In the second part of this two part article (click here for part one), Cathleen Macher, Technical Analyst at IDTechEx, looks at the very latest progress with carbon nanotubes (CNT) in East Asia. CNTs are becoming commonplace in printed electronics. On March 2, Printed Electronics World published an article covering work at Optomec, who has achieved printed transistors using CNTs which operate at over 5GHz. In this article, we explore progress in East Asia, focussing on Fujitsu and NEC.
Fujitsu's carbon nanotube transistors
Fujitsu has demonstrated a CNT transistor (not printed) with a gate width of 50nm, and an on/off ratio of 4. The biggest issue usually facing CNT transistors is how to control the orientations of the CNTs - they need to be aligned in one direction. Fujitsu claimed it achieved this work with Kyushu University, by growing the CNTs on a specific crystal plane of sapphire. The CNT strands are grown to a length of about 100microns.
Fujitsu reports, "If the miniaturization of semiconductor devices continues to progress, uniform wiring cannot be formed appropriately by using copper. We are promoting the research on the expectation that the CNT wiring can be a substitute for copper wiring."
Graphene transistors are possible
Fujitsu have also demonstrated a graphene transistor, where graphene is used as a channel layer. The transistor was made by Kazuhito Tsukakoshi, a researcher at Japan's National Institute of Advanced Industrial Science and Technology (AIST).
Graphene, in the form of a sheet, has a carrier mobility of 2,000cm2/Vs - similar to that of CNTs. Being in the form of a sheet, it can be easily laid out as the channel layer in transistor design compared to CNTs, where it is harder to control the orientation of CNTs and connect the electrodes.
Fujitsu revealed that the graphene transistor has an on/off ratio of 5-10. The company is now working on ribbon-like graphene sheets by reducing its width, in order to improve the on/off ratio.
NEC
Japan's NEC Corporation says it has successfully fabricated CNT transistors using printing on plastic film. NEC says that CNT transistors can have better properties than silicon transistors of an equivalent size. The device uses zirconium oxide rather than silicon dioxide, which has a lower dielectric constant as the gate insulator, ensuring efficient charge injection into transistor channels and reducing direct-tunnelling leakage currents.
NEC reports the following features of its transistors:
- Each component of the CNT transistors, including electrodes, insulating layers, and CNT channels, are completely printed.
- The composition of materials, solvent concentration, and conditions for insulator ink and metal ink were each optimized in order to eliminate interference between stacked layers, and to maintain printing conditions.
- Low-temperature production (below 200°C) enabled the use of plastic substrates.
- The fabricated transistors demonstrate p-type conduction and an on/off ratio of 1,000.
NEC view CNTs as a key enabling technology platform for printed electronics, and intend to increase development on the topic.
For more details and a list of the main developers and their activities, read Inorganic and Composite Printed Electronics 2009-2019 and Printed and Thin Film Transistors and Memory 2009-2029.
IDTechEx events cover all these topics: www.IDTechEx.com/peEurope.
