Low-noise near-ballistic BN-graphene-BN heterostructure field-effect transistors for energy efficient electronic applications

Ballistic electron conduction offers a new possible approach to the development of low-power high-speed logic circuits [1-3]. Ultra-high mobility of graphene allows achieving the ballistic or near-ballistic transport regime at room temperature (RT) in devices with relatively long channels [4-5]. In this presentation, we show that heterostructure field-effect transistors (HFETs) with graphene channel encapsulated between two layers of hexagonal boron nitride (h-BN) combine extremely high electron mobility (∼36,000 cm²/Vs) with a strongly reduced 1/ f noise level (f is the frequency). Low-frequency 1/f noise hampers the operation of numerous devices and can be a major impediment to development of practical low-power low-voltage applications of graphene and other 2D van der Waals materials [6-8]. The low noise level is beneficial for the low-voltage electronic applications. The prototype h-BN-graphene-h-BN HFETs were fabricated using the mechanically exfoliated h-BN and graphene flakes on Si/SiO2 wafers. The viscoelastic materials adhered to glass slides were used as transparent stamps for the layer transfer. The stamps were spin coated with poly-propylene carbonate (PPC). A micromanipulator was used for careful positioning of h-BN flakes on the stamp over graphene flakes. Following this procedure, the h-BN layer was added to the stack resulting in a fully encapsulated graphene monolayer. The stack was then released onto target Si/SiO2 substrate by heating the stage to an appropriate temperature. The resulting heterostructure was etched to expose the edges of graphene and create one-dimensional Cr/Au (10/100 nm) electrical contacts. Figure 1 shows the schematics and an optical microscopy image of a representative device. Figure 2 (a) shows the current-voltage (I-V) characteristics of the representative HFET. Both the effective and field-effect mobility extractions gave consistent results showing the mobility of ∼36,000 cm²/Vs at the carrier concentration of 7×10¹¹ cm². Figure 2 (b) presents the normalized 1/f noise spectral density of the graphene encapsulated device. The channel-area normalized noise spectral density in BN-graphene-BN HFET is factor of ×5–×10 smaller than that in typical reference graphene FETs without the channel encapsulation. The observed strong noise reduction can be explained by screening of the traps in SiO2 by the BN barrier. Other possible physical mechanisms and prospects of further noise suppression will be discussed at the presentation.