A method includes forming a vertical transistor, and the method includes forming a vertical semiconductor bar over a substrate, forming a gate dielectric and a gate electrode encircling the vertical semiconductor bar, forming a first source/drain region over a top surface of the vertical semiconductor bar, removing the substrate to reveal a bottom surface of the vertical semiconductor bar; and forming a second source/drain region contacting the bottom surface of the vertical semiconductor bar. The method further includes forming a backside power line, with the backside power line being on a bottom side of the vertical semiconductor bar. The backside power line is connected to the second source/drain region.