## 7 Discussion
Piezoelectric resonators, such as the one in [33] operating at 132.94 MHz with a \(Q\)-factor of 1099, indicate higher energy dissipations as compared to capacitive resonators, whereas another device in [34] operating at a very high frequency (VHF) of 496 MHz with a \(Q\)-factor of 3800, showing significant performance improvements at higher frequencies. Hybrid resonators, such as the one reported in [37], operate at 101.65 MHz with a \(Q\)-factor of 14,000, combining both transduction mechanisms for improved performance, while another device in [37] operates at 14.63 MHz with a \(Q\)-factor of 1952, demonstrating versatility while holding design restrictions. In contrast, this work showcases a notable performance advancement by operating at 20 MHz with an exceptional \(Q\)-factor of 28,677 while achieving this performance with a lower bias voltage of 12 V and a smaller capacitive air gap width of 84 nm. The higher \(Q\)-factor and lower bias voltage indicate superior electromechanical coupling coefficient and overall performance, thus making it a standout among reported hybrid resonator technologies. This comparison also underscores the advantages of hybrid resonators, particularly the design presented in this work, which integrates the benefits of both capacitive and piezoelectric mechanisms, achieving high performance with lower energy requirements and advanced fabrication techniques. The term \"hybrid\" describes devices that utilize both capacitive and piezoelectric transduction with separate electrodes surrounding the resonator body and physically connected to the piezoelectric layer, respectively. In contrast to reference [37], which utilizes a concave silicon bulk acoustic bar with aluminum nitride as the piezoelectric layer and polysilicon layer, this work adopts a different and more advantageous design. Despite operating at a relatively lower frequency, the resonator reported herein exhibited more than 2-fold higher quality factor while requiring about half of the DC bias voltage as compared to the device in [37]. Additionally, the entire fabrication process has been carried out at a maximum temperature of 400 \({}^{\circ}\)C, which is significantly lower than the 1100 \({}^{\circ}\)C requiredin [37] for the annealing of the doped polysilicon layer and the 850 \({}^{\circ}\)C needed to deposit the sacrificial oxide layer. The more recent prior work in hybrid devices [38] primarily employs the capacitive electrode to tune the resonance frequency. While the design has some similarities to this work, employing a capacitive gap, the hybrid resonator in this prior work operates at 14 MHz with a \(Q\)-factor of 1952 under atmospheric conditions. Moreover, the fabrication approach could only achieve a capacitive air gap of 2.5 \(\upmu\)m which is too wide to achieve a desirable electromechanical coupling coefficient. In contrast, this work not only achieves a \(Q\)-factor that is 14X higher (28,677) but also requires a much lower bias voltage, as illustrated in Table 1. This highlights the superior performance and efficiency of this design. Therefore, the in-house fabrication process used in this work offers significant advantages over other methods, allowing for lower-temperature processes, which is beneficial for integrating devices with CMOS technology. Low-temperature fabrication is crucial as it prevents damage to the metal interconnects or degradation of transistors underneath, ensuring the reliability of the entire integrated microsystem. This resonator achieves a high quality factor and efficient performance with a lower bias voltage, and effectively mitigates parasitic effects, making the design an improved and more efficient solution among all hybrid MEMS resonators. Table 2 presents key fabrication parameter differences of the hybrid resonators reported herein as compared to previous literature. The hybrid design's versatility is further strengthened by its ability to support multiple transduction configurations, which satisfy various operational requirements.