ABSTRACT
High electron mobility transistors (HEMTs) are emerging as excellent candidates for millimeter, sub-millimeter, and microwave applications. The fundamental characteristic of the HEMT is the conduction band offsets between the semiconductor materials, which forms the barrier and channel layers. Two major innovations in the 1980s would greatly expand the high-frequency capabilities of HEMTs and would open up many new application areas in wireless and fiber optical communications, radar systems, sensing, space, and military. The HEMT is also known as a heterojunction field effect transistor, modulation doped field-effect transistor, two-dimensional electron gas (2DEG) field effect transistor, or selectively doped heterojunction transistor. The electrons in the 2DEG channel are provided by the modulation doped barrier layer. A spacer layer, if used, separates the 2DEG from dopants of the electron supply layer thereby reducing electron scattering effects, which enhances electron mobility. In HEMTs are also used as low-noise amplifier monolithic microwave integrated circuits and efficient driver amplifiers.
