A test structure on a wafer is provided. The test structure includes a plurality of cells under test, a first output pad and a second output pad coupled to different cells, a plurality of first input pads, and a plurality of second input pads. The cells are arranged in rows and columns of a test array. Each of the first input pads is coupled to the cells in respective column of the test array. Each of the second input pads is coupled to the cells in respective row of the test array. A first voltage is applied to one of the first input pads and a second voltage is applied to one of the second input pads to turn on a cell, and a current flowing through the turned-on cell is measured.