| 摘要: | 本論文主旨在於發展InGaAsSb材料之磊晶成長技術及研究其材料特性，同時將其應用於InP系列電晶體元件之基極，藉由改變磊晶成長條件以及設計不同電晶體元件之結構用以研究並探討其元件特性與材料之關係。相較於傳統單異質接面電晶體結構而言 (InP/InGaAs SHBT)，使用InGaAsSb材料取代原本InGaAs材料作為基極結構，為一種嶄新的電晶體結構設計。由於在異質接面電晶體之結構中，基極材料為主導整體元件特性之重要關鍵層，選擇適當的基極材料形成良好之能帶結構，將大為提升元件之直流亦或是高頻效能。具備InGaAsSb材料為基極之InP/InGaAsSb/InGaAs電晶體，將縮減InP/InGaASb射極/基極介面能帶之導電帶不連續處，同時提高基極之導電與價電帶之位準，進而達到低導通電壓與高電流增益之良好特性。此外，於InGaAsSb/InGaAs基極/集極導電帶處將形成type–II之能帶型態，當電晶體元件操作在高電流密度時，擁有InGaAsSb基極之電晶體結構設計將同時具備延遲Kirk效應與抑制電流阻擋效應之能力，均優於其他使用傳統同質結構或是type–I基極/集極之電晶體結構。 在本實驗中，我們研究並且觀察其InGaAsSb材料中銦與銻之含量、三族與五族元素之比例 (V/III ratio)、與成長溫度對於其材料摻雜濃度與移導率之影響，以及材料成長於元件後，元件特性上的表現。研究發現，在基極中銻的含量對於電晶體元件之導通電壓、接面理想因子、以及直流特性均有顯著之影響。 我們製作其結構為InP/InGaAsSb/InGaAs之雙異質接面電晶體，成功達到0.32 V (VBE-on) 之低導通電壓。 此外，藉由小訊號等效電路模型之分析可萃取出元件內部之參數，亦證實由於InGaAsSb基極之電晶體在基/集極導電帶結構為type–II之能帶，因此造成集極處擁有較高之電子平均速率，進而達到較高之截止頻率。 電流增益/片電阻 (beta/RSH) 為一種驗證電晶體元件特性之比率數值，使用InAlAs作為電晶體元件之射極，將與InGaAsSb基極形成較高之導電帶不連續面，且仍能保持type–I能帶型態，進而維持高電流增益。相較於InP/InGaAsSb射/基極結構而言，作者設計InAlAs/InGaAsSb為其射/基極之電晶體結構，使得射/基極介面處具有較高之導電帶不連續面，提供較高之電子之注入能量因而縮短載子於基極傳輸時間。 為了同時達到高電流增益與較低之片電阻，我們對於InGaAsSb基極材料之磊晶參數也做了調整與優化，在調整磊晶參數後的InAlAs/InGaAsSb/InGaAs電晶體元件顯示，電流增益為52且片電阻778 Ohm/sq 之數值，為結構與材料同時優化之結果。 總體而言，使用InGaAsSb基極之電晶體可同時達到低導通電壓、高電流增益/片電阻之比率、高電流密度、以及高截止頻率等等好處，將可應用於低功率損耗之超高速電路中。 This dissertation describes the material growth and characterization as well as the device design, fabrication and characterization of InP-based heterojunction bipolar transistors (HBTs) with an InGaAsSb base layer. The use of InGaAsSb as a base layer in HBTs represents an entirely new technological approach as compared to the use of the conventional InP/InGaAs SHBT. Lower turn-on voltage and higher current gain, beta can be achieved with the InGaAsSb–base HBTs due to the reduction of conduction band offset and the increase of valance band offset at the InP/InGaAsSb emitter/base (E/B) junction. Moreover, the InGaAsSb/InGaAs base/collector (B/C) junction constitutes a type-II conduction band lineup. As a result of the postponed Kirk effect and the minimized current blocking effect, the InGaAsSb-base HBT is also superior to the homojunction base/collector or type-I double heterojuction bipolar transistors (DHBTs) when operated at high current densities. In this study, we have investigated and examined the effects of In and Sb composition, V/III ratio, and growth temperature on the hole mobility and concentration in InGaAsSb as well as device performance. It is found that the Sb composition in the InGaAsSb base layer has a significant impact on the VBE turn–on voltage, the junction ideality and the dc performance of the DHBTs. A record low VBE turn-on voltage of 0.32 V at 1 A/cm2 is demonstrated in this work. Analysis based on a small signal equivalent circuit model shows that the InGaAsSb-base DHBT has a higher average velocity in the collector region and a higher cut-off frequency, resulting from the type-II InGaAsSb/InGaAs B/C junction. Current gain over sheet resistance, (beta/Rsh), is a figure of merit to examine the dc performance of an HBT. For a higher current gain,beta, InGaAsSb-base DHBTs with an InAlAs emitter layer are grown and characterized. The InAlAs/InGaAsSb E/B junction has a higher conduction band offset and provides higher initial injection energy for the electrons than the InP/InGaAsSb E/B junction, thus making it possible to reduce the base transient time of the DHBTs. Besides, the growth conditions of the InGaAsSb base layer are modified in order to achieve a high current gain and a low sheet resistance simultaneously. A current gain of 52 and a RSH of 778 Ohm/sq are achieved in the InAlAs/InGaAsSb/InGaAs DHBT with a 44 nm-thick In0.24Ga0.76As0.61Sb0.39 base. Overall, we demonstrate that DHBTs with an InGaAsSb base, which exhibit low VBE turn–on voltage, high beta/Rsh, high current density operation and high cut-off frequency, are highly promising for low-power consumption THz circuits. |
