An array of multi-stack transistor structures is provided, wherein the multi-stack transistor structures are arranged in a plurality of rows and a plurality of columns in the array, wherein each of the multi-stack transistor structures includes two or more vertically arranged transistor stacks, and wherein a dam structure is formed between adjacent two rows in a same column so that a multi-stack transistor structure in one of the adjacent two rows is electrically isolated from a multi-stack transistor structure in the other of the adjacent two rows in the same column.