A semiconductor transistor structure with increased mobility in the
channel zone and a method of its fabrication are described. A
semiconductor substrate having a first dopant is formed. A diffusion
barrier layer having a second dopant is formed on the semiconductor
substrate to suppress outdiffusion of the first dopant. Next, a
semiconductor layer having substantially low dopant concentration
relative to the first layer is epitaxially grown on the diffusion barrier
layer. The semiconductor layer defines a channel in the semiconductor
transistor structure. The low dopant concentration in the semiconductor
layer increases the mobility of the carriers in the channel of the
semiconductor transistor structure. A gate electrode and a gate
dielectric are formed on the semiconductor layer with the low dopant
concentration.