We present voltage-dependent scanning tunneling microscopy (STM) data from various semiconductor surfaces. Different techniques for acquiring the data are demonstrated, and the interpretation of spectroscopic data is discussed. For the Si(111)2×1, GaAs(110), and O/GaAs(110) surfaces, we find that the voltage dependence of constant-current contours provides the most direct method for understanding the electronic and geometric structure of the surface. For the case of oxygen on n-type GaAs, we observe a reversal in the constant-current contours when the polarity of the bias voltage is reversed, which is interpreted in terms of band bending due to a negatively charged adsorbate. Theoretical calculations for a screened Coulomb potential are shown to agree with the observed reversal in the STM contours of the adsorbed oxygen.