The defects were investigated in p(+)nn(+) 4H-SiC diodes by observing the forward-bias-induced light emission through the substrate. The spatial intensity distribution, the temporal evolution and the spectral content of the electroluminescence (EL) signal have been measured in order to detect, identify and understand the defect formation during forward-bias application. It was found that, exept from the dislocations inside the epilayers, mesa etching is a main cause for the formation of extended defects. To our knowledge, for the first time, reduction of mesa-etching-induced defects is shown in this investigation.