Po-Chin Kuo

Top-emitting 230dots∕in. active-matrix polymer light-emitting diode displays on flexible metal foil substrates

A top-emitting 230dots∕in. monochrome active-matrix polymer light-emitting diode (PLED) display having the VGA format and fabricated on a flexible steel foil utilizing the polycrystalline silicon thin-film transistor (TFT) technology is reported. The pixel circuitry architecture consists of the conventional two TFT circuitries made of two p-channel metal-oxide-semiconductor (PMOS) transistors and one storage capacitor. The average field-effect hole mobility and threshold voltage of the PMOS polysilicon TFTs fabricated on the metal foil are 37(±4)cm2∕Vs and −1.9(±0.6)V, respectively. The light turn-on voltage of the PLED is 4.0V.

Effect of mechanical strain on mobility of polycrystalline silicon thin-film transistors fabricated on stainless steel foil

The effect of uniaxial tensile strain parallel to the channel on mobility of polycrystalline silicon thin-film transistors (TFTs) on stainless steel foil has been investigated. The electron mobility increases by 20% while the hole mobility decreases by 6% as the strain increases to 0.5%, and both followed by saturation as the strain increases further. The off current decreases for both types of TFTs under strain. All TFTs remained functional at the applied strain of 1.13%.

Effect of uniaxial tensile strain on electrical performance of amorphous IGZO TFTs and circuits on flexible Metal foils

Inexpensive and light-weight flexible displays and sensor electronics would be more rugged and portable than the more conventional rigid substrate-based electronics. Till date, large area flexible systems are enabled by a-Si:H or organics which suffer from low mobilities that limit their use in driver electronics that require higher current drive. This is where oxide-semiconductor (amorphous Indium Gallium Zinc Oxide, IGZO in particular) based thin-film transistors (TFTs) provide an attractive alternative to silicon-based TFTs. Therefore, one needs to study the interdependence of mechanical flexing and electrical performance of these devices as they find applications in flexible large area based electronics. This study systematically investigates the influence of tensile strain on IGZO TFTs and ring oscillators fabricated on flexible stainless steel substrates.

Process Technology for High-Resolution AM-PLED Displays on Flexible Metal-Foil Substrates

The first successful integration of a polysilicon thin-film transistor (TFT) backplane with polymer light-emitting diodes (PLEDs) onto a flexible stainless steel foil is described, and a high-resolution (230 dots/in.) monochrome active-matrix polymer light-emitting diode (AM-PLED) display is demonstrated. The process technology required to implement this high-resolution AM-PLED display onto a flexible metal-foil substrate is discussed. This technology primarily consists of the preparation of flexible metal foil, fabrication of the active matrix polysilicon TFT backplane, and integration with top-emitting PLEDs.

Electrical characteristics and mechanical limitation of polycrystalline silicon thin film transistor on steel foil under strain

Polycrystalline silicon thin-film transistor (poly-Si TFT) technology on steel foils has been successfully demonstrated as a platform for high performance flexible electronics [1] because of the high electron and hole mobility that enables CMOS circuit architectures and the compatibility of this technology with high processing temperatures. High resolution flexible displays have also been realized by this technology [2]. Depending on the application, the poly-Si TFTs in flexible electronics will be subjected to different levels of strain. In our earlier effort we investigated the effect of tensile strain on mobility of poly-Si TFTs [3]. In this paper, we further report the characteristics of poly-Si TFTs under both tensile and compressive strain as well as its limitation under strain. We fabricated both types poly-Si TFTs on steel foil substrate and applied strains to TFTs along the channel direction by bending, as shown in Fig. 1.

Effects of Mechanical Strain on Characteristics of Polycrystalline Silicon Thin-Film Transistors Fabricated on Stainless Steel Foil

The effects of uniaxial tensile strain on the performance of polycrystalline silicon thin-film transistors (poly-Si TFTs) is reported. Longitudinal strain increases the electron mobility and decreases the hole mobility, while transverse strain decreases the electron mobility and slightly decreases the hole mobility. Under longitudinal strain the off current decreases for both NMOS and PMOS TFTs and shifts in threshold voltage and substhreshold slope are observed for p-channel TFTs. A strong dependence on channel length for both electron and hole mobilities under longitudinal strain indicates the presence of a series resistance. For poly-Si TFTs, the mobility changes under strains are related to the strain effects on single crystalline silicon devices.

Laser-crystallized polysilicon TFT adder cells on flexible metal foil

In an effort to develop a large area digital system (such as a CUP) on flexible metal foil substrate utilizing polysilicon TFT technology, we extensively studied 21 different adder cells most of them novel circuits. The adder circuits were first carefully designed to be best compatible with poly-Si TFT technology and then fabricated on flexible metal foil. Adder cells were carefully tested and their performance were evaluated and compared with the goal of realizing the optimum adder structure(s) suitable for poly-Si TFT digital system development.

Electrical Response of Polycrystalline Silicon Thin Film Transistor on Steel Foil under Mechanical Strain

This work investigates the effects of mechanical strain on electrical characteristics of polycrystalline thin film transistors (poly-Si TFTs). Poly-Si TFTs were fabricated on steel foil substrate and characterized under the strain ranging from -1.2% to 1.1% induced by bending. The electron mobility increased under tensile and decreased under compressive strain while that of the hole exhibited an opposite trend. For p-channel TFTs the normalized threshold voltage and subthreshold slope were a function of strain. In both n- and p-channel TFTs the off current decreased under tensile while it increased under compressive strain. The observed mobility trends in poly-Si TFTs are similar to those reported in single crystalline silicon devices.

Mechanically strained laser crystallized poly-silicon thin film transistors and ring oscillators fabricated on stainless steel foils

In this study we evaluated NMOS and PMOS laser crystallized poly-silicon thin- film transistors (TFTs) as well as ring oscillators on stainless steel foil substrate under tensile strain. We also evaluated poly silicon TFTs with channels oriented perpendicular to the direction of applied strain. We observed that the mobility of both NMOS and PMOS devices decreased when the strain level increased while threshold voltage and subthreshold slope stayed unchanged.