GAAs High-Speed Devices: Physics, Technology, and Circuit Applications

9

HIGH ELECTRON-MOBILITY TRANSISTOR (HEMT)

9.1 INRODUCTION TO HEMT

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. Then there are the increasing demands of military and commercial applications on low-noise, high-frequency amplification. 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. 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. 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 [Col85]. HEMT shows much promise in MMICs, demonstrably outperforming the GaAs MESFET in gain, low noise, and frequency response. Devices with switching delays under 5 ps, current-gain cutoff frequencies of about 250GHz, and maximum oscillation frequencies in excess of 400GHz have been developed [Moc91]. Enhancement-mode and depletion-mode HEMTs can also be fabricated on the same wafer for digital integrated ...