A new component of DRAM soft error, in addition to the previously known cell failure component and the bitline failure component, is reported. It occurs when the cell and the bit line each collects radiation-induced charge which is insufficient to upset the cell, but which does cause an error in combination. It has been named the combined cell-bit line or CCB failure mode. Experimental results covering the three soft error rate (SER) components (cell, bit line, and CCB) have been obtained using an Americium-241 source, over a wide range of cycle time from 75 ns to 30μm is. The study indicates that, at short cycle times, CCB failure mode can dominate SER in high-density, highspeed dynamic memories.