Graphene transistors: Keeping the noise down

Ultralow-noise transistors can be fabricated using multilayer graphene.

The recent discovery of the exceptional electronic properties of graphene — a single layer of graphite carbon — has spurred intense research on nanoelectronic devices. Apart from single-layered graphene, multilayered structures of graphene could also be used for device applications. However, the dependence of device performance on the number of layers has not been thoroughly investigated.

Now, Atindra Nath Pal and Arindam Ghosh from the Indian Institute of Science in Bangalore¹ have found a clear contrast between single-layer devices and their multilayered counterparts through study of the electrical noise in graphene transistors consisting of varying numbers of graphene layers.

The researchers fabricated their transistors by depositing graphene flakes, obtained by exfoliation of graphite, onto an SiO₂ layer grown on a heavily doped silicon substrate.

Fig. 1: Scanning electron micrograph of a typical few-layer graphene device.

In the case of the single-layer device, the researchers observed maximum noise at the charge neutrality point, and lower noise associated with the production of positive or negative carriers by application of a gate voltage. The opposite behavior was observed for multilayer graphene devices.

“The band structure in multilayer graphene changes at high gate electric fields, weakening the screening of the potentials from external impurities, thereby increasing noise,” says Pal. “However, in a single-layer graphene device, the gate field does not affect the band structure, and moreover, screening improves at finite doping, leading to a reduction in noise.”

A more general observation was that the noise intensity was two orders of magnitude higher for the single-layer device than for the multilayer devices. The origin of this difference lies in the fact that the carriers in single-layer graphene are very close to the SiO₂ and ‘feel’ the potential fluctuations generated by the impurities in the substrate. In the multilayer case, carriers at a distance from the substrate are less affected by such fluctuations.

“The most important outcome of our experiments is the realization that few-layer graphene may be a better choice for low-noise electronics, without compromising the gating ability too much,” says Pal. In addition, the difference in the noise could be used to distinguish single-layer from multilayer graphene, particularly in situations where other techniques used to establish thickness do not work.