Projected Tolerances of Carbon Nanotube Current-Mode Logic to Process Variability

Radio frequency (RF) analog applications present an interesting opportunity for carbon nanotube (CNT)-based electronics. Extrinsic peak operating frequencies of up to 40 GHz for CNT field-effect transistors manufactured on wafer scale employing very moderate dimensions have been already demonstrated. However, as in any emergent technology, variability of the fabricated devices is significant. Therefore, predictions about the behavior of benchmark circuits at the process corners are required to further stimulate complex RF design and fabrication. We report here on the projected tolerance of a current-mode logic (CML) buffer/inverter designed for 2.4 GHz (5 Gb/s), 1.8-V supply voltage, 1.8-mA tail current, and a 600-mV voltage swing. The single CML stage is very robust against variations in CNT density but vulnerable to source-drain shorts introduced by metallic tubes in the channel. A corresponding five-stage CML ring oscillator shows considerably reduced output voltage swing beyond two percent metallic tube fraction indicating the importance of metallic tube reduction by technology dependent strategies like presorting, post-elimination, or type-selective growth.