Field effect transistors: FETs and HEMTs

Abstract

Abstract This chapter discusses the principles of operation and applications of field effect transistors. Field effect transistors, specifically high electron mobility transistors (HEMTs) or modulation-doped field-effect transistors are being extensively used in low noise and power amplifiers at microwave and millimeter-wave frequencies. The chapter begins with a discussion of current voltage and charge control mechanisms in HEMTs and this is followed by discussion of small-signal and large-signal equivalent circuit models of FET (which are used in circuit design). As operation at high frequencies is enabled by reduction in gate length, issues relating to device scaling are discussed. Issues related to operation of FETs as low-noise and power amplifier devices are discussed. The emergence of crystal growth techniques including molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) has enabled the fabrication of HEMTs using a variety of material systems. The selection of a material system depends on application and operating frequency. The AlGaAs/InGaAs pseudomorphic HEMT and the AlInAs/GaInAs HEMT are the two most widely used device structures. The performance of these devices as low-noise and power amplification devices is summarized. These devices have recently demonstrated their ability for insertion in high-volume commercial applications such as wireless and optical communication systems. This chapter therefore discusses the various design issues involving power amplifiers for wireless handsets and the suitability of GaAs p HEMTs for this application. The AlInAs/GaInAs HEMT with its high current density and superior high frequency performance has enabled the fabrication of digital circuits operating at 80 Gb/s. The various issues related to application of AlInAs/GaInAs HEMT for digital circuits are also discussed.