Method to guard-band static random access memory and dynamic random access memory signal margin
Original Publication Date: 1999-Nov-01
Included in the Prior Art Database: 2003-Jun-18
Disclosed is a method to guard-band dynamic random access memory (DRAMs) and static random access memory (SRAMs) and other circuits containing differential inputs such as sense amplifiers against degradation in signal margin. The method uses a stress to temporarily skew the symmetric sense amplifier devices such that an increase of signal margin is necessary. However at the end of the stress patterns, the sense amplifier devices have been stressed equally and are once again symmetric Most DRAM's and SRAM's rely on amplifying a small voltage differential "signal margin" created between two nodes (BitTrue, BitComp) (see Fig. 1). These two nodes are typically connected to a cross coupled transistor sense amplifier (see Fig 2.) which is turned on (set) after waiting long enough for an appropriate amount of voltage differential to be developed. When the sense amplifier is turned on, the small signal margin is amplified to a full voltage differential (Vdd and ground) by one of the two cross coupled devices turning on while the other remains off. There are many causes of signal margin degradation. These include differences in capacitance, resistance and defect leakage between the BitTrue and BitComp nodes. Other causes include capacitive coupling from adjacent noise sources and transistor device mismatches between the cross coupled transistor devices in the sense amplifier. Hot electrons are electrons that are traveling from the source to the drain of a transistor that are accelerated with such a high velocity that they have enough energy to embed into the transistor's gate oxide and become trapped there. Over time, more and more of these electrons become trapped in the gate oxide and cause the transistor's parameters such as threshold voltage and transconductance to change. This effect is enhanced at high voltages and low temperature.