Various embodiments provide transistor devices and fabrication methods. An exemplary transistor device with improved carrier mobility can be formed by first forming a confining layer on a semiconductor substrate to confine impurity ions diffused from the semiconductor substrate to the confining layer. An epitaxial silicon layer can be formed on the confining layer, followed by forming a gate structure on the epitaxial silicon layer. A portion of the epitaxial silicon layer can be used as an intrinsic channel region. A source region and a drain region can be formed in portions of each of the epitaxial silicon layer, the confining layer, and the semiconductor substrate.