Thick-film Barium-Strontium-Titantate varactors for RF power transistors

This work addresses the properties of Barium-Strontium-Titanate based thick-film varactors at conditions that are expected to arise when the varactors are integrated in a packaged RF power transistor i.e. varying thermal conditions in combination with large fields and high power larger than 30dBm. The varactors are characterized in a small- and large-signal environment in a wide temperature range. A considerable thermal dependency is found and its effect on a tunable pre-matching transistor assembly is investigated. At a 40 degree Celsius temperature increase, the pre-matching shunt varactor increases the gain shift from 1.0dB up to 1.3dB compared to a non pre-matched GaN. Considering these effects in the design, the additional shift can be fully re-tuned with the varactor bias voltage to re-establish the gain of the GaN-HEMT.