TFT Small Signal Model and Analysis

We present an accurate small signal model for thin film transistors (TFTs) taking into account non-idealities such as contact resistance, parasitic capacitance, and threshold voltage shift. The model gives high accuracy in s-parameters, and the predicted cutoff frequency yields 1% discrepancy compared with measurement results. In contrast, the conventional CMOS small signal model adapted for TFTs yields 12.5% error. The TFT’s cutoff frequency is also evaluated under bias stress to examine the effect of device instability on small signal behavior.This is the author accepted manuscript. The final version is available from IEEE via http://dx.doi.org/10.1109/LED.2016.257592

Conduction Threshold in Accumulation-Mode InGaZnO Thin Film Transistors

AbstractThe onset of inversion in the metal-oxide-semiconductor field-effect transistor (MOSFET) takes place when the surface potential is approximately twice the bulk potential. In contrast, the conduction threshold in accumulation mode transistors, such as the oxide thin film transistor (TFT), has remained ambiguous in view of the complex density of states distribution in the mobility gap. This paper quantitatively describes the conduction threshold of accumulation-mode InGaZnO TFTs as the transition of the Fermi level from deep to tail states, which can be defined as the juxtaposition of linear and exponential dependencies of the accumulated carrier density on energy. Indeed, this permits direct extraction and visualization of the threshold voltage in terms of the second derivative of the drain current with respect to gate voltage.Authors thank to the EU-FP7 under Project ORAMA CP-IP 246334-2.This is the final version of the article. It first appeared from Nature Publishing Group via http://dx.doi.org/10.1038/srep2256

Flexible In-Ga-Zn-O thin-film transistors with sub-300-nm channel lengths defined by two-photon direct laser writing

In this work, the low-temperature (≤ 150 °C) fabrication and characterization of flexible Indium-Gallium-ZincOxide (IGZO) top-gate thin-film transistors (TFTs) with channel lengths down to 280 nm is presented. Such extremely short channel lengths in flexible IGZO TFTs were realized with a novel manufacturing process combining two-photon direct laser writing (DLW) photolithography with Ti/Au/Ti source/drain e-beam evaporation and lift-off. The resulting flexible IGZO TFTs exhibit a saturation field-effect mobility of 1.1 cm2V -1 s -1 and a threshold voltage of 3 V. Thanks to the short channel lengths (280 nm) and the small gate to source/drain overlap (5.2 µm), the TFTs yield a transit frequency of 80 MHz (at 8.5 V gate-source voltage) extracted from the measured S-parameters. Furthermore, the devices are fully functional when wrapped around a cylindrical rod with 6 mm radius, corresponding to 0.4 % tensile strain in the TFT channel. These results demonstrate a new methodology to realize entirely flexible nano-structures, and prove its suitability for the fabrication of short-channel transistors on polymer substrates for future wearable communication electronics

Oxygen Defect-Induced Metastability in Oxide Semiconductors Probed by Gate Pulse Spectroscopy.

We investigate instability mechanisms in amorphous In-Ga-Zn-O transistors based on bias and illumination stress-recovery experiments coupled with analysis using stretched exponentials and inverse Laplace transform to retrieve the distribution of activation energies associated with metastable oxygen defects. Results show that the recovery process after illumination stress is persistently slow by virtue of defect states with a broad range, 0.85 eV to 1.38 eV, suggesting the presence of ionized oxygen vacancies and interstitials. We also rule out charge trapping/detrapping events since this requires a much smaller activation energy ~0.53 eV, and which tends to be much quicker. These arguments are supported by measurements using a novel gate-pulse spectroscopy probing technique that reveals the post-stress ionized oxygen defect profile, including anti-bonding states within the conduction band.Authors thank to the EU-FP7 under Project ORAMA CP-IP 246334-2. Also, they would like to thank Dr. J. W. Jin, University of Cambridge, UK for technical discussions.This is the final version of the article. It first appeared from Nature Publishing Group via http://dx.doi.org/10.1038/srep1490

Interactive Displays: The Next Omnipresent Technology [Point of View]

Visual display of information is an obvious requirement in today,s highly digital world, and constitutes a powerful means of conveying complex information. This stems from the ability of the human eye and brain to perceive and process vast quantities of data in parallel. The history of visualizing information can be traced to the ancient era, when our ancestors carved images on cave walls and monuments. Mosaic art form emerged in the 3rd millennium BC, using small pieces of glass, stone, or other materials in combination to display information. These pieces are similar to pixels in the modern electronic display. The electronic display has become the primary human-machine interface in most applications, ranging from mobile phones, tablets, laptops, and desktops to TVs, signage, and domestic electrical appliances, not to mention industrial and analytical equipment. In the meantime, user interaction with the display has progressed significantly. Through sophisticated hand gestures, the display has evolved to become a highly efficient information exchange device. While interactive displays are currently very popular in mobile electronic devices such as smartphones and tablets, the development of large-area, flexible electronics, offers great opportunities for interactive technologies on an even larger scale. Indeed technologies that were once considered science fiction are now becoming a reality; the transparent display and associated smart surface being a case in point. Examines the market for interactive displays as the next omnipresent technology

Device-circuit interactions and impact on TFT circuit-system design

This paper reviews the importance of device-circuit interactions (DCI) and its consideration when designing thin film transistor circuits and systems. We examine temperature- and process-induced variations and propose a way to evaluate the maximum achievable intrinsic performance of the TFT. This is aimed at determining when DCI becomes crucial for a specific application. Compensation methods are then reviewed to show examples of how DCI is considered in the design of AMOLED displays. Other designs such as analog front-end and image sensors are also discussed, where alternate circuits should be designed to overcome the limitations of the intrinsic device properties

Mono-Type TFT Logic Architectures for Low Power Systems on Panel Applications

This paper introduces novel 7-T pseudo-CMOS for enhancement mode and 6-T pseudo-CMOS for depletion mode inverter circuit architectures. The designs are built around mono-type of TFTs and consume less power consumption than existing 4-T pseudo-CMOS circuits. In addition, they provide steep transfer curves, along with embedded control for compensation of device parameter variations. Analysis of the transient behavior for the various circuit architectures is presented, providing quantitative insight into capacitive loading taking into account the effects of overlap capacitances

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

A Multi-functional Touch Panel for Multi-Dimensional Sensing in Interactive Displays

This thesis presents a flexible graphene/polyvinylidene difluoride (PVDF)/graphene sandwich for three-dimensional touch interactivity. Here, an x-y plane touch is sensed using graphene capacitive elements, while force sensing in the z-direction is by a piezoelectric PVDF/graphene sandwich. By employing different frequency bands for the capacitive- and force-induced electrical signals, the two stimuli are detected simultaneously, achieving three-dimensional touch sensing. Static force sensing and elimination of propagated stress are achieved by augmenting the transient piezo output with the capacitive touch, thus overcoming the intrinsic inability of the piezoelectric material in detecting non-transient force signals and avoiding force touch mis-registration by propagated stress. As a capacitive signal is important for force touch interpretation, optimization algorithms have been developed and implemented. With correlated double sampling (CDS) and spatial low-pass filtering (SLPF) based techniques, the signal-to-noise ratio (SNR) of the capacitive touch signal is boosted by 15.6 dB, indicating improved detection accuracy. In terms of the readout speed, fixed pattern and random pattern related down-sampling techniques are applied, giving rise to reductions in both readout time (11.3 ms) and power consumption (8.79 mW).China Scholarship Counci

Flexible Ultralow-Power Sensor Interfaces for E-Skin

Thin-film electronics has hugely benefitted from low-cost processes, large-area processability, and multi-functionality. This has not only stimulated innovation in display and sensor technology, but has also demonstrated great potential for integration of components for human-machine interfaces. For electronics to be deployed as sensor interfaces and signal processing, the quest for low power is compelling due to the inherently limited battery lifetime. This review will present the state-of-the-art in thin film electronics and demonstrate examples of low-cost printable transistors and biosensors that are flexible/stretchable for wearable and other applications. Ultralow power design for thin-film transistors will be discussed from the standpoint of reducing both operating voltage and operating current, taking into account the challenges in meeting frequency requirements. Compact models for circuit design will be reviewed along with new insights into ultralow power transistors and high gain amplifier circuits. Finally, a concept for an integrated system comprising sensors and interfacing circuits will be demonstrated, which has the potential to enable battery-less operation.EPSRC under Project EP/M013650/1 EU under Projects DOMINO 645760, 1D-NEON 685758 and BET-EU 692373 IEEE Electron Devices Society PhD Student Fellowship China Scholarship Counci