With the increased power density of gallium nitride (GaN) high electron mobility transistors (HEMTs), effective cooling is required to eliminate the self-heating effect. Incorporating diamond into GaN HEMT is an alternative way to dissipate the heat generated from the active region. In this review, the four main approaches for the integration of diamond and GaN are briefly reviewed, including bonding the GaN wafer and diamond wafer together, depositing diamond as a heat-dissipation layer on the GaN epitaxial layer or HEMTs, and the epitaxial growth of GaN on the diamond substrate. Due to the large lattice mismatch and thermal mismatch, as well as the crystal structure differences between diamond and GaN, all above works face some problems and challenges. Moreover, the review is focused on the state-of-art of polycrystalline or nanocrystalline diamond (NCD) passivation layers on the topside of GaN HEMTs, including the nucleation and growth of the diamond on GaN HEMTs, structure and interface analysis, and thermal characterization, as well as electrical performance of GaN HEMTs after diamond film growth. Upon comparing three different nucleation methods of diamond on GaN, electrostatic seeding is the most commonly used pretreatment method to enhance the nucleation density. NCDs are usually grown at lower temperatures (600–800 °C) on GaN HEMTs, and the methods of “gate after growth” and selective area growth are emphasized. The influence of interface quality on the heat dissipation of capped diamond on GaN is analyzed. We consider that effectively reducing the thermal boundary resistance, improving the regional quality at the interface, and optimizing the stress–strain state are needed to improve the heat-spreading performance and stability of GaN HEMTs. NCD-capped GaN HEMTs exhibit more than a 20% lower operating temperature, and the current density is also improved, which shows good application potential. Furthermore, the existing problems and challenges have also been discussed. The nucleation and growth characteristics of diamond itself and the integration of diamond and GaN HEMT are discussed together, which can more completely explain the thermal diffusion effect of diamond for GaN HEMT and the corresponding technical problems.

Understanding stress evolution in diamond/SiC heterostructures is critical for high-power electronic applications, yet the thickness-dependent stress transition and its thermal behavior remain underexplored. Here, a synergistic approach combining temperature-dependent Raman spectroscopy and finite element simulation to systematically investigate the stress evolution of diamond films (5–280 µm) on 4H-SiC substrates from 27 °C to 600 °C is adopted. A distinct tensile-to-compressive stress transition is observed within the critical thickness range of 100–200 µm. Thin films (5 µm and 100 µm) are dominated by tensile stress (e.g., D100 exhibits 0.50 GPa tensile stress at 27 °C), whereas thick films (200 µm and 280 µm) show compressive stress (e.g., D200 presents 0.23 GPa compressive stress). Raising the temperature to 600 °C relieves thermal stress, with reduction rates at the center and edge of the 280 µm-thick film reaching 45% and 86%, respectively. Spatial stress distribution reveals that center stress is higher than edge stress, with the difference widening with increasing temperature and film thickness. Films exhibit a stress-temperature trend of first increasing then decreasing, while the edge of the thickest film remains compressive across the entire range. FEM results agree well with thin-film experiments but show notable deviations for thick films. The optimal diamond/SiC thickness ratio is identified as 1:5–2:5, enabling low-stress diamond films by regulating the tensile-to-compressive stress transition. This work clarifies the stress evolution law of diamond/SiC heterostructures, and provides a practical design pathway towards low-stress, thickness-tailored diamond/SiC composites for next-generation high-power electronic and thermal management systems.

Submicrometer boron-doped diamond (BDD) epilayers with controlled thickness, smooth surfaces, and reliable boron incorporation are needed for high-power and radiation-hard electronics, yet these requirements are difficult to satisfy simultaneously. Here, finite-element simulations linked pit suppression to symmetric plasma fields and uniform H and CH3 distributions near the growth surface, guiding the design of a pocket-type molybdenum holder as well as optimization of gas composition and total flow rate. Further adjustment of the actual gas-phase ratios suppressed pits and reduced the surface roughness to Ra = 0.19 nm. Optical emission spectroscopy was used to stabilize the initial growth stage, where abrupt fluctuations in boron incorporation within the first 100 nm can easily occur. Introducing trimethylboron (TMB) in two steps with a 2 min interval improved boron incorporation uniformity. Under the optimized conditions, BDD epilayers with thicknesses of 300 nm - 1.5 μm were obtained. The 682 nm film showed a 004 rocking-curve FWHM of 91.98 arcsec and a sharp diamond Raman peak. SIMS gave a boron concentration of 6.6 × 1017 cm-3, while Hall measurements yielded a mobility of 111 cm2/(V·s), a carrier concentration of 3.66 × 1016 cm-3, and a resistivity of 1.53 Ω·cm. A preliminary vertical Au/P+-diamond/UID-diamond/P+-diamond/Au device also showed repeatable photoresponse from 0 to ±10 V.