**1. Introduction**

The requirement of high switching speed such as needed in the field of microwave com‐ munications and RF technology urged transistors to evolve with high electron mobility and superior transport characteristics. The invention of HEMT devices is accredited to T. Mimura who was involved in research of high‐frequency, high‐speed III–V compound semiconductor devices at Fujitsu Laboratories Ltd, Kobe, Japan. Following that, HEMT was first commercially

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

used as a cryogenic low‐noise amplifier at Nobeyama Radio Observatory (NRO), Nagano, Japan in 1985 [1].

Working toward the need of high frequency, low noise, and high power density applications, traditional MOSFETs and MESFETs require to be built with very short channel lengths so that majority of the carriers face minimum impurity scattering and performance degradation is reduced. Such applications also imply design and performance limitations requiring high sat‐ uration current as well as large transconductance, which may be achieved by heavy doping. To overcome these limitations, HEMT devices incorporate heterojunctions formed between two different bandgap materials where electrons are confined in a quantum well to avoid impurity scattering. The direct bandgap material GaAs have been used in high frequency operation as well as in optoelectronic integrated circuits owing to its higher electron mobility and dielectric constant. AlGaAs are the most suitable candidate for barrier material of GaAs possessing nearly same lattice constant and higher bandgap than that of GaAs. That is why GaAs/AlGaAs heterostructure is considered to be the most popular choice to be incorporated in HEMTs. However, AlGaN/GaN HEMT is another excellent device that has been extensively researched in recent times. It can operate at very high frequencies with satisfactory perfor‐ mance as well as possess high breakdown strength and high electron velocity in saturation [2]. GaN shows very strong piezoelectric polarization which aids accumulation of enormous car‐ riers at AlGaN/GaN interface. In these types of HEMTs, device performance depends on the types of material layer, layer thickness, and doping concentration of AlGaN layer providing flexibility in the design process. For its superiority over HEMT devices with other materials, AlGaN/GaN HEMT has been selected as an example for different topics in this chapter.

The chapter begins with brief explanation of different common structures and basic operating principle of HEMT devices. The main focus is to analyze HEMT device performance based on analytical and numerical analyses found in the literature. For example, *I‐V* characteris‐ tics of HEMTs [3], two‐dimensional electron gas (2DEG) estimation [4], short channel current collapse effect [5], capacitance calculation [6], and thermal effects [7] on HEMTs have been discussed in Section 4, which have been obtained using analytical study. Section 5 includes more rigorous methods such as drift‐diffusion modeling [8], transport calculation [9], Monte Carlo simulation [10], Green's function formalism [11], and polarization‐based shear stress analysis [12] that need significant numerical techniques to characterize HEMT device per‐ formance. Looking back into the very recent years, some up‐to‐date results have been pre‐ sented in Section 6, namely "Latest Research" section. Section 7 presents some prediction on the future research trends based on these latest results. Finally, possible application fields of HEMT devices have been discussed in the last section.
