RF microelectromechanical systems (MEMS) capacitive switches for two different dielectrics, aluminum nitride (AlN) and silicon nitride (Si3N4), are presented. The switches have been characterized and compared in terms of DC and RF performance (5-40 GHz). Switches based on AlN have higher down-state capacitance for similar dielectric thicknesses and provide better isolation and smaller insertion losses compared to Si3N4 switches. Experiments were carried out on RF MEMS switches with stiffening bars to prevent membrane deformation due to residual stress and with different spring and meander-type anchor designs. For a similar to 300-nm dielectric thickness, an air gap of 2.3 mu m and identical spring-type designs, the AlN switches systematically show an improvement in the isolation by more than-12 dB (-35.8 dB versus -23.7 dB) and a better insertion loss (-0.68 dB versus -0.90 dB) at 40 GHz compared to Si3N4. DC measurements show small leakage current densities for both dielectrics (<10(-8) A/cm(2) at 1 MV/cm). However, the resulting leakage current for AlN devices is ten times higher than for Si3N4 when applying a larger electric field. The fabricated switches were also stressed by applying different voltages in air and vacuum, and dielectric charging effects were investigated. AlN switches eliminate the residual or injected charge faster than the Si3N4 devices do.