Asymmetric Underlap in Scaled Floating Body Cell Memories

The asymmetric underlap device for a floating body cell is proposed without any extra process or photomask during fabrication. The electric field in the gate-drain underlap region is quietly relaxed. It is found that memory operation would fail in bipolar-based floating body cells because band-to-band tunneling significantly alters the body potential. Measurements show the proposed structure could indeed suppress the undesirable band-to-band tunneling greatly so that the bistable state via the parasitic bipolar junction transistor is ensured in scaled floating body cells. The parasitic capacitances in both word line and bit line are also reduced.