Abstract
A real-space quantum transport simulator for graphene nanoribbon (GNR) metal-oxide-semiconductor field-effect transistors (MOSFETs) has been developed and used to examine the ballistic performance of GNR MOSFETs. This study focuses on the impact of quantum effects on these devices and on the effect of different type of contacts. We found that two-dimensional (2D) semi-infinite graphene contacts produce metal-induced-gap states (MIGS) in the GNR channel. These states enhance quantum tunneling, particularly in short channel devices, they cause Fermi level pinning and degrade the device performance in both the ON-state and OFF-state. Devices with infinitely long contacts having the same width as the channel do not indicate MIGS. Even without MIGS quantum tunneling effects such as band-to-band tunneling still play an important role in the device characteristics and dominate the OFF-state current. This is accurately captured in our nonequilibrium Greens’ function quantum simulations. We show that both narrow (1.4 nm width) and wider (1.8 nm width) GNRs with 12.5 nm channel length have the potential to outperform ultrascaled Si devices in terms of drive current capabilities and electrostatic control. Although their subthreshold swings under forward bias are better than in Si transistors, tunneling currents are important and prevent the achievement of the theoretical limit of 60 mV/dec.
Highlights
Graphite-related materials such as fullerenes, graphene, and carbon nanotubes have generated considerable interest due to their unique electronic and optoelectronic properties
We focus on exploring the physical properties and device performance of armchair GNR MOSFETs
We described the real space quantum transport simulation for GNR FETs using NEGF approach based on a -orbital TB method
Summary
ON-current performance compared with CNT MOSFETs. little is known about the role of quantum effects in GNR MOSFET device performance. In particular, quantum effects such as tunneling, can play an important role by reducing the ratio of ION to IOFF, and increase the subthreshold slope, degrade the device performance. By 2D, we mean an infinitely long contact that is much wider than the channel.͒ We found that in the case of 2D contacts, metal-induced gap statesMIGSform, and produce localized states in the middle of the band gap These levels pin the Fermi level and contribute to tunneling when the channel is short, degrading the device performance. The light effective mass of GNRs, in combination with their small band gap and the short channel length, enhance carrier tunneling through the barrier from the source to the drain. This effect degrades the OFF-state device performance. FIG. 1. ͑Color online Density of states and transmission of an infinite 2D graphite sheet vs energy/ , where is the orbital coupling of a tightbinding model. ͑Obtained by using the recursive surface Green’s function approach.͒ The results are in a good agreement with Ref. 18
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