Gate-to-Source/Drain Nonoverlap Device for Soft-Program Immune Unified RAM (URAM)

Abstract

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.