*2.2. Getting the Heat Out*

Thanks to the high breakdown voltage and saturation velocity of carriers in the 2DEG, AlGaN/GaN HEMTs are able to handle substantial power densities, which result in selfheating and highly-localized power densities (in some cases as high as 10<sup>5</sup> W/cm<sup>2</sup> ) [49]. The resulting high temperatures decrease the MTTF, the performance, and the reliability of HEMT devices—and this makes the implementation of efficient thermal management techniques mandatory. This is particularly important for devices that have inherently small features yet process large power densities, such as high-power RF/millimeter-wave transistors and single-mode visible semiconductor lasers.

Most of the heat generated during device operation will diffuse from the hotspot through the different layers represented in Figure 1 until it reaches the heat sink attached to the back of the substrate. A few intrinsic obstacles hinder the transfer of the heat towards the heat sink. The AlGaN/GaN strain relief transition layers improve the electrical performance at the top of the GaN layer, however, if the concentration of Al is higher than 5%, the κ of the AlGaN decreases to around 1/10th that of bulk GaN, hampering the transfer of heat from the GaN buffer layer to the underlying substrate [41].

The thermal transport across the strain-relief layer/nucleation layer/substrate interfaces also plays an important role in determining the overall κ of the GaN HEMT material system. The existence of an interface between two solids results in the scattering of thermal energy carriers (electrons and phonons), which translates in the appearance of a thermal boundary resistance (*TBR*) between the different materials and in a temperature discontinuity across the interface [50,51].

Despite its low thickness in comparison to the GaN layer, the nucleation layer profoundly impacts the transport of heat from the strain relief layer towards the underlying substrate. Instead of being composed of high quality AlN, this layer contains dislocations, grain boundaries, and point defects (impurity atoms and vacancies) [52,53], within the itself or near the interfaces, that hinder heat transport by increasing phonon scattering rates and reducing the phonon mean free path [54].

The interfacial and nucleation layer thermal resistances contribute with an effective *TBR* between the strain relief layers and the substrate between 10 and 70 m<sup>2</sup> ·K/GW [38] that may cause an additional 30–50% channel temperature rise in AlGaN/GaN HEMTs [38,55].

#### *2.3. Why GaN-Diamond HEMTs?*

In the search of good thermal conductors, carbon-based materials, such as highly oriented pyrolytic graphite (HOPG), graphene, and diamond, are obvious candidates.

HOPG is an anisotropic material with high in-plane κ (κin-plane ≈ 2000 W/(m·K)) but a much lower out-of-plane κ (κout-of-plane ≈ 6–9 W/(m·K)) [56]. Despite being also an electrical conductor, graphite already found its place for thermal management of electronic components at system level, and high κ graphite films for assembling integrated circuits or CPUs, for example, are available from a few vendors. Other examples include graphite heat sinks [57] and composite graphite/metal laminates [58].

Graphene is another anisotropic material with even higher in-plane κ (κin-plane > 3000 W/(m·K)) [59]. However, the particular value of κ depends significantly on the preparation method and can be reduced greatly up to one order of magnitude compared to that of pristine graphene because of poor alignment and structural defects in the material [60]. Due to its intrinsic 2D nature, electrically conductive graphene films are more suited for integration at device level. In 2012 Yan et al. [61] reported the use of graphene quilts for the thermal management of GaN HEMTs, obtaining a ≈20 ◦C decrease in the hot spot temperature in transistors operating at ≈13 W/mm.

Diamond is an isotropic material with high κ (2200 W/(m·K), increasing to 3300 W/(m·K) in the case of isotopically pure material) while being electrically insulating, with a breakdown field 60 times greater that of Si (2 <sup>×</sup> <sup>10</sup><sup>7</sup> V/cm [13])—Table 1. Single-crystal diamond (SCD), grown by high pressure high temperature (HPHT) method, has the best thermal and electric properties; however its area is limited to a few mm<sup>2</sup> . Alternatively, polycrystalline diamond (PCD) films can be grown by chemical vapor deposition (CVD) on large-area substrates such as Si, overcoming the area limitation while still guaranteeing κ values in the range 1000–1800 W/(m·K) [62].
