IGZO-based capacitorless 2T0C DRAM operation at 77 K for cryogenic computing

In this work, an IGZO (In&#x2212;Ga&#x2212;Zn&#x2212;O) 2T0C DRAM (dynamic random access memory) is demonstrated as a cryogenic memory as low as 77 K. The effects of temperature on the IGZO TFTs electrical properties are investigated. We observe that the subthreshold swing (SS) is improved from 161 to 99 mV/dec with no penalty of on-state current (<i>I</i>_ON) @<i>V</i>_TH+1 V reduction when temperature decreased from 300 to 77 K. More importantly, the corresponding <i>V</i>_TH shift positively from &#x2212;1 to 0.5 V, indicating a transition from depletion-mode to enhancement-mode of IGZO TFTs, which is crucial for the low power operation and data retention time (DRT) optimization. By integrating this IGZO TFT to 2T0C DRAM, the retention time of the DRAM cell is significantly enhanced to 8000 s at 77 K, more than 5 times longer than the one at 300 K. The optimized data retention time also results from the lower leakage current (6 &#x00D7; 10^&#x2212;18 A/&#x03BC;m) of at 77 K due to the suppress of carriers thermally excitation and tunneling in IGZO channel at cryogenic temperature. Additionally, a large read current margin (<i>I</i>_{data&#x2018;1&#x2019;}/<i>I</i>_{data&#x2018;0&#x2019;}) of approximately 10³ is achieved across wide temperature range. This study demonstrates the potential of IGZO 2T0C DRAM cells for future cryogenic computing systems.