Figure 6
From: Phonon-Electron Interactions in Piezoelectric Semiconductor Bulk Acoustic Wave Resonators
Critical velocity ratio and its dependence on material properties.
The critical velocity ratio ηth is the threshold value at which 
. Any further increase in the velocity ratio (achieved by applying a higher electric field) would drive the resonator into the gain regime 
. The possible solution space for the gain regime (shown shaded here for the wurtzite GaN system under consideration) is dependent on the electronic properties (N,μ) and the resonant frequency. The upper bound for amplification in any real material is set by the electron saturation velocity.
