We have experimentally characterized the quantum efficiency in InGaAsP hydride CVD grown lasers operating at 1.3 mu m The observed reduction in external quantum efficiency with increasing temperature is found to be caused mostly by a reduction of the internal quantum efficiency. The experimental results are well explained with a theoretical model based on thermionic emission of carriers out of the active region. The model also helps to understand the difference in temperature performance between lasers studied in this paper and those grown by MOCVD.