The lock-in phase mapping technique with modulated reflectance is introduced for the first time. With its help, the modulated reflectance mechanisms using thermal laser in operating FETs, particularly in the pinch off region, are clarified. The free carrier absorption mechanism dominates at a high modulation frequency, while thermo-reflectance mechanisms at a low modulation frequency. In the pinch-off region, thermo-reflectance mechanism cannot be neglected due to extremely low free carrier concentration. The modulated reflectance signal was unexpectedly observed at passive poly/oxide/poly capacitance. The advantages of lockin phase mapping in dynamically operating mixed-signal IC devices are shown in several case studies.