Q factor
During early studies and experimentation phase of thin film resonators in 1967 cadmium sulfide (CdS) was evaporated on a resonant piece of bulk quartz crystal which served as a transducer providing a Q factor (quality factor) of 5000 at the resonance frequency (279 MHz). This was an enabler for tighter frequency control, for needs to use higher frequencies and utilising FBAR resonators. With the development of thin film technologies it was possible to keep the Q factor high enough, leave out the crystal and increase resonance frequency.