The study of spontaneous emission for n‐type has shown that the spectral shape and peak position may be utilized to determine the free electron concentration within ±15% near concentrations of and ±7% at . Excitation of the sample with a 1μ diameter electron beam gives such sensitivity with a 3–6µ spatial resolution. This sensitivity and spatial resolution are sufficient for investigating sample inhomogeneities that are related to the spatial variations in the quantum efficiency.