The invention is a novel way of guiding elastic waves and confining mechanical vibrational energy using phononic crystals and adiabatic (slow) transitions in a completely solid-state commercial CMOS stack. This requires that phonons, quanta of sound, are completely contained within the CMOS die. The result is a 35 times improvement in the Q-factor of the phononic crystal CMOS resonant body transistors (PnC-CMOS RBTs). The invention can also be used as a phononic trap or coupled with on-chip micro- and nano-photonics to achieve on-chip opto-mechanical coupling. The invention is applicable in all operational frequencies which can be fabricated by current CMOS technologies.