Tunneling field-effect transistors (FETs) have been intensely explored recently due to its potential to address power concerns in nanoelectronics. The recently discovered graphene nanoribbon (GNR) is ideal for tunneling FETs due to its symmetric bandstructure, light effective mass, and monolayer-thin body. In this work, we examine the device physics of p-i-n GNR tunneling FETs using atomistic quantum transport simulations. The important role of the edge bond relaxation in the device characteristics is identified. However, the device has ambipolar I-V characteristics, which are not preferred for digital electronics applications. We suggest that using either an asymmetric source-drain doping or a properly designed gate underlap can effectively suppress the ambipolar I-V. A subthreshold slope of 14mV/dec and a significantly improved on-off ratio can be obtained by the p-i-n GNR tunneling FETs.

High-field transport in graphene is studied by the Monte Carlo simulation. The results indicate velocity and current saturation in agreement with a recent experiment [I. Meric, M. Y. Han, A. F. Young, B. Oezyilmaz, P. Kim, and K. Shepard, Nat. Nanotechnol. 3, 654 (2008)]. The saturation current scales as the square root of the charge density, or equivalently, the square root of the gate overdrive voltage, which is qualitatively different from silicon field-effect transistors. By analytical fitting to the numerical simulation results, a simple expression of the field-dependent mobility is obtained at different strengths of charged impurity scattering.

High frequency performance limits of graphene field-effect transistors (FETs) down to a channel length of 20 nm have been examined by using self-consistent quantum simulations. The results indicate that although Klein band-to-band tunneling is significant for sub-100 nm graphene FETs, it is possible to achieve a good transconductance and ballistic on-off ratio larger than 3 even at a channel length of 20 nm. At a channel length of 20 nm, the intrinsic cut-off frequency remains at a few THz for various gate insulator thickness values, but a thin gate insulator is necessary for a good transconductance and smaller degradation of cut-off frequency in the presence of parasitic capacitance. The intrinsic cut-off frequency is close to the LC characteristic frequency set by graphene kinetic inductance (L) and quantum capacitance (C), which is about 100 GHz·μm divided by the gate length. KEYWORDSField effect transistor (FET), radio frequency (RF), carbon nanotube (CNT), intrinsic cut-off frequency, transconductance

High Frequency potential of graphene field-effect transistors (FETs) is explored by quasi-static self-consistent ballistic and dissipative quantum transport simulations. The unity power gain frequency fMAX and the cut-off frequency fT are modeled at the ballistic limit and in the presence of inelastic phonon scattering for a gate length down to 5 nm. Our major results are (1) with a thin high-κ gate insulator, the intrinsic ballistic fT is above 5 THz at a gate length of 10 nm. (2) Inelastic phonon scattering in graphene FETs lowers both fT and fMAX, mostly due to decrease of the transconductance. (3) fMAX and fT are severely degraded in presence of source and drain contact resistance. (4) To achieve optimum extrinsic fMAX performance, careful choice of DC bias point and gate width is needed.

Graphene nanoribbon tunneling FETs (GNR TFETs) are promising devices for post-CMOS low-power applications because of the low subthreshold swing, high I on/Ioff, and potential for large scale processing and fabrication. This paper combines atomistic quantum transport modeling with circuit simulation to explore GNR TFET circuits for low-power applications. A quantitative study of the effects of variations on the performance and reliability of GNR TFET circuits is also presented. Simulation results indicate that GNR TFET circuits are extremely competitive in performance in comparison to conventional CMOS circuits at comparable operating points, with static power consumption that is lower by 8-9 orders of magnitude. It is also observed that GNR TFET circuits are susceptible to parameter variations, motivating engineering and design challenges to be addressed by the device and CAD communities.

Graphene, which is mechanically flexible, electrically conductively, and optically nearly transparent, is a promising contact material in semiconductor devices such as solar cells and touch screen sensors. We present a method of obtaining the barrier height and transport properties of graphene-silicon contacts by self-consistently solving the Poisson equation and carrier transport equation. It is found that the contact barrier height is sensitive to the doping density of silicon and can be modulated by gating, in contrast to conventional metal-semiconductor contacts. Despite of being a continuous film, the contact resistance of a monolayer graphene to silicon can be modulated by orders of magnitude by using a bottom gate. The modulation of the contact resistance decreases significantly as the number of graphene layers increases and becomes negligible when the number of the graphene layers is larger than about 6. The results indicate the unique properties of graphene-semiconductor contacts.

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