AlxGa1−xAs injection lasers have been fabricated over the compositional range in which the alloy has a direct bandgap transition (0 <x≤ 0.34). At 300°K the threshold current density increases withxfrom 12 000 A/cm2(&lgr;L= 8600 Å) to 56 000 A/cm2(&lgr;L= 7340 Å) in uncoated devices with cavity lengths of about 10 mils. In the same range ofxthe external differential quantum efficiency gradually decreases from 40 to 16%. These are the highest efficiencies and lowest threshold current densities ever reported for room‐temperature lasers emitting in the same spectral range. The improvement is due to utilization of the newp+‐pheterojunction structure previously used by the authors in GaAs lasers to sharply reduce the internal optical loss by improving the optical confinement and reducing the absorption coefficient in thep+material adjoining the active region. At 77°K lasing has been achieved to 6450 Å and cw operation to 6900 Å with the emission of 0.4 W per diode.