In recent years, adaptive circuits are investigated to tackle the increasing complexity of modern mobile communication systems, due to new standards like LTE and 5G. The crucial element of an adaptive circuit, like a tunable filter, is the tunable component. However, available tunable capacitors - or varactors - are not able to fulfill the stringent requirements of modern mobile communication systems concerning quality factor, tuning ratio, frequency, and linearity. In this work, linear high-Q AlGaN/GaN varactors are developed based on the Fraunhofer IAF technology. A large-signal model is presented and the devices are transferred to a cheap GaN on Silicon technology. The developed devices are proposed as a powerful alternative to existing varactor concepts, as they display superior performance especially at frequencies exceeding 2 GHz.