We studied the I-V characteristics, lasing thresholds, and wall plug efficiencies of type-II "W" mid-IR diode lasers from 16 different wafers. At T=90 K, the wall plug efficiency for a 1-mm-long gain-guided device was > 10% and the slope efficiency was 142 mW/A (38% external quantum efficiency). When a 22-mu m-wide ridge was lithographically defined on a five-period W with a p-GaSb etch stop layer, the maximum cw operating temperature increased to 230K. We also investigated ten-stage interband cascade lasers containing W active quantum wells. While the low-temperature threshold current densities were somewhat higher than in the W diodes, at higher temperatures the pulsed thresholds were slightly lower. The threshold voltage was only approximate to 0.1 V larger than the photon energy multiplied by the number of stages, corresponding to a voltage efficiency of > 96%, while the differential series resistance-area product above threshold was as low as 0.21 m Omega cm(2) at 100 K. At T=78 K, the cw slope efficiency was 0.48 mW/A (126% external quantum efficiency), and a maximum cw power of 514 mW was produced by an epi-side-up-mounted 2-mm-long laser cavity with uncoated facets.