A diagnostic method of and computer system for root-cause analysis of performance variations of FETs in integrated circuits and a method and computer system for monitoring a field effect transistor manufacturing process. The diagnostic method includes measuring source currents in the linear and saturated regions of two FETs, calculating ratios of the source currents in the linear and saturated regions for the and two FETs and comparing the ratios of the two FETs to determine a probable root cause for a performance variation between the two FETs. One of the FETs has a known good performance.