The equations for the threshold-current density Jth, differential quantum efficiency ?d and maximum wallplug efficiency ηwp,max for quantum-cascade lasers (QCLs) have been modified for electron leakage and backfilling. We used a thermalexcitation model of "hot" injected electrons from the upper laser state to upper active-region energy states to calculate leakage currents. Then the calculated characteristic temperature T0 for Jth was found to agree well with experiment for both conventional and deep-well QCLs. The characteristic temperature T 1 for ηd was deduced to be due to both electron leakage and an increase in the waveguide-loss coefficient. For conventional mid-infrared QCLs ηwp,maxis found to be strongly temperature dependent which explains experimental data. By using a new concept: tapered active-region (TA), deep-well QCLs have been optimized for virtual suppression of the electron-leakage currents. In turn, at room temperature, for continuous-wave (CW)-operating, 4.5-5.0 μm-emitting TA QCLs we estimate the threshold current to decrease by ∼ 25 %, the active-region temperature rise at the ηwp,max point to decrease by ∼ 30 %, and the single-ended, ηwp,max value to become at least 22 %. Preliminary results from TA QCLs include T1 values as high as 454 K, over the 20-60 °C heatsink-temperature range.