SRAM subthreshold current recovery after unipolar AC stressing

Standby current in a Static RAM is a measure of the subthreshold current of the transistors which make the inverter pair in the SRAM. Application of stress on the transistor gates typically causes defects in the gate oxide leading to higher leakage and higher subthreshold current. We have recently found however that an AC stress cycling on the SRAM however reduces the leakage in many cases. A trapped charge model is proposed for decrease in subthreshold current leading to lower observed standby current on continuous negative unipolar write stress. Several mechanisms have been proposed earlier such as Poole-Frenkel enhanced emission from traps, trap assisted tunneling, and band-to-band tunneling to explain possible source of off-current. As devices shrink, for low Vcc and Vt devices, subthreshold current heroines critical. Low standby current becomes vital for application like cell phones. The electronic industry is trying to find methods to reduce power consumption by reducing leakage, in addition to reducing the on current. We have recently discovered that with application of unipolar negative AC stress on the gate of the SRAM, continuous increase in sub threshold leakage does not happen as expected as the oxide is being weakened. We have found that unless the applied stress is very high for a relatively long time the leakage current follows a periodic pattern when plotted with time. In addition to being an important finding for the application of SRAM, it also gives another insight into the behavior of gate oxide with respect to charge trapping and de-trapping.