New hybrid circuit could replace silicon

A new energy-efficient hybrid circuit could soon replace silicon as the traditional transistor material used in electronic chips.

Researchers from the USC Viterbi School of Engineering have overcome a major issue in carbon nanotube technology by developing a flexible, energy-efficient hybrid circuit combining carbon nanotube thin-film transistors with other thin-film transistors.

Since carbon nanotubes are more transparent, flexible and can be processed at a lower cost, this hybrid could take the place of silicon.

Electrical engineering professor Dr Chongwu Zhou and USC Viterbi graduate students Haitian Chen, Yu Cao and Jialu Zhang developed this circuit by integrating carbon nanotube (CNT) thin-film transistors (TFT) with thin-film transistors comprising indium, gallium and zinc oxide (IGZO).

Carbon nanotubes are so small that they can only be viewed through a scanning electron microscope. This hybridisation of carbon nanotube thin films and IGZO thin films was achieved by combining their types, p-type and n-type, respectively, to create circuits that can operate complementarily, reducing power loss and increasing efficiency. The inclusion of IGZO thin-film transistors was necessary to provide power efficiency to increase battery life. If only carbon nanotubes had been used, then the circuits would not be power-efficient. By combining the two materials, their strengths have been joined and their weaknesses hidden.

Zhou likened the coupling of carbon nanotube TFTs and IGZO TFTs to the Chinese philosophy of yin and yang.

The potential applications for this kind of integrated circuitry are numerous, including organic light emitting diodes (OLEDs), digital circuits, radio frequency identification (RFID) tags, sensors, wearable electronics and flash memory devices. Even heads-up displays on vehicle dashboards could soon be a reality.

The new technology also has major medical implications. Currently, memory used in computers and phones is made with silicon substrates, the surface on which memory chips are built. To obtain medical information from a patient such as heart rate or brainwave data, stiff electrode objects are placed on several fixed locations on the patient’s body. With this new hybridised circuit, however, electrodes could be placed all over the patient’s body with just a single large but flexible object.

With this development, Zhou and his team have circumvented the difficulty of creating n-type carbon nanotube TFTs and p-type IGZO TFTs by creating a hybrid integration of p-type carbon nanotube TFTs and n-type IGZO TFTs and demonstrating a large-scale integration of circuits. As a proof of concept, they achieved a scale ring oscillator consisting of over 1000 transistors. Up to this point, all carbon nanotube-based transistors had a maximum number of 200 transistors.

The next step for Zhou and his team will be to build more complicated circuits using a CNT and IGZO hybrid that achieves more complicated functions and computations, as well as to build circuits on flexible substrates. Zhou and Chen believe that carbon nanotube technology, including this new CNT-IGZO hybrid, will be commercialised in the next 5-10 years.

Their findings have been published in Nature Communications.