The IBM team has been able to build a 5-stage complementary type MOS (CMOS) ring oscillator based on an individual single-walled carbon nanotube. Including a read-out stage and a test stage, in total 14 field-effect transistors (FETs) were built side by side on the same carbon nanotube which is 18 micrometers long. Powered by a small dc voltage, the ring oscillator produces an ac output signal and the frequency can be modulated by the input.
In order to realize the CMOS scheme with a carbon nanotube, different work function metals, palladium (Pd) and aluminum (Al), are used as the top metal gates for the p-type and n-type FETs, respectively. The Schottky barrier type nanotube FETs used in this circuit layout show ambipolar behavior, i.e. they possess both hole (p-type) and electron (n-type) branches in their electrical characteristics. The critical part is that the work function difference between Pd and Al shifts the threshold voltages of the two FETs such that the n-type FET is “off” while the p-type FET is “on” and vice versa.
The source/drain metal contacts effectively “cut” the long nanotube into 14 segments. Although all these segments are connected to each other through the nanotube portions underneath the metal contacts, the 14 FETs are working without impacting each others performance, i.e. no crosstalk between devices was observed. The circuit layout is designed such that the same FET type (p or n) from adjacent inverter stages shares the same source/drain contact. The extra inverter stage following the ring oscillator functions as a “read-out” stage. It prevents any interference between the ring oscillator and the measurement set-up which is a spectrum analyzer in the present case. An independent test inverter stage has also been fabricated on the same nanotube (not shown in the animation picture). This extra inverter allows studying the electronic properties of the nanotube and testing the electrical characteristics of an individual inverter beforehand.
The carbon nanotube ring oscillator operates at 13MHz with a supply voltage of 0.5V, and the frequency increases to 52MHz at a voltage of 0.92V. This first nanotube ring oscillator that is entirely wired “on-chip” is still slower than a state-of-the-art silicon circuit, due to the non-optimized layout which results in large parasitic. The measured frequency response of the ring oscillator matches precisely the expected behavior if these parasitics are taken into account. So far no limitations in the ac performance due to the intrinsic carbon nanotube properties were observed. With further improvements of the nanofabrication process, the team believes that they will eventually unlock the full performance potential of carbon nanotubes.
 Captions: CMOS type 5-stage carbon nanotube ring oscillator circuit layout animation. The red colored sections display n-type FETs and the green ones represent p-type FETs. The FETs show dark colors (dark red or dark green) in their off-states and bright colors in their on-states. |
 |