7.8 Scattering in a Two-Dimensional Electron Gas

Electrons generated at the interfaces of heterojunctions, such as GaAs/ AlGaAs structures, are confined along the growth directions and are free to move in the perpendicular plane. A typical example of this type of heterostructure is shown in Fig. 7.22, where the barrier (AlGaAs) is doped instead of the GaAs layer. The portion of the AlGaAs layer near the interface is not doped and is called the spacer. A typical doping level in the AlGaAs layer is on the order of 10 ¹⁸ cm ^-3. Experimentally, it is found that the mobility increases as the spacer thickness increases while the two-dimensional electron gas density is decreased as the spacer thickness increases. Thus, there is a tradeoff between the electron concentration, mobility, and spacer thickness. The electron mobility for GaAs structures can reach values higher than 1 10 ⁶ cm ² V ^-1 s ^-1. The reason for the high mobility is that electron scattering from ionized impurities is negligible since scattering from ionized impurities decreases as the spacer thickness increases.

Figure 7.22: A sketch of the conduction band structure of a GaAs/AlGaAs heterojunction showing the confined states at the interfaces, the Fermi energy, the spacer, the depletion region, and the conduction band offset.

Doping in the barrier is called modulation doping, and the structure shown in Fig. 7.22 is the basis for the modulation-doped field-effect transistor (MODFET). This is also known as a...