Errors in threshold-voltage measurements of MOS transistors for dopant-profile determinations

The dopant and trap density profiles in MOSFETs can be deduced from the static and transient variations of V_T caused by changes in V_BS. This letter reports a study of errors in V_T measurements by three different methods. Relatively small errors in V_T could cause serious inaccuracies in dopant profiles. It is found that the major source of errors is the non-constancy of the surface mobility and that errors in V_T measured by a “constant V_GS?V_DS method” cannot be eliminated easily. However, with properly chosen V_GS?V_T or I_D, a “constant I_D?V_DS method” can provide almost error-free measurements of V_T. The theoretical predictions agree with experiments.