We investigated the origin of so-called "hot-spots" in large area pixelless QWIP-LED upconverter devices, which are quantum well infrared photodetectors grown in series with a light emitting diode [E.Dupont et al., IEEE Photonics TechnoL Lett. 14 (2002)182]. The hot-spots, with surface densities varying from 20 to 3000 cm(-2) for the wafers studied, are LED electroluminescence singularities, which can be readily observed at temperatures below similar to 100 K. Their spatial FWHM is less than 1 mu m and they can have peak intensities of more than three orders of magnitude higher than the uniform background emission caused by the dark current through the QWIP device. In most cases the hot-spots were observed on mirror smooth, morphological defect-free surfaces. These electroluminescence abnormalities result from highly localized electron injection channels in the QWIP portion of the device. As there is no morphological abnormality in their locations, these defects are undetectable with established wafer screening instruments such as Nomarski and Surfscan, yet may be very common in MBE-grown heterostructures. Although correlation with dislocations is observed, the great majority of dislocations do not give rise to the hot-spots, and there are many hot-spots, which are not located in the proximity of any dislocation. We therefore, propose that the hot-spots arise from a local alloy separation in ternary layers, induced by floating organic molecules or molecular contamination clusters, resulting in nanowire-like defects. Highly localized drops in the layer resistivity caused by such defects will adversely affect the performance of any devices relying on vertical transport, particularly if the device current depends very strongly on the height of the potential barriers employed, such as in quantum well infrared photodetectors or resonant tunneling diodes. Also, for devices working at high current densities, such as certain lasers or HBTs, the presence of such defects may lead to early device failure. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.