Free Standing Plastic Foil Research Articles

Influence of Semiconductor Island Geometry on the AC Performance of Flexible InGaZnO TFTs

The AC performance of flexible TFTs sufferers from parasitics caused by tolerances needed for the fabrication on free-standing plastic foils. In this context, the semiconductor island can either be wider or narrower than the source/drain contacts. Traditionally, the second configuration is expected to result in faster TFTs as the total gate overlap area is smaller. However, here it is shown that 2.5 μm long flexible InGaZnO TFTs with wide semiconductor islands exhibit better frequency performance such as a f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> of 26.1MHz (compared to f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> of 13.8MHz of TFTs with narrow semiconductor islands). This effect is confirmed for flat and bend TFTs and is caused by current spreading in the semiconductor islands, as well as the frequency dependency of the gate capacitance.

Solution-processed p-type copper(I) thiocyanate (CuSCN) for low-voltage flexible thin-film transistors and integrated inverter circuits

We report on low operating voltage thin-film transistors (TFTs) and integrated inverters based on copper(I) thiocyanate (CuSCN) layers processed from solution at low temperature on free-standing plastic foils. As-fabricated coplanar bottom-gate and staggered top-gate TFTs exhibit hole-transporting characteristics with average mobility values of 0.0016 cm2 V−1 s−1 and 0.013 cm2 V−1 s−1, respectively, current on/off ratio in the range 102–104, and maximum operating voltages between −3.5 and −10 V, depending on the gate dielectric employed. The promising TFT characteristics enable fabrication of unipolar NOT gates on flexible free-standing plastic substrates with voltage gain of 3.4 at voltages as low as −3.5 V. Importantly, discrete CuSCN transistors and integrated logic inverters remain fully functional even when mechanically bent to a tensile radius of 4 mm, demonstrating the potential of the technology for flexible electronics.

Flexible Self-Aligned Double-Gate IGZO TFT

In this letter, flexible double-gate (DG) thin-film transistors (TFTs) based on InGaZnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> and fabricated on free standing plastic foil, using self-alignment (SA) are presented. The usage of transparent indium-tin-oxide instead of opaque metals enables SA of source-, drain-, and top-gate contacts. Hence, all layers, which can cause parasitic capacitances, are structured by SA. Compared with bottom-gate reference TFTs fabricated on the same substrate, DG TFTs exhibit a by 68% increased transconductance and a subthreshold swing as low as 109 mV/dec decade (-37%). The clockwise hysteresis of the DG TFTs is as small as 5 mV. Because of SA, the source/drain to gate overlaps are as small as ≈ 1 μm leading to parasitic overlap capacitances of 5.5 fF μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Therefore a transit frequency of 5.6 MHz is measured on 7.5 μm long transistors. In addition, the flexible devices stay fully operational when bent to a tensile radius of 6 mm.

Flexible Self-Aligned Amorphous InGaZnO Thin-Film Transistors With Submicrometer Channel Length and a Transit Frequency of 135 MHz

Flexible large area electronics promise to enable new devices such as rollable displays and electronic skins. Radio frequency (RF) applications demand circuits operating in the megahertz regime, which is hard to achieve for electronics fabricated on amorphous and temperature sensitive plastic substrates. Here, we present self-aligned amorphous indium-gallium-zinc oxide-based thin-film transistors (TFTs) fabricated on free-standing plastic foil using fabrication temperatures . Self-alignment by backside illumination between gate and source/drain electrodes was used to realize flexible transistors with a channel length of 0.5 μm and reduced parasitic capacities. The flexible TFTs exhibit a transit frequency of 135 MHz when operated at 2 V. The device performance is maintained when the TFTs are bent to a tensile radius of 3.5 mm, which makes this technology suitable for flexible RFID tags and AM radios.

Flexible double gate a-IGZO TFT fabricated on free standing polyimide foil

In this paper, the concept of double gate transistors is applied to mechanically flexible amorphous indium–gallium–zinc-oxide (a-IGZO) thin film transistors (TFTs) fabricated on free standing plastic foil. Due to the temperature sensitivity of the plastic substrate, a-IGZO is a suitable semiconductor because it provides carrier mobilities around 10cm2/Vs when deposited at room temperature. Double gate TFTs with connected bottom and top gate are compared to bottom gate reference TFTs fabricated on the same substrate. Double gate a-IGZO TFTs exhibit a by 78% increased gate capacitance, a by 700mV higher threshold voltage, and therefore an up to 92% increased transconductance when characterized at the same gate voltage above threshold (over-bias voltage). The subthreshold swing and the on/off current ratios are improved as well, and reach excellent values of 69mV/dec and 2×109, respectively. The mechanical flexibility of double gate TFTs compared to bottom gate TTFs is investigated, and device operation is shown while the double gate TFT is exposed to tensile strain of 0.55%, induced by bending to a radius of 5mm.

Design Rules for IGZO Logic Gates on Plastic Foil Enabling Operation at Bending Radii of 3.5 mm

Findings obtained from bending experiments with mechanically flexible InGaZnO-based thin-film transistors are used to derive design rules for flexible InGaZnO-based n-channel metal-oxide-semiconductor logic circuits. Based on the developed design rules, flexible NAND gates, inverters, and five-stage ring oscillators are fabricated directly on free-standing plastic foils at temperatures ≤ 150 °C. Geometrically well-designed circuits operated at a supply voltage of 5 V are exposed to tensile mechanical strains, induced by bending, up to 0.72% without performance degradation. This corresponds to a bending radius of 3.5 mm. At the same time, increases in the rise time by a factor of ca 2 and reductions in the high and low output voltage levels by ca 10% and 50% have been observed for circuits with disadvantageous geometrical design. Ring oscillators designed to be operated under strain show an increase in oscillation frequency from 22.9 kHz (flat substrate) to 23.32 kHz (bending radius: 3.5 mm). This demonstrates the held-effect mobility increase in a-IGZO-based circuits under tensile mechanical strain. Long-term reliability is evaluated with 20000 cycles of repeated bending and reflattering without circuit failure.