Work function control layers are provided in in-laid, metal gate electrode, Si-based MOS transistors and CMOS devices by a process which avoids deleterious dopant implantation processing resulting in damage to the thin gate insulator layer and undesirable doping of the underlying channel region. According to the invention, an amorphous Si layer is formed over the thin gate insulator layer by a low energy deposition process which does not adversely affect the gate insulator layer and subsequently doped by means of another low energy process, e.g., low sheath voltage plasma doping, which does not damage the gate insulator layer or dope the underlying channel region of the Si-based substrate. Subsequent thermal processing during device manufacture results in activation of the dopant species and conversion of the a-Si layer to a doped polycrystalline Si layer of substantially increased electrical conductivity.