GSM, UMTS, LTE, WiFi, Bluetooth – to name just a few of the wireless standards that have become a natural part of mobile communication today. For all these wireless standards, signal processing could not be done without the filtering of frequencies. Micro-acoustic piezoelectric resonators are the dominant technology in the market for this purpose. Theory predicts excellent oscillation characteristics for these resonators, if the electrode used for the excitation of the oscillation becomes very light. And the lightest conceivable electrode is electrically conductive graphene.
"The metal electrodes, commonly used today, dampen the oscillation of the resonators through their mass – similar to the felt cover on a piano string – and therefore reduce the precision of signal separation in bandpass filters. While the metal electrodes cannot be arbitrarily thinned to reduce their mass and thus their damping, graphene still remains conductive even as an atomically thin electrode.", explains Dr. René Hoffmann, head of graphene research at Fraunhofer IAF. With such thin graphene electrodes, the mechanical quality factors come close to the theoretical ideal. If the oscillation characteristics of the piezoelectric resonators can be successfully improved and if higher coupling factors are achieved, both the signal separation precision and the energy efficiency of the filters will increase. Here, the challenge at hand is to connect the nearly massless graphene electrodes with the currently used mobile communication components based on piezoelectric aluminum nitride.