Flexible IGZO thin-film transistors with liquid EGaln gate contacts

Flexible thin-film electronics leverage technological innovations in fields such as sensors, wearable Computing and healthcare. As these Systems are required to conform to non-planar surfaces, novel approaches are developed to provide stable performance under mechanical stress, and prevent crack formation. Liquid eutectic-Galn promises the realisation of self- healing, reconfigurable and bendable circuits. Here, a liquid EGaln-gate thin-film transistor is fabricated and characterised. The device yielded a carrier mobility of 7.9 cm 2 V –1 s –1 that increased by 0.36 cm 2 V 1 s –1 when bent to a 4 mm radius. These results promote the integration of highly deformable liquid materials into thin-film devices

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

Contact resistance and overlapping capacitance in flexible sub-micron long oxide thin-film transistors for above 100 MHz operation

In recent years new forms of electronic devices such as electronic papers, flexible displays, epidermal sensors, and smart textiles have become reality. Thin-film transistors (TFTs) are the basic blocks of the circuits used in such devices and need to operate above 100 MHz to efficiently treat signals in RF systems and address pixels in high resolution displays. Beyond the choice of the semiconductor, i.e., silicon, graphene, organics, or amorphous oxides, the junctionless nature of TFTs and its geometry imply some limitations which become evident and important in devices with scaled channel length. Furthermore, the mechanical instability of flexible substrates limits the feature size of flexible TFTs. Contact resistance and overlapping capacitance are two parasitic effects which limit the transit frequency of transistors. They are often considered independent, while a deeper analysis of TFTs geometry imposes to handle them together; in fact, they both depend on the overlapping length (LOV) between source/drain and the gate contacts. Here, we conduct a quantitative analysis based on a large number of flexible ultra-scaled IGZO TFTs. Devices with three different values of overlap length and channel length down to 0.5 μm are fabricated to experimentally investigate the scaling behavior of the transit frequency. Contact resistance and overlapping capacitance depend in opposite ways on LOV. These findings establish routes for the optimization of the dimension of source/drain contact pads and suggest design guidelines to achieve megahertz operation in flexible IGZO TFTs and circuits

Fabrication, modeling, and evaluation of a digital output tilt sensor with conductive microspheres

Recent advances in wearable computing ask for bendable and conformable electronic circuits and sensors, allowing an easy integration into everyday life objects. Here, we present a novel flexible tilt sensor on plastic using conductive microspheres as gravity sensitive pendulum. The sensor provides a digital output of the measurement signal without the need for any additional electronics (e.g., amplifiers) close to the sensing structure. The sensor is fabricated on a free-standing polyimide foil with SU-8 photoresist defining the cavity for the pendulum. The pendulum consists of freely movable conductive microspheres which, depending on the sense of gravity, connect different electric contacts patterned on the polyimide foil. We develop a model of the sensor and identify the amount of microspheres as one of the key parameters in the sensor design, which influences the performance of the sensor. The presented tilt sensor with eight contacts achieves an angular resolution of 22.5° with a hysteresis of 10° and less at a tilt of the sensor plane of 50°. Analysis of the microsphere movements reveals a response time of the sensor at ~ 50 ms

Oxide thin-film transistors on fibers for smart textiles

Smart textiles promise to have a significant impact on future wearable devices. Among the different approaches to combine electronic functionality and fabrics, the fabrication of active fibers results in the most unobtrusive integration and optimal compatibility between electronics and textile manufacturing equipment. The fabrication of electronic devices, in particular transistors on heavily curved, temperature sensitive, and rough textiles fibers is not easily achievable using standard clean room technologies. Hence, we evaluated different fabrication techniques and multiple fibers made from polymers, cotton, metal and glass exhibiting diameters down to 125 µm. The benchmarked techniques include the direct fabrication of thin-film structures using a low temperature shadow mask process, and the transfer of thin-film transistors (TFTs) fabricated on a thin (≈1 µm) flexible polymer membrane. Both approaches enable the fabrication of working devices, in particular the transfer method results in fully functional transistor fibers, with an on-off current ratio >107 , a threshold voltage of ≈0.8 V, and a field effect mobility exceeding 7 cm2 V −1 s −1 . Finally, the most promising fabrication approach is used to integrate a commercial nylon fiber functionalized with InGaZnO TFTs into a woven textile

Flexible IGZO TFT Spice model and design of active strain-compensation circuits for bendable active matrix arrays

The detailed measurement and characterization of strain induced performance variations in flexible InGaZnO thinfilm transistors (TFTs) resulted in a Spice TFT model able to simulate tensile and compressive bending. This model was used to evaluate a new concept, namely the active compensation of strain induced performance variations in pixel driving circuits for bendable active matrix arrays. The designed circuits can compensate the mobility and threshold voltage shifts in IGZO TFTs induced by bending. In a single TFT, a drain current of 1 mA varies by 83 µA per percent of mechanical strain. The most effective compensation circuit design, comprising one additional TFT and two resistors, reduces the driving current variation to 1.1 µA per percent of strain. The compensation circuit requires no additional control signals, and increases the power consumption by only 235 µW (corresponds to 4.7 %). Finally, switching operation is possible for frequencies up to 200 kHz. This opens a way towards the fabrication of flexible displays with constant brightness even when bent

Flexible InGaZnO TFTs with fmax above 300 MHz

n this letter, the AC performance and influence of bending on flexible IGZO thin-film transistors, exhibiting a maximum oscillation frequency (maximum power gain frequency) fmax beyond 300 MHz, are presented. Self-alignment was used to realize TFTs with channel length down to 0.5 μm. The layout of this TFTs was optimized for good AC performance. Besides the channel dimensions this includes ground-signal-ground contact pads. The AC performance of this short channel devices was evaluated by measuring their two port scattering parameters. These measurements were used to extract the unity gain power frequency from the maximum stable gain and the unilateral gain. The two complimentary definitions result in fmax values of (304 ± 12)MHz and (398 ± 53) MHz, respectively. Furthermore, the transistor performance is not significantly altered by mechanical strain. Here, fmax reduces by 3.6% when a TFT is bent to a tensile radius of 3.5 mm

Low temperature and radiation stability of flexible IGZO TFTs and their suitability for space applications

In this paper, Low Earth Orbit radiation and temperature conditions are mimicked to investigate the suitability of flexible Indium-Gallium-Zinc-Oxide transistors for lightweight space-wearables. Such wearable devices could be incorporated into spacesuits as unobtrusive sensors such as radiation detectors or physiological monitors. Due to the harsh environment to which these space-wearables would be exposed, they have to be able to withstand high radiation doses and low temperatures. For this reason, the impacts of high energetic electron irradiation with fluences up to 1012 e-/cm2 and low operating temperatures down to 78 K, are investigated. This simulates 278 h in a Low Earth Orbit. The threshold voltage and mobility of transistors that were exposed to e- irradiation are found to shift by +0.09 ± 0.05V and -0.6 ± 0.5cm2 V-1 s-1. Subsequent low temperature exposure resulted in additional shifts of +0.38 V and -5.95 cm2 V-1 s-1 for the same parameters. These values are larger than the ones obtained from non-irradiated reference samples. If this is considered during the systems’ design, these devices can be used to unobtrusively integrate sensor systems into space-suits

Charge trapping mechanism leading to sub-60-mV/decade-Swing FETs

In this work, we present a novel method to reduce the subthreshold swing of field-effect transistors below 60 mV/dec. Through modeling, we directly relate trap charge movement between the gate electrode and the gate dielectric to subthreshold swing reduction. We experimentally investigate the impact of charge exchange between a Cu gate electrode and a 5 nm thick amorphous Al2O3 gate dielectric in an InGaZnO4 thin-film transistor. Positive trap charges are generated inside the gate dielectric while the semiconductor is in accumulation. During the subsequent de-trapping, the subthreshold swing diminishes to a minimum value of 46 mV/dec at room temperature. Furthermore, we relate the charge trapping/de-trapping effects to a negative capacitance behavior of the Cu/Al2O3 metal-insulator structure

Inferring complex textile shape from an integrated carbon black-infused ecoflex-based bend and stretch sensor array

We demonstrate how an array of custom-made strain and bend sensors could be integrated into a stretchable sleeve to infer the textile deformation. The angles and elongation measured by the sensors can be used by an optimisation-based algorithm to infer the textile geometrical model by minimising a loss function. We evaluated this on 4 shapes highlighting different body-part characteristics. We demonstrated that a 3.11 mm reconstruction error on complex geometries can be reduced up to 0.08 mm with the computation of angles. This proves the potential of the proposed prototype for capturing the shape of a body parts, muscle density measurement, body shape acquisition, the fabrication of orthoses and prostheses, or to perform movement sensing for human activity recognition, where it could be included in sports leggings for biomechanical analysis, or in everyday garments for motion and gesture sensing