NMOS processes require a variety of threshold voltages for differing applications. For this experiment, the threshold voltages of NMOS devices were altered by a using several different ion implant doses (none, 1, 2, 4, and 8e12/cm2) of boron. This shifted the threshold voltage in good agreement with literature values .
Wickham, Matthew A.
"Ion Implantation to Adjust NMOS Threshold Voltages,"
Journal of the Microelectronic Engineering Conference: Vol. 3
, Article 36.
Available at: http://scholarworks.rit.edu/ritamec/vol3/iss1/36