Continuing research interest in quantum-well inter-subband-based devices can be associated with its prospects for numerous optoelectronic applications in the long wavelength infrared region. This paper presents experimentally measured field dependence of the thermally activated effective-barrier lowering in quantum-well inter-subband photodetectors (QWIPs). This barrier lowering is considered to be the main cause of the commonly observed asymmetry in the current–voltage characteristics of QWIPs. The research results presented here are important for understanding the factors determining the dark-current mechanisms that are crucial for further improvement in the characteristics of these devices. The study of current-carrier transport phenomena in a quantum well is also of interest for developing quantum-well lasers and avalanche photodetectors based on intraband processes, and also transistors based on ballistic or hot carrier transport phenomena.