Part 2 : Nanostuctures Module 2 : Electrons in lower dimensions
Lecture 3 : Doped Hetero Junctions
 

Hetero junction
So far the effects of ionized donors and acceptors, and the Coulomb interaction of free electrons in the solutions of the bound states have been ignored. Undoped quantum well systems in which such effects are negligible, are often used in optical studies and in optoelectronic devices. However in electronic devices, free carriers are necessary for charge transport. In particular, modulation doped (MD) structures are used for mesoscopic devices for their high mobilities. In MD devices, the dopants responsible for free carriers are spatially separated from the carriers themselves. The technique usually involves growth of an undoped active layer with a smaller band gap such as GaAs onto which a doped wider band gap material such as AlxGa1−xAs is grown with an undoped spacer layer.

Fig 2.6: Schematic cross-section of High Electron Mobility
Transistor (HEMT) based on modulation doping

In this structure as shown in Fig 2.6, a layer of unintentionally doped GaAs (typically with carrier density between 1014 to 1015 cm−3) is grown on a semi insulating GaAs substrate. Then an undoped space layer of AlGaAs is grown followed by a thicker heavily doped layer of AlGaAs. A cap layer of doped or undoped GaAs is usually grown on top for ohmic contact and to reduce oxidation of AlGaAs layer. Since the free carriers forming a channel at the GaAs /AlGaAs interface are spatially remote from their parent donors, ionized impurity scattering is greatly reduced, and mobilities in excess of 1017 cm2/ (V.s) are attainable in such structures.