In this work we present spectrally resolved photon emission microscopy (SPEM) measurements originating from short-channel MOSFETs acquired through the backside of the silicon substrate. Two commonly used detectors have been chosen for the detection of electroluminescence (EL) in the visible and near-infrared spectral regime, namely Si-CCD and InGaAs. As the backside photon emission (PE) inspection is strongly influenced by the absorption of light in a substrate material, the SPEM experiments have been carried out through thinned silicon layers as obtained by mechanical grinding and local focused-ion-beam (FIB) assisted Si material removal. Intrinsic Si absorption (generation of electron-hole pairs) and absorption on free carriers have been modeled to be able to calibrate experimental results and obtain devicerelated PE spectra. The results show no evidences of specific transitions and lead to a conclusion that photon emission from MOSFETs is fully electrical field related.