The use of the SSPG method at different electric fields and background
light intensities yields values of the differential electron drift mo
bility. From the dependence of these values on the electron quasi-Ferm
i level, as determined by the background illumination, information abo
ut the shape of the density of states al approximately 0.4 eV below th
e conduction band edge was obtained. The density of states near that e
nergy falls off much more slowly towards midgap than near the mobility
edge.