We study the effect of semiconduction on mechanical-to-electrical energy conversion through a theoretical analysis on the thickness-extensional vibration of a piezoelectric semiconductor plate driven mechanically. An analytical solution is obtained. A ZnO plate is used as a numerical example. Results show that both the electrical output power and the energy conversion efficiency are sensitive to semiconduction at a moderate carrier density of 10 ¹⁵ m ⁻³ , and that the effect of the dissipation due to semiconduction can be comparable to the effect of material damping when the material quality factor is in the usual range of 10 ² –10 ³ .