Improvement of Ballistic I-V Performance of Nanoscale GFET with Drain Underlap Region for RF Applications

The absence of a bandgap and the difficulties in inducing one without substantially degrading its excellent inherent electronic properties-viz., large carrier mobility, mean free path, etc. have rendered graphene less attractive for digital applications, the interest in grapheme field effect transistors (GFETs) for high-frequency RF applications, where transistor turnoff is not critical for device operation, continues. Although long-channel GFETs have shown some quasi-saturation behavior in their output, i.e., IDS-VDS (IDS and VDS are the drain current and voltages, respectively) characteristics, which has been attributed to velocity saturation, this trend is absent in their short-channel counterparts, hence resulting in degraded output resistance and, subsequently, reduced intrinsic small-signal gain. However, it has been argued, based on the relative density of states in the channel vis-a-vis that in the drain, that such quasi-saturation should be observable even in the ballistic limit.
With the recent upsurge in experimental efforts toward fabrication of short-channel graphene field-effect transistors(GFETs) for analog and high-frequency RF applications-where the advantages of distinctive intrinsic properties of gapless graphene are expected to be leveraged-a critical understanding of the factors affecting both output and transfer characteristics is necessary for device optimization. Analyzing the device characteristics through ballistic electronic transport simulations within the non equilibrium Green's function formalism, this work show that adoping in the drain underlap region can significantly improve the quasi-saturation behavior in the GFET output characteristics and, hence, the output resistance and intrinsic gain. From this understanding, this work provides a unified and coherent explanation for seemingly disparate phenomena-quasi-saturation and the recently reported three-terminal negative differential resistance in GFETs. This work also investigates the scaling behavior of cutoff frequency and comment on some of the observed scaling trends in recent experiments.