| 摘要: | 本研究包括製作具有不同AlxGa1-xN/AlyGa1-yN超晶格緩衝層之氮化鎵高電子遷移率電晶體(High Electron Mobility Transistor, HEMT)，並探討不同超晶格緩衝層對成長於矽基板之GaN元件動態特性之影響。
第一部份之研究標的為成長於矽基板之AlInN/GaN HEMTs，緩衝層的動態響應很大程度上取決於應力緩衝層中超晶格的成分、厚度和位置，應力釋放可使緩衝層之缺陷密度下降。研究顯示具超晶格緩衝層之元件對垂直崩潰電壓方面有顯著的改善，也減緩電子陷捕效應。在背閘極負偏壓量測垂直遲滯曲線中，缺陷密度低的元件有最低的緩衝層遲滯曲線，表示電子陷捕最少。脈衝量測指出不同靜態偏壓點下(Vgsq, Vdsq) = (-1, 5)、(-1, 10)、(-4, 0)，缺陷密度與電流崩塌效應相關，缺陷密度低的元件其電流回復狀況越佳。當缺陷密度從10.55×10^8 cm-2降至5.4×10^8 cm-2，垂直崩潰電壓可從360 V提高至446 V。
第二部份之研究標的為成長於矽基板之AlGaN/GaN HEMTs，分別將不同對數之Al0.8Ga0.2N/Al0.2Ga0.8N超晶格置於緩衝層中，超晶格對數之組合由50對增至80對，緩衝層缺陷密度從8.8×10^8 cm-2降至8.01×10^8 cm-2，垂直崩潰電壓可達544 V。於此部分缺陷密度最低的元件在動態特性方面，由脈衝量測檢視電流崩塌對元件電性之影響，隨著對數的增加，顯示缺陷密度低的元件其電流回復狀況越好。
最後利用電流暫態頻譜計算出電子脫阱的時間常數並萃取活化能。以兩種條件的關閉狀態(off-state)應力偏壓後，50對的超晶格元件其活化能分別為0.37 eV與0.30 eV；60對的元件其活化能分別為0.50 eV與0.32 eV；80對的元件其活化能分別為0.35 eV與0.23 eV。推估缺陷位置可能位於GaN buffer layer。這項對Ⅲ族氮化物HEMT-on-Si的緩衝層動態研究，有望幫助提升GaN HEMTs高功率元件的可靠度。
;This study aims to investigate the effects of different AlxGa1-xN/AlyGa1-yN superlattice(SL) buffer layers on the dynamic characteristics of AlInN/GaN and AlGaN/GaN high electron mobility transistors (HEMTs) grown on low-resistivity silicon substrates.
The first part of the study focuses on the dynamic buffer response of AlInN/GaN HEMTs. The dynamic response of the buffer layer largely depends on the composition, thickness and position of the superlattice in the stress-mitigating buffer. It is observed that devices with superlattice buffer exhibit significant improvements in vertical breakdown voltage, and charge trapping effect. Negative back-gating measurements show the lowest buffer current hysteresis in the sample with the lowest dislocation density, indicating least charge trapping in the sample. Additionally, pulsed current-voltage measurements with various quiescent biases such as (Vgsq, Vdsq) = (-1, 5), (-1, 10) and (-4, 0) indicate a clear correlation between dislocation density and current collapse in the devices under study, where lower density of dislocation in the GaN buffer results in better recovery of the drain current. The vertical breakdown voltage increases from 360 V to 446 V as the dislocation density decreases from 10.55×10^8 cm-2 to 5.4×10^8 cm-2, respectively.
The second part of the study focuses on AlGaN/GaN HEMTs grown with different pairs of Al0.8Ga0.2N/Al0.2Ga0.8N superlattice layers. As the superlattice pair increases from 50 to 80 pairs, the dislocation density decreases from 8.8×10^8 cm-2 to 8.01×10^8 cm-2, resulting in an increase of vertical breakdown voltage up to 544 V. Moreover, pulsed I-V measurements show better current recovery as the superlattice increases from 50 to 80 pairs.
Finally, current transient spectroscopy method is used to determine the time constant of electron detrapping and its activation energy. The activation energies, 0.37 eV/0.30 eV, of the device with 50 pairs of SL, 0.50 eV/0.32 eV, of the device with 60 pairs of SL, and 0.35 eV/0.23 eV, of the device with 80 pairs of SL, are extracted from the current recovery curves after two different off-state stress conditions. A detailed analysis of the current transient spectroscopy indicates the trap locations in the GaN buffer layer. This systematic and detailed study of the dynamic buffer response of GaN HEMTs-on-Si is beneficial to the development of high reliability GaN HEMTs for high power device applications. |
