**1. Introduction**

Gallium nitride (GaN) is a wide bandgap compound III-V semiconductor with high breakdown electric field, high electron mobility, and high electron saturation velocity that translate in a tremendous potential for high power and high frequency applications (Table 1). The GaN high electron mobility transistor (HEMT) is a device that takes advantage of the two-dimensional electron gas (2DEG) that spontaneously forms at an aluminum gallium nitride (AlGaN)/GaN heterojunction thanks to the strong internal piezoelectric and spontaneous polarization. This 2DEG typically exhibits high values of sheet carrier density (≈10<sup>13</sup> cm−<sup>2</sup> ) and carrier mobility (1000–2000 cm2/(V·s)) [1].

GaN HEMTs can be used in power switching for information technologies, automotive, healthcare, and industrial manufacturing applications [1–3]. Thanks to the large bandgap, leakage currents in GaN power devices are orders of magnitude smaller than in silicon (Si), allowing for operation at higher temperature without thermal runaway and reducing the cooling requirements. The high breakdown electric field allows shorter drift distances for a given blocking voltage, when compared to Si devices, yielding to a drastic reduction in the specific on-resistance that in turn translates into smaller device area and correspondingly lower capacitance. This reduces switching losses and enables higher switching frequencies.

GaN HEMTS have also paved their way into mobile and satellite communications and radar systems [4]. In addition to the properties listed above, the high breakdown electric field of GaN allows higher matching impedances and circuits with broader bandwidth and high power-added efficiency (*PAE*) [5]. The ability of GaN to withstand higher temperatures further increases the power density of a given HEMT device and power amplifiers with absolute power levels of tens to hundreds of Watts have been reported [6].


**Table 1.** Selected properties of relevant semiconductors [1,6–13].

Despite the fact that commercial HEMT devices based on a different combination of processes and design technologies are currently available from a broad range of manufacturers [4], GaN-based technologies still face some challenges that affect their overall performance and limit their potential benefits.


From what was described above, it can be concluded that the capability of efficiently transferring the heat away from the localized hotspots and the consequent control of the device temperature is fundamental to achieve high levels of stability and reliability in HEMT applications [31]. Diamond has the highest thermal conductivity (κ) of any bulk material, and the integration of diamond films and GaN HEMTs as substrates or packaging has already proven to enhance the extraction of the heat generated during the devices operation, leading to a substantial decrease in the junction temperature as well as to an increase in the maximum power density the HEMTs can safely handle. This anticipates a superior high-frequency handling capacity, higher energy efficiency and flexibility, and a better utilization of the electromagnetic spectrum.

Diamond has been successfully integrated with HEMT devices following different approaches. This manuscript aims at describing each of them in detail, pointing out the technical challenges and benefits, and providing the reader with a critical discussion of the feasibility of each approach. The manuscript is organized as follows: Section 2 discusses the critical aspects that impact the thermal management of GaN HEMTs; Section 3 describes the different strategies that have been followed by different groups to integrate diamond and GaN into high performance devices; Section 4 describes the challenges faced by each integration technology and discusses their feasibility; Section 5 draws the main conclusions and Appendix A summarizes the performance of the different GaN/diamond HEMTs reported so far.
