Muhammad Asad - Academia.edu (original) (raw)
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Papers by Muhammad Asad
IEEE Transactions on Electron Devices, 2021
High-frequency performance of top-gated graphene field-effect transistors (GFETs) depends to a la... more 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 GHz and 46 GHz, respectively, were obtained for GFETs on Al 2 O 3 with 0.5 µm gate length. These values are approximately 30% higher than those for state-of-theart 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 to 2•10 7 cm/s from 1.5•10 7 cm/s.
IEEE Journal of the Electron Devices Society, 2020
This paper addresses the high-frequency performance limitations of graphene field-effect transist... more This paper addresses the high-frequency performance limitations of graphene field-effect transistors (GFETs) caused by material imperfections. To understand these limitations, we performed a comprehensive study of the relationship between the quality of graphene and surrounding materials and the high-frequency performance of GFETs fabricated on a silicon chip. We measured the transit frequency (f T) and the maximum frequency of oscillation (f max) for a set of GFETs across the chip, and as a measure of the material quality, we chose low-field carrier mobility. The low-field mobility varied across the chip from 600 cm 2 /Vs to 2000 cm 2 /Vs, while the f T and f max frequencies varied from 20 GHz to 37 GHz. The relationship between these frequencies and the low-field mobility was observed experimentally and explained using a methodology based on a small-signal equivalent circuit model with parameters extracted from the drain resistance model and the charge-carrier velocity saturation model. Sensitivity analysis clarified the effects of equivalent-circuit parameters on the f T and f max frequencies. To improve the GFET high-frequency performance, the transconductance was the most critical parameter, which could be improved by increasing the charge-carrier saturation velocity by selecting adjacent dielectric materials with optical phonon energies higher than that of SiO 2. INDEX TERMS Graphene, field-effect transistors, high frequency, transit frequency, maximum frequency of oscillation, microwave electronics, contact resistances, transconductance.
IEEE Transactions on Electron Devices, 2021
High-frequency performance of top-gated graphene field-effect transistors (GFETs) depends to a la... more 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 GHz and 46 GHz, respectively, were obtained for GFETs on Al 2 O 3 with 0.5 µm gate length. These values are approximately 30% higher than those for state-of-theart 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 to 2•10 7 cm/s from 1.5•10 7 cm/s.
IEEE Journal of the Electron Devices Society, 2020
This paper addresses the high-frequency performance limitations of graphene field-effect transist... more This paper addresses the high-frequency performance limitations of graphene field-effect transistors (GFETs) caused by material imperfections. To understand these limitations, we performed a comprehensive study of the relationship between the quality of graphene and surrounding materials and the high-frequency performance of GFETs fabricated on a silicon chip. We measured the transit frequency (f T) and the maximum frequency of oscillation (f max) for a set of GFETs across the chip, and as a measure of the material quality, we chose low-field carrier mobility. The low-field mobility varied across the chip from 600 cm 2 /Vs to 2000 cm 2 /Vs, while the f T and f max frequencies varied from 20 GHz to 37 GHz. The relationship between these frequencies and the low-field mobility was observed experimentally and explained using a methodology based on a small-signal equivalent circuit model with parameters extracted from the drain resistance model and the charge-carrier velocity saturation model. Sensitivity analysis clarified the effects of equivalent-circuit parameters on the f T and f max frequencies. To improve the GFET high-frequency performance, the transconductance was the most critical parameter, which could be improved by increasing the charge-carrier saturation velocity by selecting adjacent dielectric materials with optical phonon energies higher than that of SiO 2. INDEX TERMS Graphene, field-effect transistors, high frequency, transit frequency, maximum frequency of oscillation, microwave electronics, contact resistances, transconductance.