Disclosed are a flash memory device and method of operation. The flash memory device includes a bottom memory cell array and a top memory cell array disposed over the bottom memory cell array. The bottom memory cell array includes a bottom semiconductor layer, a bottom well, and a plurality of bottom memory cell units. The top memory cell array includes a top semiconductor layer, a top well, and a plurality of top memory cell units. A well bias line is disposed over the top memory cell array and includes a bottom well bias line and a top well bias line, The bottom well bias line is electrically connected to the bottom well, and the top well bias line is electrically connected to the top well.