High Electron Mobility Transistors(HEMTs) [340] are an advanced modification of the simple bulk FET, such as the MEtal Semiconductors Field Effect Transistor (MESFET). Typically, a semiconductor material (barrier) with a comparably wider bandgap is grown on top of a semiconductor material with a higher mobility and comparably lower bandgap. If the bandgap alignment of the two materials is appropriately chosen, a channel forms due solely to the alignmentof the band edges and not, as for silicon MOSFETs or III-V MESFETs [460], due to an oxide/semiconductor interface or doping profiles. The channel material is not intentionally doped.