A rectifier antenna (called a rectenna) is useful for wireless power charging of electronic devices. To enhance the rectifier efficiency, we propose sub-GHz bulk acoustic wave (BAW) piezoelectric transformers based on the combination of high and low dielectric constant (ε) thin films. The input layers are sputter-epitaxial lead titanate (PbTiO3) films, which possess both high electromechanical coupling coefficient kt and high ε, and the output layers are ScAlN films with high kt but low ε. The voltage gain of 12.3 in 50 MHz ranges and 40.9 in 400 MHz ranges was experimentally observed in the thickness-extensional mode BAW transformer. The experimental results are in good agreement with the theoretical predictions of the electromechanical transmission line model. The BAW transformer is promising for a high voltage gain transformer for wireless power transfer.
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