How fast can a flexible circuit operate? The question is inquired with great interest and passion by researchers worldwide. Flexible circuits are building blocks in flexible sensing systems, which interface with humans and have enormous potential in health care applications, such as biosignal monitoring. High-speed flexible circuits are in high demand, since they provide a platform for efficient in-situ signal processing, or more fashionably named, near-sensor computing. In addition, the wireless system operations call for a high-speed, therefore reasonably sized system, because the footprint of a wireless system with far-field communication capability is mainly limited by the size of the antenna. The frequency of circuit operation must be raised to above 100 MHz to shrink the size of the antenna to an acceptable size of < 100 cm. However, there is a long-standing vacancy of flexible high-speed circuits with such high operating frequencies. Carbon Nanotube based Thin-film transistor (CNT-TFT) technology is favoured to construct high-speed flexible circuits, due to the excellent electrical and mechanical properties of CNTs along with their low-temperature process capability. Scaling devices down is the most straightforward approach to promote speed for high-frequency operation.
Simple as the idea of scaling may seem, the downscaling process of flexible CNT-TFTs faces several practical problems. For example, the insufficient high purity of semiconducting-CNT, the harsh fabrication circumstances on polymer substrates (low thermal budget, restricted fabrication methodology, etc.), deteriorated interface conditions in the devices (a large amount of interface traps, low gate-control efficiency, deteriorated contacts, etc.), etc. Tackling these problems mentioned above and achieving high performance in a scaled flexible CNT-TFT device is therefore a challenge, and this challenge is even more prominent when channel lengths are scaling down to the sub-micrometer region. Flexible CNT-TFTs with sub-μm Lch that can achieve potential in performance and speed demanded by wireless systems have not been reported.
Fig. 1: CNT-TFTs fabricated on a flexible parylene substrate. a Schematic illustration of device and circuit fabrication on a flexible parylene substrate. b Schematic diagram of a flexible CNT-TFT.
With proper material and methodology, our work filled the vacancy of high-performance flexible CNT-TFTs with sub-μm Lch and high-speed flexible circuits with sub-ns stage delays. Flexible CNT-TFTs with scaled Lch of 450 nm were fabricated on a 2-μm-thick parylene substrates, as illustrated in Fig. 1a. State-of-the-art performances of high on-state current (187.6 μA μm-1) and large transconductance (123.3 μS μm-1) were obtained, which were comparable with rigid CNT-TFTs of similar size. A nonoverlapping gate structure was adopted to suppress parasitic capacitance by introducing air gaps, as shown in Fig. 1b, therefore boosting the speed of devices. Five-stage flexible ring oscillators were built to benchmark the speed of scaled devices, demonstrating sub-nanosecond stage delays, with our champion RO showing a 281 ps stage delay at a low supply voltage of 2.6 V (shown in Fig. 2a). It is for the first time a flexible circuit operating with a sub-ns stage delay (shown in Fig. 2b). Such high-speed CNT-TFTs lay a solid foundation for the realization of high-speed analogue-digital mixed signal or radio frequency front-end circuits, which composed the integrated wireless sensing systems.
Fig. 2: Flexible CNT-TFT-based RO showing sub-nanosecond stage delays. a Power spectra of multiple 5-stage ROs with Lch of 1 μm and 450 nm, showing sub-ns stage delays. b Comparison of stage delays among representative flexible ROs.
And it is notable that, our work also leaves room for further, more aggressive downscaling of flexible CNT-TFTs, toward which the performance of the device and speed of circuits should be pushed to the limit. The scaling behaviour analysis, based on the Y function method, reveals that the enhanced performance introduced by scaling is attributed to channel resistance reduction while the contact resistance remains unchanged. Meanwhile, the extracted contact resistance on a single nanotube basis of our devices is comparable to that achieved in devices on rigid substrates, indicating great potential in ultimate scaled flexible CNT-TFTs with high performance comparable to their counterparts on rigid substrates where contact resistance dominates the whole device performance. On rigid substrates, circuits based on CNT-TFT with sub-200 nm Lch were reported to operate at frequencies of several gigahertz. It is imaginable that the ultimately scaled flexible CNT-TFTs help construct the speed-boosted and function-enriched flexible wireless systems, which contribute to the multi-scenario next-generation health care applications.
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