Numerical Study of Scaling Issues in Graphene Nanoribbon Transistors
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Numerical Study of Scaling Issues in Graphene Nanoribbon Transistors
Man-Tieh Chen and Yuh-Renn Wu ∗ Graduate Institute of Photonics and Optoelectronics and Department of Electrical Engineering, National Taiwan University, Taipei, Taiwan, 10617. ∗ [email protected]
ABSTRACT
This paper addresses scaling issues in graphene nanoribbon transistors (GNRFETs) by using a two-dimensional (2-D) Poisson and drift-diffusion solver with finite element method (FEM). GNRFETs with the back gate control and the channel width down to less than 5nm have been reported to have Ion /Iof f ratio up to 106 . Our simulations show an agreement with the published experimental work and show a potential to reach unit current gain cut-off frequency, fT , up to more than 1THz with a satisfying Ion /Iof f ratio at the same time. This makes GNRFETs attractive for high speed logic. INTRODUCTION
In high speed RF applications, the lack for terahertz source has been a long term challenge for people working on this field. The feasibility of making graphene transistor has opened the possible way to overcome the terahertz barrier. Graphene is a single layer graphite and features many exceptional properties[1–3]. Due to the linear E-k relation and almost zero bandgap, the mobility of graphene is quite high, which has been experimentally demonstrated the cut-off frequency fT over 230GHz[4]. The potential fT could be even higher with a proper design. However, due to the zero bandgap, the Ion /Iof f ratio is bad and limits its potential application. There are several approaches to open a bandgap : (1) applying perpendicular electric field to bilayer graphene[5, 6]; (2) forming an epitaxial graphene on silicon carbide[7, 8]; (3) water adsorption to the graphene surface[9]; (4) applying strain to graphene[10]; and (5) patterned hydrogen adsorption in graphene[11]. Theory and experiment have indicated that if we use the vertical electric field, bandgap values can be opened up to 0.2eV. However, the approaches are not feasible for a circuit design since the high electric field and water are not welcomed for most devices. Theory predicted that to open a bandgap by applying strain requires a global uniaxial strain exceeding 20%, which is difficult to achieve in practice. Also, except for the patterned hydrogen adsorption in graphene, these approaches mentioned above cannot provide an adequate bandgap for logic applications. Theoretical study[12] has indicated that the Graphene nanoribbon (GNR) with channel width below 10nm can open up a bandgap and become semiconductor type material. Back-gated transistors based on GNR have been experimental reported to have Ion /Iof f ratio up to 106 [13]. Recently, to obtain larger Ion /Iof f and smaller subthreshold swing with a small bandgap, tunnel FETs have been studied in simulations[14, 15] but have not been experimentally reported yet. In this study, we are interested in investigating the potential high speed performance of the GNR transistor.
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FIG. 1: (a) The schematic flowchart of the self-consisten
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