Does carrier velocity saturation help to enhance fmax in graphene field-effect transistors?

TL;DR

This study uses numerical simulations to explore how carrier velocity saturation influences current saturation and maximum oscillation frequency (fmax) in graphene FETs, revealing complex interactions affecting RF performance.

Contribution

It provides a detailed microscopic analysis of the factors affecting fmax in GFETs, including velocity saturation, self-heating, and electrostatics, highlighting their interplay.

Findings

Higher drift velocity near saturation increases fmax.

Maximum fmax occurs near the unipolar-bipolar crossover, not at highest velocity.

Self-heating significantly reduces fmax from ~60 GHz to 40 GHz.

Abstract

It has been argued that current saturation in graphene field-effect transistors (GFETs) is needed to get the highest possible maximum oscillation frequency (fmax). This paper numerically investigates whether velocity saturation can help to get better current saturation and if that correlates with enhanced fmax. For such a purpose, we used a drift-diffusion simulator that includes several factors that influence output conductance, especially at short channel lengths and-or large drain bias: short-channel electrostatics, saturation velocity, graphene-dielectric interface traps, and self-heating effects. As a testbed for our investigation, we analyzed fabricated GFETs with high extrinsin cutoff frequency fT,x (34 GHz) and fmax (37 GHz). Our simulations allow for a microscopic (local) analysis of the channel parameteres such as carrier concentration, drift and saturation velocities. For biases far away from the Dirac voltage, where the channel behaves as unipolar, we confirmed that the higher is the drift velocity, as close as possible to the saturation velocity, the greater fmax is. However, the largest fmax is recorded at biases near the crossover between unipolar and bipolar behavior, where it does not hold that the highest drift velocity maximizes fmax. In fact, the position and magnitude of the largest fmax depend on the complex interplay between the carrier concentration and total velocity which, in turn, are impacted by the self-heating. Importantly, this effect was found to severely limit radio-frequency performance, reducing the maximum fmax from around 60 to 40 GHz.