I need some documents and booklets that include some information about Sub-threshold Region of CMOS. I couldn't find anything about it in Google and my references books.
You can find details about sub-threshold SRAM in the work titled "A 160mV robust Schmitt Trigger Based Subthreshold SRAM" IEEE J. of Solid state circuit, vol. 42, no. 10, Oct. 2007
Searching for the word "subthreshold" in the following link https://www.u-cursos.cl/usuario/9553d43f5ccbf1cca06cc02562b4005e/mi_blog/r/CMOS_Circuit_Design__Layout__and_Simulation__3rd_Edition.pdf might be helpful.