Embodiments of the invention are directed to a method of forming a semiconductor device. A non-limiting example of the method includes forming a channel fin over a substrate and forming a top spacer region around a top portion of the channel fin, wherein the top spacer region includes a dopant. A dopant drive-in process is applied, wherein the dopant drive-in process is configured to drive the dopant from the top spacer region into the top portion of the channel fin to create a doped top portion of the channel fin and a top junction between the doped top portion of the channel fin and a main body portion of the channel fin.