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Topside Nanocrystalline Diamond Integration on AlGaN/GaN HEMTs for High Temperature Operation
Keywords: GaN HEMT, nanocrystalline diamond, power switching
Enabling benefits of gallium nitride has trust GaN high electron mobility transistors (HEMTs) to the forefront of emerging power switching device technologies for next-generation high-efficiency power converters. As with all semiconductor devices, the performance and reliability of GaN HEMTs is dependent on temperature. Therefore, improving thermal management for high temperature operation, high thermal conductivity (> 4 W/cm⋅K) nanocrystalline diamond (NCD) films are integrated on the topside of AlGaN/GaN HEMTs as thermally stable, heat-spreading capping layers and gate electrodes. NCD films are formed over the entire sample by seeding and growth techniques reported in [1], [2], and [3], then patterned by an O2-based ICP etch, using Al as a hard mask. NCD-capped HEMTs were fabricated with a thin (10 nm) SiNx passivation layer between AlGaN surface and 500 nm NCD film. These devices exhibit a 20% decrease in peak channel temperature compared to reference HEMTs, measured by Raman thermography, as well as improved sheet carrier density, sheet resistance, Hall mobility, on-state resistance, transconductance, and breakdown voltage [4]. Thicker NCD films, as well as optimization of the interface for improved electrical passivation and reduced thermal resistance can further reduce the peak channel temperature. Process improvements for integrating NCD-capping with the HEMT process flow, are currently being pursued. Eliminating the SiNx passivation layer such that the NCD film is directly on the AlGaN barrier will place the heat-spreading layer closer to the heat source, at the drain edge of the gate. Also, the NCD film shows promise for high quality passivation of the access regions. A sacrificial gate process flow has been developed, showing promise to improved the device performance by eliminating the challenging process step of etching the NCD without damaging the AlGaN barrier layer. To place the heat-spreading layer in direct contact with the heat source, boron doped p+-NCD films were implemented as a gate electrode for the AlGaN/GaN HEMT [5]. It is essential that the doping initiates at the semiconductor/NCD interface for optimum gate control. This device demonstrates reduced ON-resistance, reduced gate leakage, and significantly increased ON-state current density compared with reference Ni/Au-gated devices from the same wafer. Also, the NCD gate electrode is beneficial for high temperature operation, since it replaces the thermally sensitive Schottky gate. Additional improvements in the NCD-gated HEMTs are in progress, such as incorporating a sacrificial layer, so that the NCD etch does not degrade the AlGaN barrier. Also, incorporating the NCD gate electrode with a NCD capping layer will provide full topside NCD heat spreading, to allow for optimum thermal management.
Andrew D. Koehler,
Naval Research Laboratory
Washington, DC

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