the electrons generated in the thin n-type algaas layer drop completely into the gaas layer to form a depleted algaas layer, because the heterojunction created by different band-gap materials forms a quantum well (a steep canyon) in the conduction band on the gaas side where the electrons can move quickly without colliding with any impurities because the gaas layer is undoped, and from which they cannot escape. the effect of this is to create a very thin layer of highly mobile conducting electrons with very high concentration, giving the channel very low resistivity (or to put it another way, "high electron mobility").