On the impact of the output conductance on high-frequency amplification in graphene transistors

In contrast to the impressive dynamic current gain and cut-off frequency demonstrated by graphene field-effect transistors (GFETs), which compete with mature high-frequency (HF) technologies, GFETs still stay behind their HF counterparts in terms of power amplification and maximum oscillation frequency. This limitation is tipycally attributed to the absence of an energy bandgap, which leads to poor current saturation and hence high output conductance (gds), despite the exceptionally high carrier mobility attained in graphene and corresponding high transconductance (gm). Indeed, the intrinsic voltage gain of GFETs (Av, i= gm/gds) is not competitive with that of conventional high-frequency technologies. As a consequence of this back-of-the-envelope reasoning, the study of the most relevant metrics of amplifiers at higher frequencies, i.e. the power gain, has been often disregarded. In this work, a comprehensive analysis of the physics-based small-signal parameters of GFETs reveals that strong current saturation and the resulting low gds are not mandatory for achieving reasonable power gain levels in the GHz range, demonstrating that Av,i is not critical for HF performance evaluation. Specifically, we project a maximum stable gain exceeding 5 dB at 2.4 GHz in fabricated 1-µm-long GFETs with Av,i < 1. This work provides a new perspective on performance improvement in GFET-based power gain amplifiers, showing that high carrier mobility and selecting the bias point with a focus on transconductance, as the most relevant figure of merit for amplifier operation, are crucial, while microwave stability must also be properly addressed.