Investigation on MEMS Designs of Noncontact Varactors for high Tuning Ratios

Controllable electrical capacitances, so-called varactors, are used in tuner circuits, high-frequency voltage-controlled oscillators (VCO), radio front-end systems, RF filters and certain sensors. CMOS-compatible varactors can also be designed using MEMS technology. Due to the high quality, the lower phase noise, and the low temperature sensitivity, they represent an alternative to the semiconductor varactors. The MEMS varactor uses the electrostatic attraction between capacitor plates for tuning, which, results in a limited tuning range due to the pull-in effect. Changes in capacitance only reach a factor of approx. 3 (tuning ratio, TR). In this design, the control voltage is also on the RF path. However, they can be decoupled with actuators, which means that the limitation caused by the pull-in effect can also be avoided. Continuous changes in capacity with a TR > 10 seem conceivable. The paper compares MEMS varactor designs exemplarily. It also deals with the design of the MEMS varactors and identifies some key parameters for achieving a high TR.