*2.1. GaN HEMT*

The concept of mobility enhancement through modulation doping of an aluminum gallium arsenide (AlGaAs)/gallium arsenide (GaAs) multilayer heterojunction was introduced by Dingle et al. in 1978 [32]. Since the energy of the GaAs conduction band is lower than the energy of the AlGaAs donor states, electrons from the later move into the GaAs regions, forming a 2DEG. By introducing a rectifying contact on top of the heterojunction, Mimura et al. [33] were able to control, by field effect, the concentration of the 2DEG. Soon after the deposition of high quality GaN films on sapphire substrates by Metalorganic Chemical Vapor Deposition (MOCVD) [34] and using the same principle, the AlGaN/GaN HEMT was reported by Khan and co-workers [35]. In a conventional AlGaN/GaN HEMT, the current flowing in the 2DEG channel between the source and drain Ohmic electrodes is modulated by a negative bias applied to the gate Schottky contact. – –

The cross-section view of a general GaN HEMT structure is shown in Figure 1 and the κ of the materials typically used are listed in Table 2. The actual structure, composition, and thickness of each layer in a particular HEMT depend on its specific purpose and/or the vendor's manufacturing practices. Since the review and discussion of the literature presenting different devices modifications is out of the scope of this work, only the layers considered relevant to the thermal transport are represented in Figure 1. Other possible layers (not shown) would include the spacer and cap layers.

**Figure 1.** Simplified representation of an AlGaN/ GaN HEMT structure; drawing not to scale.

– **Ther** 1. Substrate. Homoepitaxial growth of GaN-based films is hampered by the limited availability of GaN substrates in standard wafer sizes. As a consequence, the different layers are typically deposited by either Molecular Beam Epitaxy (MBE) or MOCVD onto sapphire, Si, or silicon carbide (SiC) substrates. Epitaxial films with dislocation densities of 10 8 cm−<sup>2</sup> are typically obtained [36]; dislocation densities lower than 10 7 cm−<sup>2</sup> involve hydride vapor phase epitaxy (HVPE) growth.

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**Table 2.** Typical thermal conductivity of the materials present in an AlGaN/GaN HEMT [1,7,9–11,46–48].
