GaN基高电子迁移率场效应管的可靠性研究

摘 要：虽然GaN基高电子迁移率场效应管（HEMT）在高频大功率器件方面具有突出的优势并已经在应用领域取得了重要的进展，但由于GaN基HEMT器件的材料缺陷密度高、高电场工作环境、GaN基异质结特有的强极化效应以及工艺复杂等问题，使得GaN基HEMT的可靠性问题十分突出。这些问题导致现有GaN基HEMT器件的实际表现一直与其理论值有一定差距。因此，要想实现GaN基HEMT器件全面、广泛的商业应用，必须对其可靠性做深入的研究分析。在该文中，我们利用不同的测试方法，从不同的角度对AlGaN/GaN HEMT的可靠性问题进行了研究。首先，我们观察并讨论了AlGaN/GaN HEMT器件输运过程中由载流子的俘获导致的Kink效应。实验表明：载流子俘获过程是由高场诱导的栅极电子注入到栅极下AlGaN势垒层中深能级的过程；载流子的去俘获过程则是由沟道中的热电子碰撞离化被俘获的电子引起的。 其次，我们对GaN基HEMT器件进行了阶梯电压应力可靠性测试，并观察到了一个临界应力电压，超过这个临界电压后AlGaN/GaN HEMT可以从之前的低电压应力引起的退化中逐渐恢复。研究发现：低电压应力引起的器件的退化是由电子在表面态或者体缺陷中被俘获引起的；器件性能的恢复则是由于电场诱导的载流子去俘获机制。 最后，我们利用不同的测试方法详细分析了关态下AlGaN/GaN HEMT的击穿特性和电流传输机制。我们在三端测试中观察到了与缓冲层相关的AlGaN/GaN HEMT的过早硬击穿现象；然后我们利用漏极注入测试方法验证了源-漏间缓冲层漏电的存在。分析结果表明缓冲层漏电是由电场引起的势垒下降、能带弯曲和缓冲层缺陷共同决定的。在电场足够高的时候，缓冲层漏电可以引起AlGaN/GaN HEMT过早发生硬击穿。

关键词：氮化镓 AlGaN/GaN 高电子迁移率场效应管 可靠性 载流子俘获/去俘获 击穿机制

Abstract：GaN-based high-electron-mobility transistors （HEMTs） are the most promising candidate for high-frequency and high-power microwave applications due to the intrinsic material advantanges of GaN-based semiconductors. However， imperfect material quality， strong polarization effect， high field operation enviroment and variation of processing technologies make the reliability problem of GaN-based HEMTs very complicated. As a result， GaN-based HEMTs have actual performance not up to their ideal level. These reliability challenges will hinder the extensive commercial applications of GaN-based HEMTs. In this work， we studied the reliability behaviors of AlGaN/GaN HEMTs from different perspectives. Firstly， we studied a carrier-trapping related anomalous kink effect in AlGaN/GaN HEMTs. The kink is found largely caused by trapping electrons from the gate leakage current by deep levels within the AlGaN barrier at high drain bias， while the carrier trapping process mainly occuring within the AlGaN barrier under the gate has to be induced by a high drain bias leakage current. The carrier detrapping process is caused by hot electrons injected into the AlGaN barrier which “knock off” the trapped electrons. Secondly， we observed a critical stress voltage in the step stress tests， beyond which the AlGaN/GaN HEMTs start to recover from drain current slump induced by the lower voltage stress. The recovery process is explained by a high field induced carrier detrapping mechanism， while the device degradation in low-to-medium stress voltage range is caused by carrier trapping in surface or bulk states. Thirdly， we comprehensively studied off-state breakdown and current transport process of GaN-based HEMTs. Premature hard-breakdown can be observed in three-terminal tests， illustrating that buffer layer plays an important role in off-state breakdown. Measurement using drain current injection technique proves the existence of drain-to-source buffer leakage. Based on detailed tests at different current injection levels and different temperatures， we find that the buffer leakage is determined by a combined effect of electric field induced energy barrier lowering， band bending and inherent defects in the buffer layer. This buffer leakage current can induce premature hard-breakdown when stress electric field is high enough.

Key Words：GaN；AlGaN/GaN HEMT；Reliability；Carrier-trapping/detrapping；Breakdown mechanism

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