INTEGRATED PIEZO-ACOUSTIC CIRCUITS FOR RADIO-FREQUENCY FRONT-ENDSINTEGRATED PIEZO-ACOUSTIC CIRCUITS FOR RADIO-FREQUENCY FRONT-ENDS

Gallium Nitride (GaN) is simultaneously a semiconductor and a piezoelectric material. These properties are singularly valuable but the benefit of both together is still under investigation. There has been much research into metal oxide semiconductor (MOS) applications of GaN such as radio-frequency (RF) amplifiers. However, these GaN amplifiers are made using materials and processes which are incompatible with those used to make modern RF filters, which use quartz or lithium tantalate (LiTaO3). This presents significant problems when the application of RF filtering and amplification is combined with strict space constraints, like smartphone design, where microwave and RF filtering and amplification together form the technology behind wireless data transfer. The ideal solution to this problem is to produce filters and amplifiers on the same chip. To facilitate this the piezoelectric resonator of the filter and the substrate of the MOS amplification circuit need to compatible processes. Research from the University of Bristol illuminates GaN as a suitable material from which to produce surface acoustic wave (SAW) RF filters. This research demonstrates that despite the inferior piezoelectric properties of GaN compared to lithium tantalate, high frequency filters can be produced by exploiting the material's specific properties. The impedance step at the interface between a GaN layer and a hard substrate produces interesting reflections of acoustic energy, which result in a confined Lamb wave. The acoustic energy of this Lamb wave is guided by the GaN layer which enables small, high Qmech resonators to be produced. Following this research, GaN appears to be an ideal material from which to produce monolithic radio-frequency chips. The proposed research would continue the investigation of this design of GaN SAW filters. Filters of decreasing size should be fabricated and their radio-frequency performance analysed. The ideal device size comes from a compromise of size and performance. The reasons for changing performance with decreasing size should also be established as this may indicate opportunities for modification of the design. The research should also concern the integration of these filters with GaN MOS amplifiers and other GaN RF acoustic components. This project falls within the EPSRC Quantum Technologies research area and is affiliated with the QET Labs research group at the University of Bristol.