We developed a SQUID-based, noninvasive method for the investigation of semiconductor wafers. It is based on the detection of excited photocurrents via their magnetic field by means of a highly sensitive SQUID system. Our method allows the visualization of small growth-related fluctuations of the doping level of the semiconductor with a spatial resolution of a few tens of a micrometre determined by the excitation spot size. Numerical simulations of the magnetic signals have been performed, the results of which show a reasonable agreement with the experiments.