Dynamic Thermal Coupling in GaN MMIC Power Amplifiers

The influence of dynamic thermal coupling on gallium nitride (GaN) monolithically microwave integrated circuit (MMIC) power amplifiers (PAs) is investigated through transient measurements, numerical simulations, and equivalent circuit modeling. The measured thermal coupling exhibits a low-pass-filtered response, where the magnitude and cutoff frequency decrease with increasing separation from the heat source. The coupling between two neighboring transistor channels shows a fractional order transient response and a pronounced temperature increase after $\approx 1~\mu $ s in the measurements. The coupling between transistors on the same MMIC is close to a first-order transient response and shows a pronounced temperature increase after $100~\mu $ s to 2.6 ms for the measured structure. It is shown that the thermal coupling causes the transistors in the PA to operate at different temperatures, where the transient response of the PA exhibits five distinct time regions. An equivalent linear network is extracted to model the effect efficiently in a circuit simulator. Here, it is shown that the thermal coupling between neighboring transistors can change the thermal response of the PA considerably below 10 kHz. The outlined results give guidelines for predicting the dynamic self-heating in GaN PAs.