A dynamic random access memory (DRAM) device has an array of DRAM cells of rows by columns. Each DRAM cell of the array is coupled with a wordline of a corresponding row and a bitline of a corresponding column. Entry into and an exit from the self-refresh mode is detected by a mode detector and a self-refresh mode signal is provided. An oscillation circuit generates in response to the self-refresh mode signal a basic time period. A first frequency divider/time period multiplier changes the basic time period in accordance with a process variation factor relating to the DRAM device. A second frequency divider/time period multiplier further changes the changed time period in accordance with a temperature change factor. In the self-refresh mode, data stored in the DRAM cells is refreshed. In accordance with the two factors, the DRAM devices perform and achieve reliable self-refresh for variable DRAM cell retention time.