Enhanced High-Frequency Performance of Top-Gated Graphene FETs Due to Substrate- Induced Improvements in Charge Carrier Saturation Velocity

The high-frequency performance of top-gated graphene field-effect transistors (GFETs) depends to a large extent on the saturation velocity of the charge carriers, a velocity limited by inelastic scattering by surface optical phonons from the dielectrics surrounding the channel. In this work, we show that, by simply changing the graphene channel surrounding dielectric with a material having higher optical phonon energy, one could improve the transit frequency and maximum frequency of oscillation of GFETs. We fabricated GFETs on conventional SiO
₂
/Si substrates by adding a thin Al
₂
O
₃
interfacial buffer layer on top of SiO
₂
/Si substrates, a material with about 30% higher optical phonon energy than that of SiO
₂
, and compared performance with that of GFETs fabricated without adding the interfacial layer. From S-parameter measurements, a transit frequency and a maximum frequency of oscillation of 43 and 46 GHz, respectively, were obtained for GFETs on Al
₂
O
₃
with 0.5-
$\mu \text{m}$
gate length. These values are approximately 30% higher than those for state-of-the-art GFETs of the same gate length on SiO
₂
. For relating the improvement of GFET high-frequency performance to improvements in the charge carrier saturation velocity, we used standard methods to extract the charge carrier velocity from the channel transit time. A comparison between two sets of GFETs with and without the interfacial Al
₂
O
₃
layer showed that the charge carrier saturation velocity had increased from
$1.5\cdot 10^{{7}}$
to
$2\cdot 10^{{7}}$
cm/s.