Researchers at IBM have demonstrated that high-performance, flexible carbon nanotubes TFTs and complementary integrated circuits can be fabricated on flexible substrates, paving the road for future integration of electronic skin in smart robotics and prosthetic solutions.

Over the past decades, the scaling of transistors made on rigid silicon wafers has steadily boosted the performance of personal electronics and supercomputers. For emerging applications like real-time analytics and Internet of Things (IoT), high-performance logic circuits and sensors made on flexible or unconventional substrates are needed in order to enable the true computation at the edge.

These are several examples of growing areas where flexible nanomaterials, like carbon nanotubes (CNTs), could offer many appealing advantages over rigid silicon, such as low cost, low power, large-area fabrication or even roll-to-roll production.

Although CNTs have been widely considered as superior candidates for flexible electronics due to their high mobility, their practical applications have been limited by the lower performance of flexible CNT thin-film transistors (TFTs) compared to those built on rigid substrates (such as silicon wafer or glass).

In a recent journal article, Flexible CMOS integrated circuits based on carbon nanotubes with sub-10 ns stage delays, published on Nature Electronics, IBM researchers demonstrates that high-performance CNT TFTs and complementary integrated circuits can be fabricated on flexible substrates. The researchers have have addressed key challenges in the fabrication of high-performance flexible CNT electronics, including purity and density of semiconducting CNTs, reliable n-type doping technique for complementary logic, as well as process yield and variation on flexible substrates.

According to Dr. Jianshi Tang, researcher at IBM Thomas J. Watson Research Center, the fabricated flexible CNT TFTs have shown state-of-the-art performance, highlighted by the high current densities (>17 mA/mm), large current ON/OFF ratios (>106), small subthreshold slopes (<200 mV/dec), high mobilities (~50 cm2/Vs) and also excellent flexibility-when wrapping on a finger, the flexible TFTs can still work with no performance degradation.

Integrating all the pieces together, the rsearchers then took one step further to demonstrate high-speed CMOS ring oscillator-a standard benchmark circuit in any logic technology. The functional 5-stage CMOS ring oscillator exhibits stage delays down to only 5.7 nanoseconds, showing nearly 1000X improvement over previous carbon nanotube work. It also represents the fastest flexible ring oscillator ever made with any nanomaterials including CNTs, organic polymers, oxide semiconductors, and nanocrystals.

The superior performance and integration-level demonstration here highlight the potential of using CNTs for future applications such as IoT, edge computing, flexible displays and sensors. An example of such applications is presented in another journal article, Large-area high-performance flexible pressure sensor with carbon nanotube active matrix for electronic skin, recently published on Nano Letters.