GaN基高電子遷移率場效應(yīng)管的可靠性研究

陸海　付立華　劉新宇　王燕

摘 要：雖然GaN基高電子遷移率場效應(yīng)管（HEMT）在高頻大功率器件方面具有突出的優(yōu)勢并已經(jīng)在應(yīng)用領(lǐng)域取得了重要的進(jìn)展，但由于GaN基HEMT器件的材料缺陷密度高、高電場工作環(huán)境、GaN基異質(zhì)結(jié)特有的強(qiáng)極化效應(yīng)以及工藝復(fù)雜等問題，使得GaN基HEMT的可靠性問題十分突出。這些問題導(dǎo)致現(xiàn)有GaN基HEMT器件的實際表現(xiàn)一直與其理論值有一定差距。因此，要想實現(xiàn)GaN基HEMT器件全面、廣泛的商業(yè)應(yīng)用，必須對其可靠性做深入的研究分析。在該文中，我們利用不同的測試方法，從不同的角度對AlGaN/GaN HEMT的可靠性問題進(jìn)行了研究。首先，我們觀察并討論了AlGaN/GaN HEMT器件輸運(yùn)過程中由載流子的俘獲導(dǎo)致的Kink效應(yīng)。實驗表明：載流子俘獲過程是由高場誘導(dǎo)的柵極電子注入到柵極下AlGaN勢壘層中深能級的過程；載流子的去俘獲過程則是由溝道中的熱電子碰撞離化被俘獲的電子引起的。 其次，我們對GaN基HEMT器件進(jìn)行了階梯電壓應(yīng)力可靠性測試，并觀察到了一個臨界應(yīng)力電壓，超過這個臨界電壓后AlGaN/GaN HEMT可以從之前的低電壓應(yīng)力引起的退化中逐漸恢復(fù)。研究發(fā)現(xiàn)：低電壓應(yīng)力引起的器件的退化是由電子在表面態(tài)或者體缺陷中被俘獲引起的；器件性能的恢復(fù)則是由于電場誘導(dǎo)的載流子去俘獲機(jī)制。 最后，我們利用不同的測試方法詳細(xì)分析了關(guān)態(tài)下AlGaN/GaN HEMT的擊穿特性和電流傳輸機(jī)制。我們在三端測試中觀察到了與緩沖層相關(guān)的AlGaN/GaN HEMT的過早硬擊穿現(xiàn)象；然后我們利用漏極注入測試方法驗證了源-漏間緩沖層漏電的存在。分析結(jié)果表明緩沖層漏電是由電場引起的勢壘下降、能帶彎曲和緩沖層缺陷共同決定的。在電場足夠高的時候，緩沖層漏電可以引起AlGaN/GaN HEMT過早發(fā)生硬擊穿。

關(guān)鍵詞：氮化鎵 AlGaN/GaN 高電子遷移率場效應(yīng)管 可靠性 載流子俘獲/去俘獲 擊穿機(jī)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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