AlGaN/GaN high electron mobility transistors (HEMT) capped with nanocrystalline diamond (NCD) have been demonstrated in the past to outperform electrically and thermally their SiN-passivated counterparts [1, 2]. However, a major process limitation for the integration of a diamond heat spreader has been the O₂-plasma damage in the gate opening associated with etching the diamond cap. To reduce plasma damage, Anderson et al. have demonstrated a two-step diamond etch process, where the plasma power is reduced towards the end of the etch to minimize damage to the AlGaN surface [3]. While this process has enabled improved electrical performance of NCD-capped HEMTs, optimal passive heat spreading requires NCD films thicker than 0.5 µm which challenges the two-step etch approach [4]. Further complications such as on-wafer variations in NCD thickness have motivated the search for a more robust gate process. Recently, we have proposed the use of a sacrificial gate (SG) prior to diamond growth [5, 6]. The SG stack consisted of a 10 nm thick ALD-grown Al₂O₃ layer followed by a 100 nm thick PECVD-grown SiN_x layer. The SiN_x layer served as an etch stop for the NCD layer, and the Al₂O₃ served as an etch stop for the SiN_x layer. The Al₂O₃/SiN_x stack was patterned with the dimensions of the gate and a blanket 500 nm thick CVD NCD film was conformally-grown over the feature. The gate feature was then realigned and a high power O₂-plasma was used to clear the NCD film in the gate region. This resulted in a much more reliable gate opening since the window for NCD over-etch was much larger. The sacrificial SiN_x finger was subsequently etched away using a SF₆-ICP process. The Al₂O₃ layer was removed in dilute HF solution with no measureable degradation to the rest of the device. Finally, the Ni/Au gate metal stack was lifted off. On HEMTs with a SG, the addition of a NCD cap did not cause any significant degradation in mobility, carrier density, or sheet resistance. By contrast, when a SG process was not used to protect the AlGaN surface, the NCD-capped HEMT experienced a decrease in mobility and an increase in sheet resistance. For the SG NCD-capped HEMT, the off-state current increased by less than an order of magnitude and threshold voltage shifted by about 1 V. However, on-state drain current levels remained comparable and degradation in dynamic on resistance (R_ON,DYN) upon an off-state drain bias stress was improved, indicating potential improvement in AlGaN surface passivation by the NCD film [2, 3]. Such improvement is also corroborated by the improvement in breakdown voltage in NCD-capped HEMTs with a conventional gate opening design. This presentation will review progress in simulation, process integration, channel temperature mapping, and electrical characterization of diamond-capped devices. We will also review several novel NCD concepts, such as in-plane control of NCD thermal conductivity, as well as a diamond-gated AlGaN/GaN HEMT [7, 8]. [1] M.J. Tadjer, et al., IEEE Electron Dev. Lett., vol. 33, no. 1, pp. 23-25, 2012. [2] D.J. Meyer, et al., IEEE Electr. Dev. Lett., vol. 35, no. 10, pp. 1013, 2014. [3] T.J. Anderson et al., 70th Dev. Research Conf. Proc., pp. 155-156, 2012. [4] A. Wang, M.J. Tadjer, and F. Calle, Semicond. Sci. Technol., vol. 28, pp. 055010 (2013). [5] T.J. Anderson et al., CS Mantech Conf. Digest, pp. 205-208, 2013. [6] M.J. Tadjer, et al., Phys. Status Solidi A, 2015, in press. [7] T.J. Anderson, et al., IEEE Electr. Dev. Lett., vol. 34, no. 11, pp. 1382, 2013. [8] J. Anaya et al., Acta Materialia 103 (2016) 141-152.