Sai-Kong Chin

A Simulation Study of Graphene-Nanoribbon Tunneling FET With Heterojunction Channel

The device physics and performance of heterojunction (HJ) graphene-nanoribbon (GNR) tunneling field-effect transistors (TFETs) with different designs are investigated in this letter. Due to the width-dependent energy bandgap (EG), a single GNR with spatially dependent width naturally yields an HJ structure to improve the device performance of a GNR TFET. By adding a small-EG region in the channel near the source and a large-EG region in the middle of the channel, the ON- and OFF-state currents (ION and IOFF, respectively) can be tuned. Last, we have studied the effect of channel length scaling on an HJ GNR TFET, and it has been observed that an ION/IOFF ratio of four orders of magnitude can be achieved with a channel length of 10 nm and a drain bias of 0.6 V.

Device Physics and Characteristics of Graphene Nanoribbon Tunneling FETs

We present a detailed simulation study on the current-voltage characteristics of ballistic graphene nanoribbon (GNR) tunneling FETs of different widths with varying temperatures and channel length. Our model uses the self-consistent nonequilibrium Greens function and the quasi-2-D Poisson solver with the material details of the GNRs modeled by the uncoupled mode space Dirac equation. We find that, in general, the GNR tunneling FETs from the 3p + 1 family have better ION/IOFF characteristics than those from the 3p family due to smaller effective masses of the former. A lower drain doping concentration relative to that of the source enhances the ION/IOFF. Most significantly, we find that a higher doping concentration at the source enhances ION but degrades the subthreshold swing (SS). As a function of temperature, the SS shows highly nonlinear behaviors. In terms of intrinsic delay and power-delay product, the GNR tunneling FETs show very promising scaling behaviors and can be optimized to meet the International Technology Roadmap for Semiconductors roadmap requirements through adjustment in doping concentrations and other parameters.

Quantum transport simulations of graphene nanoribbon devices using Dirac equation calibrated with tight-binding π-bond model

We present an efficient approach to study the carrier transport in graphene nanoribbon (GNR) devices using the non-equilibrium Greens function approach (NEGF) based on the Dirac equation calibrated to the tight-binding π-bond model for graphene. The approach has the advantage of the computational efficiency of the Dirac equation and still captures sufficient quantitative details of the bandstructure from the tight-binding π-bond model for graphene. We demonstrate how the exact self-energies due to the leads can be calculated in the NEGF-Dirac model. We apply our approach to GNR systems of different widths subjecting to different potential profiles to characterize their device physics. Specifically, the validity and accuracy of our approach will be demonstrated by benchmarking the density of states and transmissions characteristics with that of the more expensive transport calculations for the tight-binding π-bond model.

Graphene Nanoribbon Tunneling Field-Effect Transistors With a Semiconducting and a Semimetallic Heterojunction Channel

We present a computational study of the device performance of graphene nanoribbon tunneling field-effect transistors (TFETs) with a heterogeneous channel. By varying the length and the energy bandgap (EG) of the heterogeneous region, the on- and off-state currents (ION and IOFF) can be effectively optimized independently. Both semiconducting and semimetallic heterogeneous regions are studied to understand the effects of EG engineering on device behaviors. In addition, the effect of gate coverage (GC) over the heterogeneous region is also investigated. We found that device performance is greatly affected by the positioning of the gate to modify the region where band-to-band tunneling occurs. For a given ION/IOFF of eight orders, our results show that, for the semiconducting heterojunction, a higher ION can be obtained by having the gate partially covering the heterogeneous region. This is due to a combination of a short tunneling length and resonant states, which leads to an increase in carrier concentration for the tunneling mechanism. On the other hand, for the semimetallic case, a similar ION/IOFF is only attainable when the heterogeneous region is not covered by the gate. A large IOFF is observed for even small GC due to the valence electrons from the source traveling to the conduction bands of the semimetallic region, enhancing the carrier transport toward the drain. Our study highlights the device design consideration required when optimizing the device performance of heterojunction TFETs.

Effect of Ribbon Width and Doping Concentration on Device Performance of Graphene Nanoribbon Tunneling Field-Effect Transistors

The device performance of graphene nanoribbon (GNR) tunneling field-effect transistor (TFET) is studied using the self-consistent non-equilibrium Greens function (NEGF) and quasi-two dimensional Poisson solver based on the Dirac equation model. The effects of different GNR widths and doping concentrations at the source and drain on the device characteristics are investigated and the electronic property of the GNR TFET is found to be strongly dependent on its width. A comprehensive characterization of this dependence is expected to be crucial to the designs and fabrications of GNR TFETs. Furthermore, the doping concentrations at the source and drain is found to play a crucial role on the ON- and OFF-state currents (ION and IOFF) respectively. Therefore, the ability to control the doping concentrations allows the tailoring of the drive current, the ION/IOFF ratio and the subthreshold swing of GNR TFETs to meet different design requirements.

Self-Consistent SchrÖdinger–Poisson Simulations on Capacitance–Voltage Characteristics of Silicon Nanowire Gate-All-Around MOS Devices With Experimental Comparisons

We simulate room temperature capacitance-voltage characteristics of silicon (Si) nanowire gate-all-around MOS structures with radius les 10 nm using a self-consistent Schrodinger- Poisson solver in cylindrical coordinates with full treatment of the transverse quantum confinement. In this paper, we compare our simulation results with the latest capacitance measurements on single Si nanowire pMOS and nMOS devices in the subfemtofarad range. We also propose to probe the density-of-states features of the Si channel from the capacitance-voltage characteristics at room temperature measurements using dC/dV dependence and illustrate the idea by employing the latest measurements, our quantum and Medici (Synopsys) simulations, as well as a simplified analytical model.

Source/drain doping influence on heterojunction graphene nanoribbon tunneling field effect transistors

We investigate the device performance of heterojunction graphene nanoribbons tunneling field-effect transistors as a function of the doping concentrations based on the non-equilibrium Greens function. We observe that variation in source doping changes the OFF-state currents (IOFF), the ON-state currents (ION) and the subthreshold slope (SS) significantly while variation in drain doping changes mainly the IOFF. Additionally, low SS and large ION/IOFF ratio can be achieved by applying proper asymmetric source-drain doping.

Performance Evaluation of Graphene Nanoribbon Tunneling Field Effect Transistors

Fig. 1: (a) A side view schematic of a double-gate graphene nanoribbon tunneling field effect transistor (GNR TFET). The gate insulator is silicon dioxide with a thickness (tins) of 1nm in this work. (b) The top view of the GNR TFET. The source and drain are doped with holes and electrons respectively and different ribbon width are used. The channel length of the device is kept at a constant of 10nm. (c) A schematic of the energy band of the p-i-n structure during (left) OFF-state and (right) ON-state. The conduction and valance bands are represented by the blue and red lines, respectively. Ideally, OFF-state current is low due to the long tunneling width while ON-state current is high due to the short tunneling width.