A Low-<i>V<sub>t</sub></i> Small-Offset Gated-Preamplifier for Sub-1-V DRAM Mid-Point Sensing

Abstract

A gated sense amplifier (GSA) consisting of a low-Vt gated preamplifier (LGA) and a high-Vt sense amplifier (SA) is proposed. The gating scheme of the LGA enables quick amplification of an initial cell signal voltage (vS0) because of its low Vt and prevents the cell signal from degrading due to interference noise between data lines. As for a conventional sense amplifier (CSA), this new type of noise causes sensing error, and the noise-generation mechanism was clarified for the first time by analysis of vS0. The high-Vt SA holds the amplified signal and keeps subthreshold current low. Moreover, the gating scheme of the low-Vt MOSFETs in the LGA drives the I/O line quickly. The GSA thus simultaneously achieves fast sensing, low-leakage data holding, and fast I/O driving, even for sub-1-V mid-point sensing. The GSA is promising for future sub-1-V gigabit dynamic random-access memory (DRAM) because of reduced variations in the threshold voltage of MOSFETs; thus, the offset voltage of the LGA is reduced. The effectiveness of the GSA was verified with a 70-nm 512-Mbit DRAM chip. It demonstrated row access time (tRCD) of 16.4ns and read access (tAA) of 14.3ns at array voltage of 0.9V.