Carbon nanotubes form the basis of a new type of logic

By Gary Elinoff, contributing writer  

One of the fundamental properties of an electron is its spin, which can be up or down, suggesting “1” or “0” to any red-blooded electrical engineer. This phenomenon is the heart of a newly emerging area of inquiry into a possible future path for semiconductors called, appropriately enough, spintronics. It holds forth the promise of clock speeds in the terahertz range along with the potential for extremely small size and greatly diminished power requirements.

A team of scientists working at the University of Texas have come up with a design based on carbon nanotubes. They also demonstrated an important must-have if the concept is to achieve practicality — the ability to cascade — for the output of one device to serve as the input to another directly and without any intermediation.

Graphene nanoribbon transistors  

The basic configuration of a graphene nanoribbon transistor (GNR) is illustrated below. The three “wires” are constructed of graphene nanotubes. The center wire, the GNR itself, is partially “unzipped,” so it’s essentially a two-dimensional nanoribbon. Tiny electrical currents, I _CTRL pass thru the two controls (CNT), parallel and on either side of the GNR. Current passing through any wire will generate magnetic fields, and the fields generated here are designated as B . The magnetization of the GNR is stronger, of course, closest to the respective CNTs, and the magnetization decreases closer to the center of the ribbon.

Graphene nanoribbon transistor. Source: Nature.

In this specialized graphene structure, the magnetization can change the ferromagnetic ordering of the GNR, related to the actual spins of the electrons. The two possible states, antiferromagnetic ordering (AFM) and ferromagnetic ordering (FM), determine the conductance through the GNR. In an AFM state, conductance is low, while in FM, conductance is high. A steady voltage is always maintained through the GNR, so a change in conductance translates directly into a change in current. The change from low conductance and low current to high conductance and high current is quite sharp, as it is in a conventional silicon device, and can signify one and zero. 

Quantum computing and computing based on nano structures are active areas of research, but at this stage of the game, most are stand-alone devices. The truly exciting thing about GNRs is that their output current can be directly connected to the next GNR transistor. One GNR can supply the I_CTRL  to one of a subsequent GNR’s control nanotubes, while another GNR can supply the I_CTRL  to the other control nanotube of that subsequent GNR. Thus, two GNRs directly supply the “inputs” to a third GNR. 

Logical function with GNR transistors 

When the current flowing through both control nanotubes is of the same magnitude and is flowing in the same direction, magnetization on both edges of the GNR is the same. That causes an AFM state, and the conductance of the GNR will be low, causing an output of zero. If one of the control nanotubes carries a high current, and the other a low current, the magnetizations on the edges of the GNR will be different. An FM state occurs, causing an output of one. 

In this manner, combinational logic can be affected. 

Additionally, more complex geometries are possible, combining the logical function of more than one transistor into one device. And current is the state variable in GNR transistors, rather than voltage, as is the case with silicon devices. This leads to astonishingly fast switching time, with clock frequencies in the 2-terahertz range being attainable. And finally, power consumption in GNRs is, unlike in the case of silicon devices, largely independent of frequency.

Spintronics-based components are still a long way away from commercialization. However, the researchers have demonstrated what will clearly be the basis for a technology that will loom large in the not-too-distant future.