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Rapid (nanosecond-scale) electrical pulsing is used to study drift-velocity saturation in graphene field-effect devices. In these experiments, high-field pulses are utilized to drive graphene's carriers on time scales much faster than that on which energy loss to the underlying substrate can occur, thereby allowing the observation of the highest saturation velocities reported to date. In a dramatic departure from the behavior exhibited by conventional metals and semiconductors, as the electron or hole density is reduced toward the charge-neutrality point, the drift velocity is found to reach values comparable to the Fermi velocity itself. Corresponding current densities are as large as 109 A/cm², similar to the values reported for carbon nanotubes and for graphene-on-diamond transistors. In essence, our approach of rapid pulsing allows us to "free" graphene from the deleterious influence of its substrate, revealing a pathway to achieve the superior electrical performance promised by this material. The usefulness of this approach is not merely limited to graphene but should extend also to a broad variety of two-dimensional semiconductors. [ABSTRACT FROM AUTHOR]

Weinvestigate energy relaxation of hot carriers in monolayer and bilayergraphene devices, demonstrating that the relaxation rate increasessignificantly as the Dirac point is approached from either the conductionor valence band. This counterintuitive behavior appears consistentwith ideas of charge puddling under disorder, suggesting that it becomesvery difficult to excite carriers out of these localized regions.These results therefore demonstrate how the peculiar properties ofgraphene extend also to the behavior of its nonequilibrium carriers. [ABSTRACT FROM AUTHOR]