Direct Measurement of Correlation Between SRAM Noise Margin and Individual Cell Transistor Variability by Using Device Matrix Array

Noise margin, characteristics of six individual cell transistors, and their variability in static random-access memory (SRAM) cells are directly measured using a special device-matrix-array test element group of 16-kb SRAM cells, and the correlation between the SRAM noise margin and the cell transistor variability is analyzed. It is found that each cell shows a very different supply voltage
$V_{dd}$
dependence of the static noise margin (SNM), and this scattered
$V_{dd}$
dependence of the SNM is not explained by the measured threshold voltage
$V_{\rm th}$
variability alone, indicating that the circuit simulation taking only the
$V_{\rm th}$
variability into account will not predict the SRAM stability precisely at low supply voltage.