Design Concepts for Semiconductor based Ultra-Linear Varactor Circuits

For the implementation of RF tunable components, semiconductor based varactors provide advantages in terms of low control voltage, high capacitance density, low packaging costs, high reliability and technology compatibility. In this paper, an overview is given of the linearization approaches for semiconductor based ultra-linear varactors. Implementation issues regarding the optimum doping profiles are discussed. Design considerations for dedicated bias networks that provide optimum thirdorder intermodulation cancellation for the various varactor configurations are presented. To give an indication of the system-level responses for linear varactors, a varactor-based “true” time delay phase shifter is designed and the system-level linearity parameters, like adjacent channel power ratio (ACPR) and error vector magnitude (EVM), are evaluated for various application conditions.