Asymmetrically Engineered Nanoscale Transistors for On-Demand Sourcing of Terahertz Plasmons.

Terahertz (THz) plasma oscillations represent a potential path to implement ultrafast electronic devices and circuits. Here, we present an approach to generate on-chip THz signals that relies on plasma-wave stabilization in nanoscale transistors with specific structural asymmetry. A hydrodynamic treatment shows how the transistor asymmetry supports plasma-wave amplification, giving rise to pronounced negative differential conductance (NDC). A demonstration of these behaviors is provided in InGaAs high-mobility transistors, which exhibit NDC in accordance with their designed asymmetry. The NDC onsets once the drift velocity in the channel reaches a threshold value, triggering the initial plasma instability. We also show how this feature can be made to persist beyond room temperature (to at least 75 °C), when the gating is configured to facilitate a transition between the hydrodynamic and ballistic regimes (of electron-electron transport). Our findings represent a significant step forward for efforts to develop active components for THz electronics.

Transient Response of h-BN-Encapsulated Graphene Transistors: Signatures of Self-Heating and Hot-Carrier Trapping.

We use transient electrical measurements to investigate the details of self-heating and charge trapping in graphene transistors encapsulated in hexagonal boron nitride (h-BN) and operated under strongly nonequilibrium conditions. Relative to more standard devices fabricated on SiO2 substrates, encapsulation is shown to lead to an enhanced immunity to charge trapping, the influence of which is only apparent under the combined influence of strong gate and drain electric fields. Although the precise source of the trapping remains to be determined, one possibility is that the strong gate field may lower the barriers associated with native defects in the h-BN, allowing them to mediate the capture of energetic carriers from the graphene channel. Self-heating in these devices is identified through the observation of time-dependent variations of the current in graphene and is found to be described by a time constant consistent with expectations for nonequilibrium phonon conduction into the dielectric layers of the device. Overall, our results suggest that h-BN-encapsulated graphene devices provide an excellent system for implementations in which operation under strongly nonequilibrium conditions is desired.

Gate-Controlled Metal-Insulator Transition in TiS3 Nanowire Field-Effect Transistors.

We explore the electrical characteristics of TiS3 nanowire field-effect transistor (FETs), over the wide temperature range from 3 to 350 K. These nanomaterials have a quasi-one-dimensional (1D) crystal structure and exhibit a gate-controlled metal-insulator transition (MIT) in their transfer curves. Their room-temperature mobility is ∼20-30 cm2/(V s), 2 orders of magnitude smaller than predicted previously, a result that we explain quantitatively in terms of the influence of polar-optical phonon scattering in these materials. In the insulating state (<∼220 K), the transfer curves exhibit unusual mesoscopic fluctuations and a current suppression near zero bias that is common to charge-density wave (CDW) systems. The fluctuations have a nonmonotonic temperature dependence and wash out at a temperature close to that of the bulk MIT, suggesting they may be a feature of quantum interference in the CDW state. Overall, our results demonstrate that quasi-1D TiS3 nanostructures represent a viable candidate for FET realization and that their functionality is influenced by complex phenomena.