Array of vertical transistors, an array of memory cells comprising an array of vertical transistors, and a method used in forming an array of vertical transistors

A method used in forming an array of vertical transistors comprises forming laterally-spaced vertical projections that project upwardly from a substrate in a vertical cross-section. The vertical projections individually comprise an upper source/drain region, a lower source/drain region, and a channel region vertically there-between. First gate insulator material is formed along opposing sidewalls of the channel region in the vertical cross-section. One of (a) or (b) is formed over opposing sidewalls of the first gate insulator material in the vertical cross-section, where (a): conductive gate lines that are horizontally elongated through the vertical cross-section; and (b): sacrificial placeholder gate lines that are horizontally elongated through the vertical cross-section. The one of the (a) or the (b) laterally overlaps the upper source/drain region and the lower source/drain region. The first gate insulator material has a top that is below a top of the channel region and has a bottom that is above a bottom of the channel region. An upper void space is laterally between the one of the (a) or the (b) and both of the upper source/drain region and the channel region. A lower void space is laterally between the one of the (a) or the (b) and both of the lower source/drain region and the channel region. Second gate insulator material is formed in the upper and lower void spaces. Other embodiments, including structure independent of method, are disclosed.