Abstract: A tunable high frequency AC scanning tunneling microscope (ACSTM) has been utilized to image and to record spectra for semiconductor characterization. A difference frequency mixing technique sensitive to dopant type and concentration is applied both to uniformly doped and to patterned semiconductor substrates. Uniformly doped silicon substrates were used to characterize the difference frequency spectral signature for both p- and n-type Si. Comparison of the measured difference frequency to such signature can be used for distinguishing between the two types of dopants in samples with unknown dopant type. Patterned substrates were then fabricated, and a spectroscopic imaging mode was used to map out dopant density at ultrahigh resolution, and to distinguish between areas of different concentration and different dopant type.