The two-dimensional electron gas concentration in InAs channel modulation-doped heterostructures can be controlled by using an In 0.5 Al 0.5 As 0.56 Sb 0.44 quaternary alloy — doped with Si as a donor — as an electron-supplying layer. When lattice-matched with InAs, an electron concentration of more than 4 × 10 18 cm −3 was obtained in the quaternary layer. The two-dimensional electron gas concentration of an undoped-In0.5Al0.5As0.56Sb0.44 undoped-InAs heterostructure was 7 × 10 11 cm −2 and that of a Si-doped In0.5Al0.5As0.56Sb0.44 undoped-InAs heterostructure was 1.24 × 10 12 cm −2 . The electron mobility was low, though, due to the roughness of the heterointerface. If the interface can be made smooth, however, In 0.5 Al 0.5 As 0.56 Sb 0.44 will be a useful material for forming the electron-supplying layer of InAs channel modulation-doped heterostructures.