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By tailoring the active-region quantum wells and barriers of 4.5-5.0-μm-emitting quantum cascade lasers (QCLs), the device performances dramatically improve. Deep-well QCLs significantly suppress carrier leakage, as evidenced by high values for the threshold-current characteristic temperature T0 (253 K) and the slope-efficiency characteristic temperature T1 (285 K), but, due to stronger quantum confinement, the global upper-laser-level lifetime τ4g decreases, resulting in basically the same room-temperature (RT) threshold-current density Jth as conventional QCLs. Tapered active-region (TA) QCLs, devices for which the active-region barrier heights increase in energy from the injection to the exit barriers, lead to recovery of the τ4g value while further suppressing carrier leakage. As a result, experimental RT Jth values from moderate-taper TA 4.8-μm emitting QCLs are ~14% less than for conventional QCLs and T1 reaches values as high as 797 K. A step-taper TA (STA) QCL design provides both complete carrier-leakage suppression and an increase in the τ4g value, due to Stark-effect reduction and strong asymmetry. Then, the RT Jth value decreases by at least 25% compared to conventional QCLs of same geometry. In turn, single-facet, RT pulsed and continuous-wave maximum wallplug-efficiency values of 29% and 27% are projected for 4.6-4.8-μm-emitting QCLs.