Chapter 3 Two-Dimensional Electron Gas FETs: Microwave Applications

Abstract

Publisher Summary This chapter discusses the theory of two-dimensional electron gas field-effect transistors (TEGFET). The TEGFET can theoretically work with the Schottky gate on GaAs or on AlGaAs, but experimental results have shown that the latter structure is more suitable for various reasons. (1) When the n–doped AlGaAs is underneath, its thickness has exactly controlled until the complete depletion of electrons, otherwise parallel conduction occurs. There is no way to recalibrate the thickness of this AlGaAs layer as can be done in a recessed structure when the AlGaAs layer is at the top. (2) Under gate bias, hot electrons in the 2DEG channel are pushed by the gate electric field toward the heterojunction interface. Since the heterojunction barrier height is 0.3 eV hot electrons with energy above 0.3 eV can be injected into AlGaAs, the low electron velocity material. This real-space transfer mechanism has been shown to occur in GaAs/AlGaAs quantum-well structures and to a smaller extent in a TEGFET structure. This injection mechanism, which would give rise to a large output conductance, is undesirable. (3) When GaAs is grown on top of the AlGaAs n-doped layer, the modulation-doped heterojunction exhibits low mobility. The origin of this poor mobility is not clearly understood. Recently, by using a superlattice structure in the spacer layer, some mobility enhancement was observed.