CNTs and Graphene-Based Diodes for Microwave and Millimeter-Wave Circuits on Flexible Substrates

This paper presents the fabrication and characterization of carbon nanomaterials, i.e., carbon nanotubes (CNTs) and reduced graphene oxide (r-GO), based diodes on flexible substrates for high-frequency circuit applications. CNTs and graphene are good candidates as they possess excellent electronic and mechanical properties. For high-frequency circuits, diodes with lower parasitics and optimal impedance are required to achieve a high cutoff frequency and ease of impedance matching. Here, multiple CNTs are arranged in parallel to reduce the series resistance and to lower the impedance value, whereas 2-D graphene readily provides the desired impedance values. CNT and r-GO diodes with dissimilar metal contacts are fabricated using a novel process on high-frequency-compatible flexible substrates. The process utilizes an undercut and self-alignment approach that allows fabrication of submicrometer-size devices using an all-photolithographic process. The fabricated diodes demonstrate nonlinear current-voltage characteristics with current in the microampere range. Both types of diodes work efficiently as microwave rectifiers showing a near-ideal behavior with a rectification sensitivity of 4 V/W (18 GHz) and 33 V/W (22 GHz) for the CNT and r-GO, respectively. In addition, the results for r-GO-based frequency multiplication at fundamental frequencies ranging from 2 to 6 GHz and frequency mixing for 1.5 and 1.0 GHz are also presented.