6 September 2010


UCLA sets record cut-off frequency of 300GHz for graphene transistors

Researchers at University of California, Los Angeles (UCLA) have reported record intrinsic cut-off (transit) frequency of fT = 100–300GHz for graphene transistors, comparable to transistors using high-electron mobility materials (such as gallium arsenide or indium phosphide) with similar gate lengths, it is claimed (Lei Liao et al, Nature; doi:10.1038/nature09405).

Graphene (a one-atom-thick layer of graphitic carbon) has the highest known carrier mobility, giving it the potential to make radio-frequency electronic devices faster and smaller. However, the material's unique properties have led to difficulties in integrating it into such electronic devices. Conventional fabrication processes cannot readily be applied to produce graphene transistors because they often introduce significant defects into the monolayer of carbon lattices and severely degrade the device performance. The researchers now say that they have overcome some of these difficulties.

Led by professor of chemistry and biochemistry Xiangfeng Duan, the UCLA team has developed a new fabrication process for graphene transistors that uses a Co2Si–Al2O3 core–shell nanowire as the gate, with the source and drain electrodes defined through a self-alignment process and the channel length defined by the nanowire diameter (self-aligned gates were developed to deal with problems of misalignment encountered due to the shrinking scale of transistors).

To develop the new fabrication technique, Duan teamed with two other researchers from the California NanoSystems Institute at UCLA, assistant professor of materials science and engineering Yu Huang and professor of electrical engineering Kang Wang, both of the at the Henry Samueli School of Engineering and Applied Sciences.

“This new strategy overcomes two limitations previously encountered in graphene transistors,” Duan says.

“First, it [the physical assembly of the nanowire gate] doesn’t produce any appreciable defects in the graphene during fabrication, so the high carrier mobility is retained,” he says.

“Second, by using a self-aligned approach with a nanowire as the gate, the group was able to overcome alignment difficulties previously encountered and fabricate very short-channel devices with unprecedented performance,” Duan adds. The self-alignment process ensures that the edges of the source, drain and gate electrodes are automatically and precisely positioned so that no overlapping or significant gaps exist between the electrodes, minimizing access resistance.

Graphene transistors with a channel length as low as 140nm have been fabricated with record scaled on-current (3.32mA/μm) and transconductance (1.27mS/μm ). On-chip microwave measurements show that the self-aligned devices have an intrinsic cut-off (transit) frequency fT=100–300GHz. However, the extrinsic fT (of a few gigahertz) is largely limited by parasitic pad capacitance.

“We are currently taking additional efforts to scale up the approach and further boost the speed,” says the lead author, postdoctoral fellow Lei Liao. Such high-speed radio-frequency electronics based on graphene could find wide applications in microwave communication, imaging and radar technologies. 

Funding for the UCLA research came from the US National Science Foundation (NSF) and the National Institutes of Health (NIH).

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