RF performance projections for 2D graphene transistors: Role of parasitics at the ballistic transport limit

Abstract

The modeled device structures are shown in Figure 1, (a) top-gated structure εox = 20 and tox = 1.5nm, and (b) back-gated structure with 90nm thick SiO2. The contact resistance and parasitic capacitance have also been taken into consideration in the simulations. Figure 2 shows the effect of M-G contacts on the transfer characteristics and transconductance, gm for the top-gated structure. The on-current, Ion, can be increased with strong M-G coupling strength Δ or heavy contact induced doping (i.e. larger ΔEcontact). The off-current, Ioff, does not increase. Large ΔEcontact increases gm, but the maximum gm does not show a strong dependence on Δ. We use Δ=50meV and ΔEcontac = −0.4eV for rest of the simulations. Figure 3 shows the IDS vs. VGS and gm vs. VGS at different VDS for the top-gated structure. Large VDS yields a higher maximum gm, but low VDS shows better linearity with a broader ƒT peak. Transfer characteristics with different channel lengths are shown in Figure 4. For the top-gated structure excellent gate electrostatics helps avoid short channel effects (SCE). Ion remains the same for all channel lengths. Ioff increases about 1.5 times when Lch decreases from 100nm to 15nm due to direct source to drain tunneling. The rise in Ioff leads to gm degradation at Lch=15nm. In the back-gated structure, the on/off ratio is degraded at shorter channel lengths, and the minimum conduction point shifts. Figure 5 shows the effect of contact resistance on ID - VGS characteristics at Lch = 100nm. At VDS = 0.3V, compared with the intrinsic case, when RS/D = 0.5Ωmm, on/off ratio decreases 3x for the top-gated structure and 1.2x for the back-gated structure. Ion reduces 22x for the top-gated structure and 6x for the back-gated structure. Figure 6 shows the comparison of ƒT -VGS with different channel lengths at VDS = 0.3V. The cutoff frequency is calculated as ƒT = 1/2πτtot, where τtot = LchCgs/gm + Cgd/gm + Cgd(RS+RD), Cgs = ∂Qch/∂VGS, RS/D = 0.5Ωmm, and Cgd = 2pF/cm and 0.5pF/cm for the top-gated and the back-gated structures, respectively. Charging/discharging process is faster at shorter channel lengths, thus the peak ƒT increases. In the back-gated structure, SCE is strong, thus the on/off ratio decreases and gm drops dramatically at short Lch. When Lch is shorter than 30nm, even the peak ƒT drops. Figure 7 summarizes the ƒT vs. Lch at VDS = 0.3V. The intrinsic ƒT = <v>/2πLch is added as a reference with the average ballistic velocity <v> = 2vF/π in 2D graphene. With RS/D = 0.5Ωmm and Cgd = 2pF/cm, ƒT drops 2x at Lch=100nm and 8x and Lch=15nm for the top-gated structure. For the back-gated structure, with RS/D = 0.5Ωmm and Cgd = 0.5pF/cm, when Lch is below 70nm, ƒT does not increase, and it even decreases when the channel length is below 30nm. Thus, parasitics currently dominate the performance, and major gains are expected with their reduction. This work is supported by the Semiconductor Research Corporation Nanoelectronics Research Initiative and the National Institute of Standards and Technology through the Midwest Institute for Nanoelectronics Discovery (MIND).