What Are CNTFETs?

CNTFETs, or carbon nanotube field-effect transistors, are transistors in which the conducting channel is formed by one or more semiconducting carbon nanotubes (CNTs) rather than bulk silicon or other conventional semiconductor materials. A carbon nanotube is a cylindrical arrangement of carbon atoms in a hexagonal lattice, and when rolled into a single-walled tube with the appropriate chirality, the nanotube exhibits semiconducting behavior with a bandgap inversely proportional to its diameter. The device structure mirrors that of a conventional MOSFET, with source and drain contacts at the ends of the nanotube and a gate electrode that controls the carrier density in the channel. CNTFETs were first demonstrated experimentally in 1998 and have since been studied as a potential successor to silicon CMOS in the post-scaling era.

The motivation for CNTFETs comes from two properties of semiconducting single-walled carbon nanotubes: their extremely high carrier mobility, which exceeds silicon by a factor of roughly 200, and their near-ballistic transport, in which carriers traverse the channel with few scattering events because the quasi-one-dimensional geometry eliminates lateral scattering paths.

Carbon Nanotube Channel Physics

The electrical properties of a carbon nanotube channel are determined by its diameter and chirality, which together set the bandgap, carrier effective mass, and intrinsic capacitance. Semiconducting nanotubes with diameters in the range of 1 to 2 nm produce bandgaps of 0.5 to 1 eV, suitable for switching applications. The one-dimensional density of states concentrates carriers into discrete subbands, and transport in the channel approaches the ballistic limit in short devices, where the channel length is shorter than the carrier mean free path of several hundred nanometers. Quantum capacitance, the contribution to total gate capacitance arising from the finite density of states in the nanotube, becomes significant in CNTFETs because the geometric gate capacitance is low for a nanometer-scale channel. When quantum capacitance is smaller than the geometric capacitance, it becomes the limiting factor in gate control, and its proper modeling is essential for accurate CNTFET circuit simulation. The Nature Electronics paper on fabricating CNTFETs in commercial silicon manufacturing facilities describes how these quantum mechanical properties are preserved when nanotubes are integrated into back-end-of-line CMOS processing.

Device Fabrication and Integration

CNTFET fabrication requires placing semiconducting nanotubes with controlled density and orientation between source and drain contacts, and forming a gate dielectric and gate electrode over the nanotube channel. Early devices used individual nanotubes isolated by atomic force microscopy, but scalable manufacturing requires depositing arrays of aligned nanotubes from solution or growing them by chemical vapor deposition on catalyst-patterned substrates. A critical challenge is the separation of semiconducting nanotubes from metallic tubes in the as-grown material; metallic nanotubes short the channel and degrade device switching behavior. Techniques including density gradient ultracentrifugation and gel chromatography achieve semiconducting purity above 99.9 percent. IBM and Stanford researchers have demonstrated that carbon nanotube FET fabrication is compatible with standard CMOS photolithography and deposition equipment, an important step toward integration with existing manufacturing infrastructure.

Performance and Scaling Outlook

Benchmarking studies project that CNTFET logic circuits operating at scaled supply voltages would consume three to five times less energy per switching event than comparable silicon CMOS at equivalent performance. The ScienceDirect review of CNTFETs for ultra-low power applications surveys the experimental and projected performance, noting that fabrication uniformity and nanotube alignment remain the primary barriers to high-yield production. Wrap-around gate structures, in which the gate electrode surrounds the full nanotube circumference, improve short-channel electrostatics and are the device geometry of choice for sub-10 nm channel lengths.

Applications

CNTFETs are being explored for applications in several technically demanding areas, including:

• Post-silicon logic circuits for energy-efficient computing at advanced technology nodes
• Radio frequency transistors where high carrier velocity enables gain at millimeter-wave frequencies
• Chemical and biological sensors, exploiting the nanotube surface area and sensitivity to molecular adsorption
• Flexible and transparent electronics on polymer substrates, enabled by the mechanical flexibility of carbon nanotubes
• Non-volatile memory elements incorporating nanotube electromechanical switching