In this paper, Photon Emission Microscopy (PEM) and micro-Raman Spectroscopy (μRS) are applied for temperature profile measurements and failure characterization in gg-nMOS ESD protection devices. The measurements were carried out in avalanche and snapback biasing conditions. A correlation between the temperature profile obtained by μRS and the light emission location, measured by PEM, is observed for non-degraded devices. In addition, ESD-degraded devices were studied. PEM, μRS, Spectroscopic Photon Emission Microscopy (SPEM) and electrical measurements were used to investigate the origin of the light emitted at the failure site. They showed that the light emission occurring at the failure site is due to impact ionization.