Chapter 3 - Field Effect Transistors: FETs and HEMTs

Abstract

This chapter discusses the principles of operation and applications of field effect transistors (FET). Field effect transistors, specifically high electron mobility transistors (HEMTs) or modulation-doped field-effect transistors are extensively used in low-noise and power amplifiers at microwave and millimeter-wave frequencies. The chapter describes the mechanisms of current voltage and charge control in HEMTs. It also provides an overview of small-signal and large-signal equivalent circuit models of FET that are used in circuit design. The chapter also discusses the issues related to device scaling and operation of FETs as low-noise and power amplifier devices. 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 performance of these devices as low-noise and power amplification devices is summarized. These devices have demonstrated their ability for insertion in high-volume commercial applications such as wireless and optical communication systems. This chapter also focuses on the various design issues involving power amplifiers for wireless handsets and the suitability of GaAs pHEMTs for this application. The AlInAs/GalnAs 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 with application of AlInAs/GalnAs HEMT for digital circuits are also discussed.