Electronics 2022, 11, 3683. https://doi.org/10.3390/electronics11223683 www.mdpi.com/journal/electronics Communication Developing PCM-Based Microwave and Millimetre-Wave Switching Networks by Optimised Building Blocks Rodica Ramer and King Yuk Chan * The School of Electrical Engineering and Telecommunications, The University of New South Wales, Sydney, NSW 2052, Australia * Correspondence: kyc@unsw.edu.au Abstract: The implementation of microwave and millimetre-wave switching networks using phase change material (PCM) is presented in this paper. We propose integrating a combination of ultra- wide bandwidth-optimised building cells into a unique semi-T type switch. The construction of arrays with different dimensions is enabled. The present paper selected GeTe for the PCM-based switches, which are 150 nm GeTe thin-film offering on- and off-state  = 37,203,703 S/m and ff = 94.97 S/m conductivities by a customised eight-step fabrication process. The integrated semi-T switch cell with two, thru, and turn operational states allows easy expansion into the form of a staircase switch matrix. The simulated results for the semi-T type switch show excellent insertion loss of better than 0.8 dB, return loss of better than 20 dB, and isolation of 40 dB for both the thru and turn paths from DC to 120 GHz. The proposed 4 × 4 staircase switch matrix with a dimension of only 510 × 510 μm 2 is also the smallest in its class. The switch matrix exhibits better than 17 dB return loss and 40 dB isolations across all possible combinations and paths. Keywords: phase change material (PCM); switch; switching networks; switch matrix; monolithic microwave integrated circuits 1. Introduction Switches represent one of the most fundamental building blocks of electronic systems. They are ‘everywhere’ and in ‘everything’. At the same time, wireless communications systems are continually progressing and expanding to meet the demands for reliable, high-data-rate operation in multiple frequency ranges, evolving into complex hardware architectures with carrier aggregation and multiple-input multiple-output (MIMO) antennas [1]. For instance, a 4G cellular radio-frequency (RF) front end needs to support more than 16 bands, 60 RF ports, and 30 RF switches per RF port [2]. Given the rising consumer demand for wireless ubiquity and the desire for anytime, anywhere access, billions of connections are yet to be made. The 5G mobile and upcoming 6G networks expand into higher frequency ranges (from 28 GHz towards 1 THz) with massive MIMO antennas that make the RF front end much more complex and challenging. The increased complexity of RF front ends poses severe design and layout challenges where reconfigurable RF front ends and tuneable antennas are solutions to maintain the RF systems [3,4]. How these breakthrough changes to networks and telecommunications affect the switching routing and communication networks remain to be seen. An inevitable fact is that architectures of high-performance, power-aware, low- loss, linear, minimised RF switches are integral parts of these tuneable RF systems and redundancy networks. The current state of commercial switches demonstrates that the most used mechanical and semiconductor-type switches offer numerous advantages but come with several trade-offs. Mechanical RF switches demonstrate excellent RF performance but are substantial in size and expensive. The semiconductor switches are compact but have poor Citation: Ramer, R.; Chan, K.Y. Developing PCM-Based Microwave and Millimetre-Wave Switching Networks by Optimised Building Blocks. Electronics 2022, 11, 3683. https://doi.org/10.3390/ electronics11223683 Academic Editor: Riccardo Bernardini Received: 21 October 2022 Accepted: 9 November 2022 Published: 10 November 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/license s/by/4.0/).