We report on the variation in the response time in scanning tunneling microscopy (STM) measurements across a Si pn junction, and its effect on STM images of such devices. The response time of the tip height subsequent to a stepwise change in the voltage at a fixed demanded current varies by almost two orders of magnitude across the junction, with the slowest response of nearly 80 ms corresponding to the tip-sample junction in strong inversion. Measurements at decreasing separation show an increase in response time up to a saturation point, consistent with screening due to the inversion charge. The slow response in regions where the tip-sample junction is in inversion explains observations of anomalously deep features in STM topography images and scan direction-dependent features in STM conductance maps. (C) 2003 American Institute of Physics.