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

• Published in: IEEE transactions on electron devices Vol. 68; no. 2; pp. 899 - 902
• Main Authors: Asad, Muhammad, Jeppson, Kjell O., Vorobiev, Andrei, Bonmann, Marlene, Stake, Jan
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.02.2021; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Aluminum oxide; Buffer layers; Charge carriers; Current carriers; Field effect transistors; Field-effect transistors (FETs); Frequency measurement; Graphene; Inelastic scattering; Logic gates; maximum frequency of oscillation; Optical buffering; optical phonons; Phonons; Saturation; saturation velocity; Semiconductor devices; Silicon dioxide; Silicon substrates; Substrates; transit frequency; Transit time; Velocity
• ISSN: 0018-9383; 1557-9646; 1557-9646
• DOI: 10.1109/TED.2020.3046172

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-<inline-formula> <tex-math notation="LaTeX">\mu \text{m} </tex-math></inline-formula> 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 <inline-formula> <tex-math notation="LaTeX">1.5\cdot 10^{{7}} </tex-math></inline-formula> to <inline-formula> <tex-math notation="LaTeX">2\cdot 10^{{7}} </tex-math></inline-formula> cm/s.