Channel Length Scaling in Microwave Graphene Field Effect Transistors

Abstract

In order to operate at microwave frequencies and higher, the channel length of a field effect transistor must be made very short ($\sim25$nm at 1THz) to minimize input capacitance and the drift time of carriers through the channel. This scaling cannot continue indefinitely, however, as short channel effects limit FET performance at very short gate lengths. These effects are largely due to a degradation in the ability of the gate to electrostatically control channel doping as the gate-to-channel distance and the finite thickness of the 2DEG channel become comparable to the channel length. This geometrical limitation can be avoided by utilizing graphene, a zero-bandgap two-dimensional material consisting of carbon atoms on a hexagonal lattice, as a channel material due to its high carrier mobility, truly atomic thickness, and ability to be integrated with other Van der Waals materials for ultra-thin gate dielectrics. Presently, the difficulty of producing low-resistance electrical contacts to graphene and the absence of saturating behavior in short-channel graphene transistors lead to poor RF performance especially for short channel devices. In this thesis, we aim to study the channel-length scaling behavior in monolayer graphene FETs, and to examine the prospects for their use as very high frequency transistors. In order to achieve the highest intrinsic mobility and saturation velocity and to minimize the parasitic effects of contact resistance, we utilize state-of-the-art fabrication techniques to create exfoliated, hexagonal Boron-Nitride encapsulated edge-contacted graphene field effect transistors. We embed these transistors in a microwave circuit designed to allow accurate calibration and de-embedding. Measurements of the DC and AC linear response of several GFETs of gate lengths between 1$\mu$m and 140nm were made and analyzed by fitting the complex S-parameter data to a small signal FET equivalent circuit model. These measurements exhibit saturating characteristics in the 1$\mu$m device, likely brought on by velocity saturation through the energy-dependent emission of optical phonons. For the short channel devices, however, a distinct loss of saturating characteristics is observed, possibly arising from a longer than expected optical phonon emission mean free path or other high-bias induced conduction mechanisms that compete with optical phonon emission. Even so, the graphene FETs fabricated in this thesis represent state-of-the-art f$_T$ and f$_{max}$ performance for their channel lengths, notably a 140nm GFET with an f$_T > 300$GHz. In contrast to the high-field, high-carrier density operation of GFET amplifiers, when biased near the zero density of states Dirac point, the low field conductivity of graphene exhibits a distinctly non-linear behavior with respect to gate voltage. Utilizing this non-linearity to rectify incident voltage waves, a GFET can be made to be a very broadband power detector. In a 1$\mu$m gate length GFET at 30K, a responsivity from 1--20GHz of 55V/W is achieved with a noise equivalent power limited by the room temperature readout to 1nW/$\sqrt{\mathrm{Hz}}$.