The correlation between the contour maps of thermal-wave signals and those of carrier concentrations in GaAs substrates was investigated to understand which portion of an implanted profile mainly corresponds to the generation of thermal-wave signals. It was found that the thermal-wave signals on a Si-implanted GaAs wafer showed good correlation with the peak concentration in the carrier profile obtained from capacitance-voltage (C-V) measurements. This method is useful for the threshold voltage monitoring of GaAs metal-semiconductor field-effect transistors with a thin implanted channel layer, which cannot be characterized by conventional C-V measurement.