A metal-oxide semiconductor field effect transistor (MOSFET) includes a substrate and a well over the substrate, the well including dopants of a first conductivity-type. The well includes an anti-punch-through (APT) layer at an upper section of the well, the APT layer including the dopants of the first conductivity-type and further including carbon. The MOSFET further includes a source feature and a drain feature adjacent the APT layer, being of a second conductivity-type opposite to the first conductivity-type. The MOSFET further includes multiple channel layers over the APT layer and connecting the source feature to the drain feature, wherein the multiple channel layers are vertically stacked one over another. The MOSFET further includes a gate wrapping around each of the channel layers, such as in a gate-all-around device, wherein a first portion of the gate is disposed between a bottommost one of the channel layers and the APT layer.