Inverters based on Carbon-nanotube Transistors with Switching Frequencies Above 1 MHz on Glass.
Nanoscale field-effect transistors (FETs) based on individual semiconducting carbon nanotubes are of interest for circuits with high integration densities that can be made on inexpensive, large-area substrates, such as glass or flexible plastics. While the static performance of FETs based on individual carbon nanotubes has been discussed many times [1-3], there are only a few reports on the dynamic performance of carbon-nanotube circuits [4-6]. Bachtold et al. and Javey et al. measured signal delays of 30 msec  and 750 µsec , but the switching speed of their circuits was limited by off-chip interconnects that introduced with large parasitic capacitcances. Chen et al. realized fully integrated circuits with a record delay of 1.9 nsec , but these complementary circuits required p-channel and n-channel carbon-nanotube FETs, the latter of which are difficult to obtain and are usually not stable in air. We have fabricated arrays of p-channel carbon-nanotube FETs on glass substrates, integrated the FETs with on-chip load resistors based on vacuum-deposited amorphous carbon films, and measured signal delays as small as 12 nsec. First, an array of probe pads was defined by electron-beam lithography, Ti/AuPd evaporation, and lift-off. Gate electrodes were then defined by e beam lithography and deposition of 30 nm thick Al. The Al gates were briefly exposed to an oxygen plasma to create a 3.6 nm thick AlOx layer, and a 2.1 nm thick phosphonic acid monolayer was then allowed to self-assemble from solution. The total thickness of the AlOx/SAM gate dielectric is 5.7 nm. Carbon nanotubes produced by arc discharge were then deposited from a suspension. Using scanning electron microscopy an individual nanotube was located on each gate, and a pair of AuPd source/drain contacts was defined by e beam lithography for each device. The channel length is ~400 nm. Some of the devices are metallic, but many show useful FET characteristics with ON/OFF ratios up to 107 and ON-state drain currents >1 µA at 1 V. To realize logic circuits we fabricated load resistors on the same substrate by evaporating a thin layer of amorphous carbon, patterned by e-beam lithography. The resistors have excellent linearity and resistances between 105 and 108 Ω, depending on the geometry and film thickness. Inverters composed of a carbon-nanotube FET and an amorphous-carbon load resistor have full output swing and small-signal gain up to 15. To estimate the dynamic performance of the FETs we extracted the time constants from the measured output-signal transitions. When the FETs switch from the OFF-state to the ON-state, the time constant is about 12 nsec, which suggests a maximum frequency of ~10 MHz.  Javey et al., Nano Lett. 2005, 5, 345.  Appenzeller et al., Phys. Rev. Lett. 2004, 93, 196805.  Chen et al., Appl. Phys. Lett. 2005, 86, 123108.  Bachtold et al., Science 2001, 294, 1317.  Javey et al., Nano Lett. 2002, 2, 929.  Chen et al., Science 2006, 311, 1735.