Electrons in the interface

One remarkable example of a discontinuity-related effect is the interfacial spike in the conduction band edge, shown in Figure 3.22. The wider gap AlAs is doped n-type in this case. Enough electrons flow into the GaAs that near the heterojunction it is made so n-type that it becomes a metal. With careful design, this metallic layer can be rendered very thin such that the electrons find it very hard to move perpendicular to the heterojrmction. In other words, the potential traps them adjacent to the interface. Because they are free to move randomly in this plane and have a moderate to high concentration they are referred to as a two-dimensional electron gas. Because their motion is restricted, it is more difficult for them to scatter since they must remain in the interface plane. The consequence is that the electrons in the interface have a higher mobility than electrons in the bulk semiconductor. 

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