**2. Common HEMT structures**

#### **2.1. GaAs‐based HEMTs**

A typical GaAs‐based HEMT structure is shown in **Figure 1**. With a view to separating the majority carriers from ionized impurities, an abrupt hetero‐structure is created between the

**Figure 1.** Structure of GaAs‐based HEMTs.

used as a cryogenic low‐noise amplifier at Nobeyama Radio Observatory (NRO), Nagano,

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

A typical GaAs‐based HEMT structure is shown in **Figure 1**. With a view to separating the majority carriers from ionized impurities, an abrupt hetero‐structure is created between the

HEMT devices have been discussed in the last section.

**2. Common HEMT structures**

**2.1. GaAs‐based HEMTs**

Japan in 1985 [1].

46 Different Types of Field-Effect Transistors - Theory and Applications

wide bandgap material AlGaAs and lower bandgap material GaAs while the wide bandgap material is doped (e.g., doping density, *n* = 7 × 1017 cm−3). Thus, a channel is formed at the inter‐ face of GaAs/AlGaAs heterojunction. To reduce coulombic scattering, a thin layer of undoped AlGaAs is used as spacer layer. At the bottom, the Si or GaAs layer serves as a substrate.

#### **2.2. GaN‐based HEMTs**

GaN‐based HEMTs have the similar layered structure to conventional GaAs‐based HEMTs as shown in **Figure 2**. But no intentional doping is required in AlGaN/GaN HEMTs. Rather elec‐ trons come from surface states due to the spontaneous polarization found in wurtzite‐struc‐ tured GaN. This accumulation of free carrier forms high carrier concentration at the interface leads to a 2DEG channel. **Figure 2** also indicates donor‐like surface traps (empty) on top and thereby the positively polarized charge at AlGaN/GaN interface. The 2DEG is an explicit func‐ tion of the surface barrier, AlGaN thickness and the bound positive charge at the interface.

#### **2.3. InP‐based HEMTs**

InP HEMTs result in lower electron effective mass in InGaAs channel layer compared to con‐ ventional GaAs‐based HEMTs. These HEMTs contain comparatively large conduction band offset (approximately 0.5 eV) between the channel layer and adjacent barrier layer, InAlAs

**Figure 2.** Structure of GaN‐based HEMTs.

[13]. Hence, InP‐based HEMTs show high electron mobility, high electron saturation velocity, and high electron concentration. The device usually consists of an InGaAs/InAlAs composite cap layer for enhanced ohmic contact, an undoped InAlAs as Schottky barrier and an InGaAs/ InAs composite channel for superior electron transport properties as depicted in **Figure 3**.

**Figure 3.** Structure of InP‐based HEMTs.
