Disclosed are systems and methods of operation for capacitive radio frequency micro-electromechanical switches, such as CMUT cells for use in an ultrasound system. An RFMEMS may include substrate, a first electrode connected to the substrate, a membrane and a second electrode connected to the membrane. In some examples, there is a dielectric stack between the first electrode and the second electrode and flexible membrane. The dielectric stack design minimizes drift in the membrane collapse voltage. In other examples, one of the electrodes is in the form of a ring, and a third electrode is provided to occupy the space in the center of the ring. Alternatively, the first and second electrodes are both in the form of a ring and there is a support between the electrodes inside the rings.
