8.4 μm-emitting quantum cascade lasers (QCLs) have been designed to have, right from threshold, both carrier-leakage suppression and miniband-like carrier extraction. The slope-efficiency characteristic temperature T1, the signature of carrier-leakage suppression, is found to be 665 K. Resonant-tunneling carrier extraction from both the lower laser level (ll) and the level below it, coupled with highly effective ll-depopulation provide a very short ll lifetime (~0.12 ps). As a result the laser-transition differential efficiency reaches 89%, and the internal differential efficiency ηid, derived from a variable mirror-loss study, is found to be 86%, in good agreement with theory. A study of 8.8 μm-emitting QCLs also provides an ηid value of 86%. A corrected equation for the external differential efficiency is derived which leads to a fundamental limit of ~90% for the ηid values of mid-infrared QCLs. In turn, the fundamental wallplug-efficiency limits become ~34% higher than previously predicted.