Various acronyms have been coined for these devices (MODFET, HEMT, TEGFET, SDHT, GAGFET, etc.) all describe either the technology employed in creating the structure or the resultant electronic properties. This device has the superior transport properties of electrons moving along the two-dimensional electron gas (2DEG) formed at the heterojunction interface between two compound semiconductor materials. This chapter describes the basic principle of the field-effect transistor in terms of selectively doped heterojunctions yielding high electron mobility and velocity, which at the device terminals give rise to a high transconductance FET that can be operated at a millimeter-wave frequency range with ultra-low noise. Then there are the increasing demands of military and commercial applications on low-noise, high-frequency amplification. In today's technical and business world, we need high-speed computers to solve problems calling for high-volume data processing, real-time signal processing, graphics, and remote imaging. 9 HIGH ELECTRON-MOBILITY TRANSISTOR (HEMT) 9.1 INRODUCTION TO HEMT
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