Unconventional Carrier Transport and Switching with Graphene PN Junction

Sajjad, Redwan, Electrical Engineering - School of Engineering and Applied Science, University of Virginia
Ghosh, Avik, Department of Electrical and Computer Engineering, University of Virginia

Graphene is considered to be a wonder material for its unique physical properties. In graphene, record-breaking numbers have been shown for the thermal and electrical conductivities, mechanical strength, electronic mobility, chemical sensing, filtering and optoelectronic properties. Therefore, it has potential for various electronic, spintronic and photonic applications. In this dissertation, we investigate graphene's potential as a channel material for digital logic applications using electro-statically built graphene pn junction (GPNJ). Despite graphene's high electrical conductivity and other useful properties, the lack of bandgap makes it difficult to accomplish logic implementation, which requires a large amount of current modulation with gate voltage. In graphene pn junction, the linear, photon like energy dispersion combined with zero bandgap leads to an electron transport much like optical refraction and carrier trajectories are governed by an equivalent Snell's law. Determined by the wave-function dynamics, it also has a unique angle (transverse mode) dependent transmission through a pn junction. This research aims to manipulate such angle dependent transport with gate geometry for switching. We show that such scheme is capable of switching without having to open a structural bandgap, but with what we call a `Transmission Gap'. We show multiple device concepts that manipulate the angle dependent transmission with device geometry and produce the gap by suppressing transmission of all propagating modes. The tunability of the gap leads to a new way of beating thermal switching limit. Combined with graphene's high current carrying capability, these properties make the switch energy efficient. The device designs are complemented with our benchmarking of recent experiments on angle dependent transport in GPNJ. We also show an intriguing implication of the pn junction based conductance control in another novel material: topological insulator (TI) which has similar bandstructure on its surface as graphene. A TI based pn junction is shown to produce high spin current with low charge current, following a very similar tunneling physics in GPNJ. Such gate controlled spin current can have implication in spin based systems, where a spin polarized current is needed to rotate a ferromagnet with as low charge current as possible to decrease dissipation. Throughout the dissertation, we show simulation results from a sophisticated quantum mechanical numerical modeling platform based upon Non-Equilibrium Green's Function (NEGF) formalism, developed to augment the analytical formalism. The numerical platform is optimized so that it can perform calculations that the analytical model cannot possibly do: model all kinds of transport (e.g. electron, spin) for small to large scale devices (up to experimental device size) for both ballistic and diffusive transport regime including non-idealities such as charge impurity scattering and edge effects.

PHD (Doctor of Philosophy)
graphene, bandgap, NEGF, topological insulator, quantum transport
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