Current dependence of the piezoresistive coefficients of CMOS FETs on (100) silicon

Orthogonal pairs of MOS transistor are well known to make excellent high-resolution stress sensors on (100) silicon. In order to properly design these sensors, circuit designers need an understanding of the operating point dependence of the piezoresistive coefficients (pi-coefficients) of the PMOS and NMOS devices. This paper presents the new results for the drain current dependence of the key pi-coefficients needed for application of CMOS stress sensors, namely Π₄₄^p and Π_Dⁿ. It is demonstrated that uniaxial stress can be utilized to calibrate both the normal stress and shear stress sensors if the temperature is maintained sufficiently constant during calibration. Coefficient values are related to drain current, thereby providing the information necessary to make appropriate operating point choices for the design of CMOS stress sensors, and are also correlated with surface mobility in the devices.