A method for manufacturing a semiconductor device includes forming a source layer on a semiconductor substrate, forming a channel layer on the source layer, and forming a drain layer on the channel layer. The source, channel and drain layers are patterned into at least one fin, and a cap layer is formed on a lower portion of the at least one fin. The lower portion of the at least one fin includes the source layer and part of the channel layer. The method further includes forming a gate structure comprising a gate dielectric layer and a gate conductor on the at least one fin and on the cap layer. The cap layer is positioned between the lower portion of the at least one fin and the gate dielectric layer.