Optimisation of integrated RF varactors on a 0.35 μm BiCMOS technology

Integrated varactors are becoming a common feature for many RF designs and in particular RF voltage controlled oscillators (VCOs). Optimisation of the quality of both the inductor and the varactor from the VCO core is essential. This work details the characterisation and optimisation of a number of varactor types available on a typical sub-micron BiCMOS process. Engineering of the bottom plate of the varactor was used to optimise the quality factor of the varactor. No additional mask layers or processing steps were required to achieve this. Integrated isolated diode varactors with quality factors of 30 at 2 GHz have been demonstrated with tuning capacitance ranges of 2.5. Integrated MOS capacitor varactors with quality factors of 50 at 2 GHz have been demonstrated with tuning capacitance range of 5.