Semiconductor devices and methods of forming the same include forming a bottom source/drain region in a semiconductor substrate under a semiconductor fin. First charged spacers are formed on sidewalls of the semiconductor fin. A gate stack is formed on the fin, over the first charged spacers. Second charged spacers are formed on sidewalls of the fin above the gate stack. The fin is recessed to a height below a top level of the second charged spacers. A top source/drain region is grown from the recessed fin.