Abstract
We have previously demonstrated AlGaN/GaN high-electron-mobility transistors (HEMTs) that used a coating of high thermal conductivity nanocrystalline diamond (NCD) to reduce channel temperature for a given dissipated power and simultaneously improve electrical performance compared to devices with conventional silicon nitride (SiN) passivation only. Here we examine the effect of a protective SiN interlayer of varying thickness inserted between the semiconductor surface and NCD coating on large-signal output power density as well as the reduction in channel temperature. For an InAlN/AlN/GaN HEMT with a >1 µm NCD coating and SiN interlayer ranging from 5 to 50 nm in thickness, the output power density and power-added efficiency at 4 and 10 GHz are found to be maximized for the thickest SiN interlayer. At the same time, we find the reduction in channel temperature provided by the NCD coating is not strongly dependent on thickness, which is confirmed by numerical simulation.
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