An integrated circuit device includes a substrate, an isolation feature, a memory cell, and a semiconductor device. The substrate has a cell region, a peripheral region, and a transition region between the cell region and the peripheral region. The isolation feature is in the transition region. A top surface of the isolation feature has a first portion and a second portion lower than the first portion, the second portion of the top surface of the isolation feature is between the cell region and the first portion of the top surface of the isolation feature, and a bottom surface of the isolation feature has a step height directly below the second portion of the top surface of the isolation feature. The is memory cell over the cell region of the substrate. The semiconductor device is over the peripheral region of the substrate.