We investigate theoretically three double‐heterojunction modulation‐doped field‐effect transistor structures with different channel composition. All of these transistors have an InxGa1−xAs channel sandwiched between two doped Al0.3Ga0.7As barriers with undoped spacer layers. In one of the structures,xvaries from 0 from either heterojunction to 0.15 at the center of the channel quadratically and in the other two, constant values ofxof 0 and 0.15 are used. We solve Poisson’s and Schro¨dinger’s equations self‐consistently for the electron wave function for all three cases. The results showed that the two‐dimensional electron gas (2DEG) concentration in the channel of the quadratically graded structure is higher than thex=0 one and slightly lower than thex=0.15 one, and the mean distance of the 2DEG is closer to the center of the channel for this transistor than the other two. These two effects have important implications on the electron mobility in the channel.