High Frequency Generation from Carbon Nanotube Field Effect Transistors Used as Passive Mixers
Tunnell, Andrew Jacob
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The high mobilities, low capacitances (due to small sizes), and high current densities of carbon nanotube field-effect transistors (CNT FETs) make them valid candidates for high frequency applications. The high cost of high frequency measurement equipment has been the largest hurdle to observing CNT transistor behavior at frequencies above 50 GHz. One economic solution to this barrier is to use an external harmonic mixer to convert high frequency signals to lower frequencies where they can be detected by a standard spectrum analyzer. By using this detection method, a new regime of high frequency CNT FET behavior is available for study. In this dissertation, we describe the design and fabrication of CNT FETs on quartz substrates using aligned arrays of CNTs as the device channel. The nonlinear input voltage to output drain current behavior of the devices is explained and approximated to the first order by using a Taylor expansion. For the high frequency mixing experiments, two input voltages of different frequencies are sourced on the gate of the devices without any device biasing. The input frequencies are limited to 100 kHz to 40 GHz by the signal generators used. The nonlinearities of the fabricated CNT FETs cause the input frequencies to be mixed together, even in the absence of a source-drain bias (passive mixing). The device output is the drain current, which contains sum and difference products of the input frequencies. By using an external harmonic mixer in combination with a spectrum analyzer to measure the drain current, output frequencies from 75 to 110 GHz can be observed. Up to 11th order mixing products are detected, due to the low noise floor of the spectrum analyzer. Control devices are also measured in the same experimental setup to ensure that the measured output signals are generated by the CNTs. The cutoff frequencies from previous passive mixing experiments predict that our devices should stop operating near 13 GHz, however our measurement setup extends and overcomes these cutoffs, and the generation of high frequency output signals is directly observed up to 110 GHz. This is the highest output frequency observed in CNT devices to date.