Dynamic Thermal Response of GaN MMICs

Doctoral thesis, 2026

Gallium nitride (GaN) monolithic microwave integrated circuits (MMICs) are a key technology for wireless infrastructure, where channel temperature constrains both output power and lifetime. Predicting this temperature is difficult for two reasons. First, it depends on every layer from the channel to the ambient, many of which are uncertain and require in situ characterisation. Second, the dissipated power varies in time during operation. As heat diffuses, each level of integration contributes on a largely distinct time scale, from nanoseconds near the channel to seconds and longer at the system level. Consequently, the dynamic thermal response both complicates the prediction and enables a solution to resolve individual contributions.

This thesis aims to improve channel-temperature prediction in packaged GaN MMICs by using the dynamic thermal response to separate the relevant thermal contributions. Following the heat path, it combines transient measurements on dedicated test structures with calibrated finite-element simulations. At the integrated-circuit level, the study investigates the dynamics of thermal coupling between heat sources and the role of the through-substrate via in layout consideration. At the package level, a thermal test chip is developed to match the GaN MMIC in material system and layout-level heat-dissipation, enabling in situ characterisation without optical access. At the system level, an experimental setup for additively manufactured, liquid-cooled baseplates is implemented. Collectively, these approaches demonstrate how the device, integrated circuit, and package levels shape the dynamic thermal response and provide methods to separate these contributions in situ.