Semiconductor devices and methods are provided. A semiconductor device of the present disclosure includes a bias source, a memory cell array including a first region adjacent to the bias source and a second region away from the bias source, and a conductive line electrically coupled to the bias source, a first memory cell in the first region and a second memory cell in the second region. The first memory cell is characterized by a first alpha ratio and the second memory cell is characterized by a second alpha ratio smaller than the first alpha ratio.