Monolithic Integration of GeTe Switches and BST Varactors for Reconfigurable Millimeter-Wave Devices

Abstract

Reconfigurable microwave and millimeter-wave systems require tunable components that provide low loss, high resolution, and compact implementation. Conventional approaches based on semiconductor devices, microelectromechanical systems (MEMS), and purely analog or digital tuning techniques face limitations in loss, tuning range, resolution, and integration complexity. Therefore, alternative approaches are needed that can overcome these challenges while remaining compatible with integrated fabrication processes. This thesis presents the development of tunable RF components based on the monolithic integration of ferroelectric barium strontium titanate (BST) varactors and phase-change material (PCM) germanium telluride (GeTe) switches. BST varactors provide continuous analog tuning with low power consumption, while GeTe switches enable discrete, nonvolatile reconfiguration. The combination of these technologies enables a hybrid analog–digital tuning approach that improves tuning range and flexibility.

The work begins with the development and optimization of fabrication process for BST thin-film varactors, followed by their application in tunable circuits. A monolithic fabrication process is then developed to integrate BST varactors and GeTe switches. The challenges associated with material compatibility and process conditions are addressed, and both BST and GeTe devices are fabricated and characterized. Using this platform, hybrid analog–digital varactors with enhanced tuning range are demonstrated. Finally, the hybrid tuning approach is applied to the design and implementation of phase shifters, including true-time-delay (TTD) and reflective-type architectures. These designs combine the advantages of analog and digital tuning to achieve improved phase control, compact implementation, and reduced loss compared to conventional approaches. The results presented in this thesis demonstrate the effectiveness of combining BST varactors and GeTe switches for the realization of reconfigurable millimeter-wave components, providing a practical approach for next-generation tunable RF systems.