Very highly efficient GaAs–AlGaAs GRIN–SCH lasers were grown on ann‐GaAs substrate as well as on a semi‐insulating GaAs substrate by MBE. The threshold current densityJthof the lasers was found to be minimum when the thickness of the GaAs quantum well active layer is 6 nm. The lowestJthof 260 A/cm2was achieved for the broad‐area Fabry–Perot laser (the Al composition of the cladding layerx=0.7, the cavity lengthL=400 μm). A ridge‐waveguide (5 μm wide stripe) GaAs–AlGaAs GRIN–SCH laser, which is monolithically integrated with GaAs MESFET’s on a semi‐insulating GaAs substrate, exhibited cw operation with a threshold current as low as 19 mA at room temperature.