July 18, 2015

Pathway to terahertz graphene transistors

. A team of scientists at the Max Planck Institute for Polymer Research (MPI-P) discovered that electrical conduction in graphene on the picosecond timescale - a picosecond being one thousandth of one billionth of a second - is governed by the same basic laws that describe the thermal properties of gases. This much simpler thermodynamic approach to the electrical conduction in graphene will allow scientists and engineers not only to better understand but also to improve the performance of graphene-based nanoelectronic devices.

The researchers found that the energy of ultrafast electrical currents passing through graphene is very efficiently converted into electron heat, making graphene electrons behave just like a hot gas. "The heat is distributed evenly over all electrons. And the rise in electronic temperature, caused by the passing currents, in turn has a strong effect on the electrical conduction of graphene" explains Professor Mischa Bonn, Director at the MPI-P.

Essentials of the thermodynamic effect on ultrafast transport in graphene.

Nature Communications - Thermodynamic picture of ultrafast charge transport in graphene



"The results of this study will help improve the performance of graphene-based nanoelectronic devices such as ultra-high speed transistors and photodetectors" says Professor Dmitry Turchinovich, who led the research at the MPI-P. In particular they show the way for breaking the terahertz operation speed barrier - i.e. one thousand billions of oscillations per second - for graphene transistors.

Abstract

The outstanding charge transport properties of graphene enable numerous electronic applications of this remarkable material, many of which are expected to operate at ultrahigh speeds. In the regime of ultrafast, sub-picosecond electric fields, however, the very high conduction properties of graphene are not necessarily preserved, with the physical picture explaining this behaviour remaining unclear. Here we show that in graphene, the charge transport on an ultrafast timescale is determined by a simple thermodynamic balance maintained within the graphene electronic system acting as a thermalized electron gas. The energy of ultrafast electric fields applied to graphene is converted into the thermal energy of its entire charge carrier population, near-instantaneously raising the electronic temperature. The dynamic interplay between heating and cooling of the electron gas ultimately defines the ultrafast conductivity of graphene, which in a highly nonlinear manner depends on the dynamics and the strength of the applied electric fields.

SOURCES: Max Planck Institute, Nature Communications

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