A soft-program immune structure for a unified RAM
(URAM) is presented. A unique feature of URAM is the multifunctionality
of a flash and capacitorless 1T-DRAM in a single
transistor. However, charge trapping into O/N/O during a cyclic
1T-DRAM operation can cause an undesirable threshold voltage
shift, resulting in an unstable URAM operation called a soft
program. In a gate-to-source/drain nonoverlap structure with a
nonextended O/N/O layer under the gate spacer, the impact ionization
region is steered out from the gate, which is located under
the spacer. In the 1T-DRAM mode of URAM, the programming
biases are selected so that impact ionization can occur under the
gate spacer, thereby alleviating the soft program. The nonoverlap
device relieves the operational voltage constraint imposed by the
soft program. In addition, nonvolatile flash memory and capacitorless
1T-DRAM perform an acceptable performance without
interference.