GaN基HBT射频性能提升的研究

GaN-based radio-frequency (RF) transistors combine the advantages of high-power handling capability, high-voltage and high-temperature operation stability. Therefore, they have been actively researched and developed for high-power switches and RF amplifiers which enable higher output power density and higher power efficiency, with compact system size, weight, and reduced thermal dissipation requirement. However, as one of the most important RF transistors, the development progress on the npn GaN-based heterojunction bipolar transistors (HBTs) has been slow. A number of obstacles inhibit their RF performance. For npn HBTs, the major issues are the high recombination current in the base region, the low-conductivity base layer caused by the difficulty of achieving high free-hole concentration, and the plasma-induced dry-etching damage on the extrinsic base region. The objective of this proposal is to develop high-power RF GaN/InGaN/GaN npn Double-HBT (DHBT). The DHBT epitaxial structures will be grown on semi-insulating (SI) 6H-SiC substrates with lower lattice/thermal mismatch and higher thermal conductivity than commonly used sapphire substrates. The advantages of SiC substrates will reduce the base recombination current and consequently increase the current gain at the high current density condition. Also, the epitaxial recipe will be carefully optimized to suppress the V-defect density especially on Indium-rich p-type InGaN base layers. With high indium concentration and low defect density, the p-type InGaN base is able to provide higher free-hole concentration as well as faster hole mobility, which are both critical for HBT's high-power RF performance. To avoid the dry-etching damage to the extrinsic base and BE junction, the GaN emitter will be selectively re-grown on top of the InGaN base. Moreover, based on the mushroom-shape of the re-growth emitter mesa, a self-aligned process will be developped to minimize device sizes and parasitic resistance and capacitance. Finally, small-area high-power-density GaN/InGaN/GaN DHBTs will be fully fabricated and their impressive RF power performance will be demonstrated.

GaN基射频电子器件有输出功率大、耐高压高温等性能，可提升通讯系统的输出功率和效率，并降低其体积、重量和散热要求。npn型GaN基HBT是一种重要射频器件，但受困于基区复合电流大、空穴浓度低、器件制备工艺不成熟等问题，射频性能尚远低于预期。本项目以实现高性能npn型GaN/InGaN/GaN 射频功率DHBT为目标，拟采用半绝缘6H-SiC衬底代替蓝宝石衬底外延GaN材料，利用SiC衬底低晶格失配度、低热失配度、高热导率的优势，降低基区复合电流，提高大电流下的电流增益。研究外延材料中应力缓解及位错控制的方法，降低高In组分p型InGaN基区中V坑密度，提高空穴浓度和迁移率，进而提升HBT的射频功率输出性能。为了解决干法刻蚀对基区的损伤，研究GaN发射区在InGaN基区上的选区二次外延技术；并结合二次外延材料的特点，开发器件制备的自对准工艺，最终研制出小尺寸高功率密度的GaN基射频HBT。

本项目组成员按照项目任务书中所述研究内容、研究方案及年度研究计划开展了相关研究，顺利完成了 GaN基HBT射频性能提升的研究，并按照计划完成了相关研究目标。本项目组依次研究了：p型基区材料外延及高效掺杂技术研究，具体包括p型GaN以及p型InGaN基区的外延及掺杂研究；开发了小尺寸选区外延技术，深入研究了小尺寸选区外延的生长动力学，系统研究了选区n-GaN及n-AlGaN发射区的外延生长工艺；研究了AlGaN/GaN HBT器件制备工艺，重点开发了基区欧姆接触、钝化层工艺等关键技术；深入研究了器件复合电流较高的物理问题。获得了高空穴浓度的p型GaN及p型InGaN基区材料，空穴浓度最高达2.4×10^18/cm^3，迁移率达到16 cm^2/V.s。实现了均匀的、表面光滑无V坑的选区n型GaN和AlGaN发射区，在国内首次实现了电流密度超过4kA/cm^2、功率密度达到60kW/cm^2、击穿电压145V的HBT器件。在本项目支持下，共发表标注基金课题号的相关文章13篇，并参加国内国际会议10次，分别作会议展报、口头报告以及邀请报告。在本项目支持下共申请专利15件，其中授权专利5项。