A Millimeter-Wave Analog–Digital Variable Capacitor With High Tuning Ratio Realized by Monolithic Integration of BST Varactors and GeTe Switches

This letter presents an analog–digital variable capacitor realized by monolithic integration of barium strontium titanate (BST) ferroelectric material and phase change material (PCM) technologies. The proposed concept enables the creation of tunable components with high performance at microwave and millimeter-wave frequencies, addressing the challenging need for large tuning range and fine resolution, with minimal loss. This letter presents the fabrication and analysis of an analog–digital variable capacitor with a size of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$80 \times 120~\mu \text {m}$</tex-math> </inline-formula>, comprising three BST analog variable capacitor monolithically integrated with three PCM germanium telluride (GeTe) switches. The measurement results for the integrated analog–digital variable capacitor show a tuning ratio of 1:10.8 with an analog resolution over a wide frequency range, while having a quality factor of 12.5 at 30 GHz. To our knowledge, this letter represents the first demonstration of monolithic integration of BST and PCM technologies.