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Electron injection into the upper state of the laser transition in quantum cascade lasers is studied by investigating the electrical and optical characteristics of a set of electroluminescent devices. These devices exploit the active region of an (Al)GaAs laser based on a diagonal–anticrossed transition scheme with emission wavelength at 9.5 μm, and are identical except for the injection barrier thickness which varies from 3.9 up to 8.0 nm. We find that for thin barriers electron wave functions in the injector are directly coupled with those of the continuum. This leads to a parallel current path which strongly reduces the injection efficiency of electrons into the active region. The current leak is suppressed at low temperatures for samples with the thickest barriers, but it is still observable at high temperatures when electrons are thermally activated from the injector miniband into the continuum. © 2001 American Institute of Physics.