Design of bendable high-frequency circuits based on short-channel InGaZnO TFTs

A unique requirement of flexible electronic systems is the need to simultaneously optimize their electrical and mechanical performance. Amorphous InGaZnO thin-film transistors (TFTs) fabricated on free-standing large-area plastic substrates address this issue by providing a carrier mobility >10 cm 2 /Vs, and bendability down to radii as small as 25 μm. At the same time, limitations such as a constrained minimum lateral feature size, the lack of appropriate p-type materials, or the influence of strain have to be considered when designing circuits. Here, models describing the scaling and bending behavior of flexible InGaZnO TFTs, together with the design of strain insensitive circuits operating at megahertz frequencies are presented

Modulating Thin Film Transistor Characteristics by Texturing the Gate Metal.

The development of reliable, high performance integrated circuits based on thin film transistors (TFTs) is of interest for the development of flexible electronic circuits. In this work we illustrate the modulation of TFT transconductance via the texturing of the gate metal created by the addition of a conductive pattern on top of a planar gate. Texturing results in the semiconductor-insulator interface acquiring a non-planar geometry with local variations in the radius of curvature. This influences various TFT parameters such as the subthreshold slope, gate voltage at the onset of conduction, contact resistance and gate capacitance. Specific studies are performed on textures based on periodic striations oriented along different directions. Textured TFTs showed upto ±40% variation in transconductance depending on the texture orientation as compared to conventional planar gate TFTs. Analytical models are developed and compared with experiments. Gain boosting in common source amplifiers based on textured TFTs as compared to conventional TFTs is demonstrated

Materials, Device, and System Integration of Amorphous Oxide Semiconductor TFTs

Amorphous oxide semiconductor (AOS) thin-film transistors (TFTs) have great potential for use in the next-generation of electronics. AOS TFTs can be used to fabricate circuits and sensors on various substrates, due to unique properties, including high mobility, excellent uniformity, and it requiring a low-temperature process. Currently, indium gallium zinc oxide (IGZO) is the predominant AOS used in the display industry as a TFT semiconductor. Although the IGZO technology is very mature, the development of AOS continues. Additional AOSs are being investigated to reduce cost and improve stability. Considering availability and the potential of materials, indium silicon oxide (ISO) was selected for this project. ISO uses silicon to suppress the instability originating from the oxygen vacancy. The silicon-oxygen bond has a higher dissociation energy, which improves retention of oxygen atoms in the film, and thus, increases the transistor’s stability. This detailed study follows a bottom-up approach. It starts with the fabrication and characterisation of materials. Basic material properties of the ISO film are discussed within, including amorphicity, bandgap, stoichiometry, and Hall-effect parameters. Based on the characterisation results, different deposition recipes for the TFT were developed and tested. The interface quality and etching selectivity were investigated. Uniformity and stability data were extracted from a TFT array using the developed photo-lithography process, which was used to verify and quantitate the capability of the process in system integration and circuit design. A Monte-Carlo simulation environment was established based on the extracted data. The two urgent challenges in all-TFT analogue circuit design, the lack of proper active load and the large parasitic capacitance, were investigated. In-depth analysis on these two issues and applicable solutions were presented. Investigation on system integration of TFT circuits and sensors were conducted, since the device demonstrated the required performance and uniformity. An all-TFT differential-input amplifier was designed and verified, as the first mixed signal all-TFT circuit.China Scholarship Council Cambridge Trus

Design and analysis of high-gain amplifiers in flexible self-aligned a-IGZO thin-film transistor technology

This paper presents two high-gain amplifiers fabricated in a flexible self-aligned amorphous indium gallium zinc oxide thin-film transistor (TFT) technology. One common-source amplifier relies on positive feedback to provide a voltage gain of 17 dB, and a bandwidth of 79 kHz from a dc power of only 0.76 mW. One cascode amplifier provides a voltage gain of 25 dB, and a bandwidth of 220 kHz from a dc power of 2.32 mW. The chip areas of the amplifiers are 7.5 and 10.3 mm2, respectively. By using a gain-enhancement technique in the first amplifier, gain, dc power consumption, and chip area are greatly improved. The presented amplifiers are designed for using as audio pre-amplifiers in a radio receiver. The presented measurements confirm that the amplifiers meet the requirements for this purpose. The circuits are designed using the Verilog-A Rensselaer Polytechnic Institute-amorphous TFT model; circuit simulations are also presented for comparison with the hardware characterization. Additionally, the impact of process variations on the amplifiers is analyzed and discussed in details