GaN surface acoustic wave filter with low insertion loss

Surface acoustic wave (SAW) filter with a low insertion loss (IL) of 4.415 dB has been demonstrated on Carbon-doped semi-insulating c-plane bulk GaN without external lumped element matching. The center frequency, 3 dB bandwidth, out-of-band attenuation, return loss of the filter are 477.05 MHz, 0.308 MHz, 32.5 dB, and -9.72 dB, respectively. The electromechanical coupling coefficient (K_t²), and temperature coefficient of frequency (TCF) of the filter are 0.21 % and -26.0 ppm/°C, respectively. The impact of the number of interdigital transducers (N_IDT) and acoustic propagation direction on filter performance has been studied. The IL of filters reduces from 16.07 dB to 4.415 dB with the increase of N_IDT from 50 to 150 due to the enhanced acoustic superposition. The numerical distribution of elastic stiffness ([c_ij]), and piezoelectric constants ([e_ik]) of GaN has been calculated in Euler angle space, indicating that they are isotropic on c-plane. The small performance difference of filters along the m- and a-direction on c-plane bulk GaN can be attributed to the small offset angle of 0.5° of the bulk GaN wafer or IDT quality variation.