Wide Tuning Range RF-MEMS Varactors Fabricated Using The PolyMUMPs Foundry

Expert Analysis

Semiconductor varactor diodes are widely used throughout the electromagnetic spectrum, but their performance limits their applications. One technology being pursued by industry and academia as a potential solution to these problems is RF-MEMS varactors. This article introduces the reader to fundamental device research conducted at McGill University concerning the performance limitations of RF-MEMS varactors fabricated in the commercially available PolyMUMPs (Polysilicon Multi-user MEMS Process) foundry. Several varactors were designed, fabricated and characterized, which demonstrated wide capacitance variations (49 to 433 percent) and high Q-factors (Q = 25 to 90) for devices operating at 2.4 GHz. The issues and procedures presented here are easily applied to the development of other devices being developed for commercial applications.

Semiconductor varactor diodes are commonly used throughout RF, microwave and millimeter-wave spectrums for a variety of applications, including the tuning of critical circuits such as voltage-controlled oscillators (VCO), filters, matching networks and phase-shifters in RFIC and MMIC circuits. However, semiconductor varactor diodes are limited in their applications by performance issues. Fundamentally, the quality factor (Q) for varactor diodes varies inversely to the tunable capacitance range of the diode. The Q suffers from the high series resistance of the varactor diode junction, which results in a decrease in Q when the diode capacitance is increased. Therefore, circuits requiring high Q values, such as high quality VCOs, do not rely solely upon varactors as their only source of capacitance. Varactors are often used in conjunction with off-chip discrete capacitors to maintain a combined minimum capacitance and Q. The addition of off-chip discrete components increases the size, complexity and cost of the circuits, so alternative solutions to this problem have been sought. One technology being investigated as a potential solution to this problem is RF-microelectromechanical system (RFMEMS) variable capacitors (varactors).

To see the resume of the expert associated with this case study, see the link below.