The excellent electrical properties of graphene, such as its high carrier mobility, gate tunability, and mechanical flexibility makes it a very promising material for radio frequency (RF) electronics. Here we study the impact of top and bottom gate control on the essential performance metrics of graphene RF transistors. We find that the maximum cut-off frequency improves as the bottom gate voltage is tuned towards the same polarity as the top gate bias voltage. These results can be explained by the bottom-gate tunable doping of the graphene underneath the metal contacts and in the under-lap region. These effects become more dramatic with device down-scaling. We also find that the minimum output conductance occurs, when the drain voltage roughly equals an effective gate voltage (V e f f ≈ V T G + V B G ṡ C B G / C T G, where VTG and VBG are top and bottom gate voltage, CTG and C BG are the respective gate capacitance). The minimum output conductance is reduced as the bottom gate bias increases, due to the stronger control of the channel from the bottom gate, lessening the influence of the drain voltage on the drain current. As a result of these two influences, when the bottom gate voltage is tuned towards the same polarity as the top gate voltage, both the maximum oscillation frequency (fmax) and the intrinsic gain significantly improve. The intrinsic gain can increase as high as 3-4 times as the gain without the bottom gate bias. Tuning the bottom gate to enhance fmax and gain will be very important elements in the effort to enable graphene RF devices for practical use.
Bibliographical noteFunding Information:
We would like to thank J. Bucchignano and S. Dawes for their contributions to device fabrication. We also thank Y. Wu, Y-M. Lin, M. Freitag, F. Xia, H. Yan, and V. Perebeinos for their insightful discussions and help on this project. The authors would also like to thank DARPA for partial financial support through the Open Manufacturing Program (contract number HRs0011-12-C-0038).
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