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Graphene, which consists of just a single atomic layer of carbon atoms bound into crystal lattice, is the hottest new material system considered for applications in future electronics and sensors. The properties, which make graphene so desirable for future electronics, are its extremely high electrical and thermal conductivities. For any transistor to be useful for communications or information processing, the level of the electronic low-frequency noise (also referred to as 1/f or flicker noise) has to be reduced to an acceptable level defined by the Hooge parameter. Although modern electronic devices such as cell phones and radars operate at very higher frequencies (GHz range), the low-frequency 1/f noise is extremely important. Due to unavoidable non-linearities in devices and systems, the low frequency noise up-converts to higher requencies, and contributes to the phase noise of the system, thus limiting its performance. The same is true for the proposed applications of graphene as a material for ultra-sensitive detectors.
A team of researchers from the University of California - Riverside (UCR) and Rensselaer Polytechnic Institute (RPI) led by UCR electrical engineering professor Alexander A. Balandin built and demonstrated the first top gate graphene transistor, which satisfies the low-noise requirements for graphene practical applications. The transistors were fabricated with electron beam lithography from single atomic layer graphene placed on Si/SiO2 substrates and coated with HfO2 (a new gate dielectric recently adopted by electronic industry). The top gate separated from graphene channel by HfO2 allowed for precise control of electron transport. The combined action of the bottom and top gates allowed the team to investigate the noise sources.
The discovery that electronic noise in the properly designed top-gate graphene transistors can be reduced to low levels came as a bit of a surprise. Graphene is just a single atomic layer of material. When it is embedded between two oxide layers (SiO2 and HfO2) its electronic transport and noise are strongly affected by defects and impurities in the oxides, which act as traps for electrons and contribute to noise. The analysis performed by UCR - RPI team suggests that the flicker noise in graphene transistors originates from the carrier number fluctuation in the device channel rather than from the mobility fluctuations. The obtained experimental date also indicates that the noise level can be reduced even further with improvements in graphene device fabrication technology. The demonstration of the low-noise top-gate graphene transistors is a crucial requirement for all proposed communication, digital and sensor applications of graphene.
The results are published in Applied Physics Letters.
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Alexander A. Balandin, PhD
Professor, Department of Electrical Engineering
Chair, Materials Science and Engineering Program
Director, Nano-Device Laboratory
University of California - Riverside
Riverside, CA 92521 USA
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