The disclosure relates to semiconductor structures and, more particularly, to one or more devices with an engineered layer for modulating voltage threshold (Vt) and methods of manufacture. The method includes finding correlation of thickness of a buffer layer to out-diffusion of dopant into extension regions during annealing of a doped layer formed on the buffer layer. The method further includes determining a predetermined thickness of the buffer layer to adjust device performance characteristics based on the correlation of thickness of the buffer layer to the out-diffusion. The method further includes forming the buffer layer adjacent to gate structures to the predetermined thickness.