應用於高功率電子之高電子遷移率氮化鎵電晶體其設計、模擬與開發 之研究

Abstract

在過去幾年間，以氮化鎵(GaN)高電子遷移率電晶體 (High Electron Mobility Transistors，HEMTs)所開發之高功率開關元件技術，已經有許多重大的突破。然而，由於GaN HEMT具有高二維電子氣密度(two dimensional electron gas density，2DEG)之特性，以至於天生具有常開式(normally on)的特性，同時，此特性也限制了在高功率電路的應用。由於高效率之功率元件，必須伴隨常閉式(normally off)的操作，並具高崩潰電壓及低導通電阻之特性。本論文專注於設計不同的氮化鎵元件架構，以提升元件之臨界電壓 (Vth)、崩潰電壓(VB)以及汲極電流(drain current)。並以量子力學修正元件模擬(quantum mechanically corrected device simulation)進行此創新結構元件之電性分析。本研究可分為四部份，分別為氮化鋁(AlN)間隔層對AlGaN/GaN HEMT之影響研究、步級緩衝層在常閉式p-InAlN 閘極之AlGaN/GaN HEMT應用、SiO2電流阻絕層在垂直式AlGaN/GaN HEMT元件上之效應，以及運用有機金屬化學氣相沉積(MOCVD)技術成長InGaN以降低接觸電阻之研究。 首先，為開發出高汲極電流之氮化鎵元件，通常採用AlN插入AlGaN屏障層及GaN通道層之間。結果顯示，AlN有明顯增加片載子濃度(sheet carrier concentration)以及降低散射機制而提高二維電子氣之載子遷移率。由於散射機制與AlN間隔層之厚度息息相關，當採用厚度為0.5nm之AlN間隔層時，因有最小的合金錯排散射(alloy disorder scattering)，而得到最大之載子遷移率。這些結果提供了合金的作用對二維電子氣之影響，有更深入了解。第二部分，為開發高性能常閉式HEMT，採用新穎之p-InAlN閘極，並設計以Al0:25Ga0:75N/Al0:08Ga0:92N步階式緩衝層架構，進行理論計算探討。研究結果顯示，藉由增加InGaN/AlGaN p-n接面之空乏層寬度，以及於GaN/Al0:25Ga0:75N界面感應電洞濃度提升之協同效應，元件之閾值電壓明顯提升至2.5V。另外，步階式緩衝層在此結構中可提高通道之載子侷限效應，將崩潰電壓提高至1080 V。此元件結構與傳統之常閉式p-InAlN閘極之AlGaN/GaN HEMT比較，在最小化汲極電流條件下，臨界電壓及崩潰電壓分別可達 127% 及 36% 之增益。第三部分之研究為， 採用了二氧化矽(SiO2)層作為電流阻擋層(current blocking layer, CBL)，應用於垂直AlGaN/GaN HEMT元件，以克服傳統之垂直式元件於CBL易有縱向漏電之問題。我們發現由於SiO2具有寬能隙且高電阻率特性，即使是在高汲極偏壓下仍可有效抑制縱向漏電，進而提高崩潰電壓至1270 V。此數值較傳統縱向GaN HEMT提高了154%。此外，本論文中亦描述了創新的平行通道孔設計，以有效提高汲極電流。為達成最佳之汲極電流與崩潰電壓間之消長，以達最佳元件性能，CBL之厚度、長度及電流通道孔數量等，均作理論優化。其崩潰電壓及汲極電流縱向GaN HEMT分別可達1260V 及78 mA。 最後，則為以MOCVD於AlGaN/GaN HEMT源極及汲極區域成長InGaN，以降低歐姆界觸電阻之實作研究。由於高載子濃度對於達成低接觸電阻非常重要，因此藉由優化MOCVD成長條件，以得到高載子濃度之InGaN。其結果顯示，高In組成並未能確保InGaN有高載子濃度。因為碳的濃度及鎵的空缺，也會抑制InGaN之電子濃度。 另外，為了探討分析接觸電阻，採用以金屬於樣品上製作TLM (Transmission line method)。分析TLM結果顯示，載子濃度對於得到低接觸電阻而言最為重要。量測結果顯示，當InGaN之片載子濃度在4.5x1016 cm-2 時可得到最低接觸電阻0.13 Ω-mm。此結果將有助於提升GaN HEMT之性能。 結論，本論文已對不同新穎結構之氮化鎵HEMT元件，進行設計、模擬以及分析研究，以改善其臨界電壓、崩潰電壓以及汲極電流。InGaN磊晶部分，亦作探討。這些研究成果，對開發高性能之常閉式GaN HEMT功率元件產業言，提供了重要之參考依據。

During the past few years, tremendous success has been achieved in the development of high power switching devices by using Gallium nitride (GaN) based High Electron Mobility Transistors (HEMTs). However, inherent normally on operation of GaN HEMT due to high two-dimensional electron gas density limits its journey toward high power applications. Since efficient power device is associated with normally-off operation with high off-state breakdown voltage keeping on-state resistance as low as possible, this study is concentrated on designing different novel structures of GaN-based HEMTs in order to enhance threshold voltage (Vth), breakdown voltage (VB) and drain current. The elec- trical characteristics of the innovative structures are examined by solving experimentally calibrated quantum mechanically corrected device simulation. This work can be divided into four parts, namely Aluminum Nitride (AlN) spacer layer AlGaN/GaN HEMT, p- doped Indium Aluminum Nitride (p-InAlN) gate normally-off AlGaN/GaN HEMT with step buffer, Silicon dioxide (SiO2) current blocking layer AlGaN/GaN vertical HEMT and Indium Gallium Nitride (InGaN) grown by Metalorganic Chemical Vapor Deposition (MOCVD) for low ohmic contact resistance in AlGaN/GaN HEMT. At first, to achieve high drain current, AlN layer was inserted in between AlGaN barrier and GaN channel layer of AlGaN/GaN HEMT. Results demonstrated that AlN layer significantly improve sheet carrier concentration and two dimensional electron gas (2DEG) mobility by reducing scattering mechanism. Since scattering mechanism strongly depends on AlN spacer layer thickness, the minimum scattering and maximum mobility are observed when AlN spacer thickness is 0.5 nm. These results provided insight into the role of alloy scattering on 2DEG properties. Secondly, for the sake of achieving high performance normally-off HEMT, novel p-InAlN gate AlGaN/GaN HEMT with Al0.25Ga0.75N/Al0.08Ga0.92N step buffer was designed and computationally studied. Synergy effect of the large depletion width in InAlN/AlGaN p-n junction with induced hole concentration at GaN/Al0.25Ga0.75N interface beneath the gate gives signicantly large Vth (2.5 V) which is 127 % higher than conventional p-AlGaN normally-off AlGaN/GaN. Moreover, step buffer increases carrier connement in the channel and hence enhance the breakdown voltage over 1080 V. The proposed device recorded 36% higher VB than the conventional p-AlGaN normally-off AlGaN/GaN with minimum trade off of drain current. Thirdly, a new AlGaN/GaN vertical HEMT with SiO2 current blocking layer (CBL) was designed in order to suppress the excessive vertical leakage through CBL in conventional p-GaN CBL vertical HEMT. It was found that large barrier due to SiO2 effectively suppresses the vertical leakage even at high drain bias condition and enhances the breakdown voltage over 1270 V which is 154 % higher than that of conventional p-GaN CBL vertical HEMT. Besides, innovative parallel apertures were described in order to enhance drain current. In order to achieve best trade-off between drain current and breakdown voltage, CBL thickness, length and number of current conducting aperture were theoretically optimized. The optimized device offers 152 % higher BV with significantly large drain current compare to conventional p-GaN CBL vertical HEMT. Finally, InGaN samples were grown by MOCVD for low resistance in ohmic contact region of AlGaN/GaN HEMT. Since high carrier concentration is indispensable for low contact resistance, growth conditions for InGaN with high carrier concentration were optimized. Results disclose that higher indium composition InGaN sample does not always have high electron concentration. In the samples grown at low pressure, low temperature, low ammonia low and high gas phase ratio, carrier concentrations are limited by carbon related defects and gallium vacancies which are the major trapping sites for electrons. Additionally, study of metal contacts deposited on the grown samples by transmission line method (TLM) shows that carrier concentration is the most important factor to obtain low contact resistance. The mea- sured contact resistance for the sample with 4.51016 cm􀀀2 carrier concentration is 0.13 Ω mm which proves that n-InGaN can be the best material for source and drain contact region of the AlGaN/GaN HEMT in order to improve device performance. In summary, this dissertation has investigated the device design, simulation and analysis of different novel GaN HEMT structure along with development of epitaxial region to improve threshold voltage, breakdown voltage and drain current. The results can be a useful reference for the semiconductor industry to develop the high performance GaN based normally-off high power device.