HIGH FREQUENCY GENERATION BASED ON CARBON NANOTUBE FIELD-EFFECT TRANSISTORS

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Date
2014
Authors
Song, Da
Advisor
Cumings, John
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Abstract
Carbon nanotubes (CNTs) are promising materials in radio frequency (RF) applications due to their high mobility, high current density and low capacitance. Over the past several years, extensive experimental and theoretical works have been focused on increasing the cut-off frequency of carbon nanotube field effect transistors (CNTFETs). However, there is limited study aiming for understanding the linearity of CNTFETs, which is an important aspect when radio frequency transistors are working in multiple frequency environments. In this dissertation, CNTFETs are fabricated based on horizontally aligned carbon nanotubes grown on quartz substrate. DC characterization shows three conduction regions in the transfer curve of the device, p-type and n-type linear regions, and ambipolar nonlinear region. The single tone excitation measurement shows extra harmonic generations as a result of the nonlinearity of the device. Same measurement is conducted with control devices without carbon nanotubes in the channel and confirms the nonlinearity is from the carbon nanotubes in the channel. Comparison between the 1st order harmonic amplitude and the 2nd order derivative of current with respect to gate voltage indicates that nonlinear transconductance is the cause of nonlinearity in the device. In order to understand the nonlinearity thoroughly, an elementary model based on 1D electronic transport and Drude model is built. The model can accurately predict the DC performance and nonlinearity of the device. Taking advantage of the transitions between linear and nonlinear transfer regions, we build our CNTFETs into gate controlled radio frequency mixers. Two-tone mixing measurement shows clearly that intermodulation terms in the output spectrum are strong in ampibolar regions and suppressed to noise floor in the linear regions. We further perform passive mixing (no source/drain voltage applied) in higher frequency regime and demonstrate the generation of harmonic and intermodulation signals in the output frequency range between 75 and110GHz, which is the among the highest output frequency observed from CNTFETs to date.
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