems capable of noninvasive measurements on the surface of the beating heart. A thirty channel custom data acquisition system enables spatiotemporal pH mapping with a single potentiostat. In vitro testing reveals super-Nernstian sensitivity with excellent uniformity (69.9 ± 2.2 mV/pH), linear response to temperature (-1.6 mV °C(-1) ), and minimal influence of extracellular ions (<3.5 mV). Device examples include sensor arrays on balloon catheters and on skin-like stretchable membranes. Real-time measurement of pH on the surfaces of explanted rabbit hearts and a donated human heart during protocols of ischemia-reperfusion illustrate some of the capabilities. Envisioned applications range from devices for biological research, to surgical tools and long-term implants.

Comprehensive Study on the Transport Mechanism of Amorphous Indium-Gallium-Zinc Oxide Transistors

High-performance thin-film transistors TFTs, in which the channel material consisted of amorphous indium-gallium-zinc oxide a-IGZO with a bottom gate architecture, were fabricated for array applications. It was found that the dependence of the field-effect mobility on the channel length was greatly affected by the value of the contact resistance RC. A high contact resistance RCW 200 cm resulted in a significant drop 22.3% in the normalized field-effect mobility for the short channel device 10 m, while contact-limited behavior was hardly seen for the device with a low contact resistance RCW 23 cm. The difference in the channel length dependence of the field-effect mobility was comprehensively investigated based on the conduction mechanism. The fabricated n-channel a-IGZO TFTs with W/L =1 0/10 m exhibited a field-effect mobility of 12.6 cm 2 /V s, threshold voltage of 4.7 V, on/off ratio of 10 8 , and subthreshold gate swing of 0.56 V/decade.

Bulk-Limited Current Conduction in Amorphous InGaZnO Thin Films

The current conduction mechanism in radio frequency sputtered amorphous indium gallium zinc oxide (a-IGZO) films was investigated using model devices designed to mimic the carrier injection from an electrode to an a-IGZO channel in thin-film transistors. Interface-limited mechanisms, such as thermionic emission and Fowler-Nordheim tunneling, clearly fail to fit the current-voltage (I-V) curves. Instead, the I-V characteristics of the a-IGZO devices fit well within the framework of space-charge-limited current, whereas the conduction is enhanced by the Frenkel effect at high field (>0.1 MV/cm).

Polymorphism as an emerging design strategy for high performance organic electronics

Organic electronics is a promising field spanning a wide range of applications, with unrivaled advantages in low production cost, versatility in material synthesis, and compatibility with a wide range of substrates including flexible polymeric materials. Organic molecules are characterized by weak van der Waals interactions, which grant access to multiple crystalline packing states (crystal polymorphism) at near ambient conditions. Different polymorphs, even with the slightest changes in their molecular packing can have electronic properties that differ by orders of magnitude. Therefore, accessing metastable polymorphs can serve as a novel design strategy for attaining high device performance. Recently, this strategy has been successfully applied to small organic molecules such as 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) and 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) and high hole mobilities have been attained in organic field-effect transistors fabricated using their metastable structures. In addition, polymorphism serves as an excellent platform for advancing the fundamental understanding of charge transport in π-conjugated systems. The relationship between molecular packing and charge transport can be unequivocally established since the chemical structures are identical amongst polymorphs, leaving molecular packing as the only variable in the case of packing polymorphism.