The temperature dependences of the threshold current and slope efficiency, as represented by their respective characteristic temperature coefficients T 0 and T 1, are discussed for quantum cascade lasers (QCLs) emitting in the 3.0-3.8 μm, 3.9-5.0 μm, 8-10 μm, and 12-16 μm wavelength ranges. Carrier-leakage mechanisms are treated with emphasis on shunt-type leakage within active regions (ARs); the dominant leakage path in state-of-the-art devices. Carrier-leakage suppression, best evidenced by the T 1 value, is shown to have been the key to effectively doubling the room-temperature pulsed and continuous-wave (CW) wallplug efficiencies for 4.5-5.0 μm emitting QCLs. By employing deep-well and/or tapered-active (TA)-type AR designs, for carrier-leakage suppression, T 0 values as high as 278 K at λ = 4.8 μm and 242 K at λ = 8.4 μm have been achieved for devices of moderately high injector doping, as required for watt-range room-temperature CW operation. Similarly, TA-type QCLs have led to record-high T 1 values: 797 K at λ = 4.8 μm, and 561 K at λ = 8.8 μm, for low-threshold (∼1.6 kA cm-2) devices at room temperature. Step-taper TA (STA) AR designs for 8.4 and 8.8 μm emitting QCLs have resulted in both carrier-leakage suppression as well as fast and efficient carrier extraction. That, in turn, led to internal-differential-efficiency values in the 85-90% range; that is, 30-40% higher than for any previously reported 7-10 μm emitting QCLs. We further show that 4.6 μm emitting STA-type devices hold the potential for room-temperature CW wallplug efficiency values in excess of 27%. Should the internal differential efficiency reach theoretical limits (87-89%) at λ = 4.6 μm, single-facet, room-temperature CW wallplug efficiency values in excess of 40% become possible.
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