Bulk acoustic wave transformer based on the combination of the high-ε epitaxial PbTiO₃ and low-ε ScAlN thin films

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 k_t and high ε, and the output layers are ScAlN films with high k_t 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.