The dependence of the characteristic temperature T0 on the cavity length and lasing wavelength is theoretical and experimentally analyzed. The devices are straight separate confinement heterostructure lasers with active layer thickness of 30 and 12 nm grown by low temperature liquid phase epitaxy. The recent observation that for very short lasers T0 is an increasing function of the cavity length is confirmed, and explained in terms of the threshold current cavity length dependence. The temperature dependence of the threshold current Ith gives T0 as high as 307 K for a quantum well laser with cavity length L=168 μm. © 1995 American Institute of Physics.