- 學位論文
應變矽技術與矽鍺光電元件
Strained-Si Technology and SiGe Optoelectronics Devices
摘要
本論文中，主要專注於矽鍺異質接面雙載子電晶體在光電元件與電路的應用，以及利用外加機械力的方式引入應變矽技術，並針對使用金氧半場效電晶體之數位與類比電路及雙載子電晶體元件特性受外加機械力的影響，作一全面且深入之探討。 首先，光偵測器是光通訊電路應用中非常重要的單元。因此，為了要描述矽鍺異質接面光電晶體作為光偵測器的元件特性，必需要使用準確的雙載子電晶體元件模型。Gummel-Pool模型是最常用的元件模型，但是由於其過於簡化的模型，使得它在元件高電流操作區，及元件頻率響應之預測都表現的相當不理想。因此針對VBIC、Hicum、和Mextram這三個模型來比較，希望能找出最適合的雙載子電晶體模型。我們最後選擇了Mextram模型，因為重要的元件物理現象都有被這個模型包含，並且對元件高電流操作和元件的頻率響應都有相當好的預測能力。Mextram模型的參數也是這三個模型當中最少的，所以本論文接下來針對矽鍺異質接面光電晶體所作的元件模型化都是使用Mextram模型為基礎。 矽鍺異質接面光電晶體可以被整合於矽鍺異質接面雙載子電晶體製程當中，在電路模擬之中也可使用我們所提出的改良型Mextram模型。利用累增崩潰效應可以增加元件的光電轉換能力，在相同光強度照射下也有”爾利電壓縮小”的效應，nkT復合電流和基板效應也可以增加矽鍺異質接面光電晶體的速度。這些效應都可被包含於我們所提出的改良型Mextram模型，最後由實驗結果與模擬對照，可以看出這個改良型Mextram模型確實對矽鍺異質接面光電晶體受光照射時，不管是直流電流或是元件的頻率響應，都有相當好的元件特性描述能力。 為了要了解矽鍺異質接面光電晶體確實可以作為光偵測器，並整合於光通訊電路之中，我們利用矽鍺製程先開發出具有10GHz頻寬的轉阻放大器。經實驗量測，本電路有~500Ω的轉阻增益，並達到10GHz的頻寬。以頻寬10GHz的2^15-1不歸零偽隨機二進制序列資料輸入之眼圖亦得到很好的量測結果，表示所設計的轉阻放大器確實有達到原先設計的10GHz頻寬需求，並可被應用於OC192光通訊電路之中。在完成矽鍺轉阻放大器後，嘗試以矽鍺異質接面光電晶體來和轉阻放大器作整合。電路製造完成後所量測到的850奈米光脈衝響應的頻寬是~0.5GHz，主要受限於所使用的矽鍺製程並沒有可以用來增強光吸收之多重量子井結構。 另一方面，我們也針對外加機械力在應變矽技術上的應用作一深入之探討。適當的外加機械力可以增加元件與電路的效能。外加機械力的好處在於其可以外加任何方向、單軸、雙軸、張力、壓縮力的各種應變力，而且可以在元件製作完成後再施加。對於250奈米製程，經實驗量測，以平行於n型金氧半場效電晶體之單軸張力，對n型與p型金氧半場效電晶體通道互相垂直之環型振盪器可有7.4%的速度增加。傳統通道互相平行的環型振盪器只有不到1.5%的速度增加。對於180奈米製程，以外加雙軸張力或平行於n型金氧半場效電晶體之單軸張力，對轉阻放大器可有~5%的頻寬增加。這是由於轉阻放大器中，主動電感的高峰頻率被外加機械力所調整而造成頻寬的增加。是故，在將來數位或類比電路設計中，外加機械力可以是一新的設計參數。 最後，針對矽鍺異質接面雙載子電晶體和傳統矽雙載子電晶體的元件特性受外加機械力改變的效應作一徹底的探討與討論。矽鍺異質接面雙載子電晶體的電流增益β在0.028%的外加雙軸壓縮力和張力機械力分別有4.2%和–7.8%的改變。而傳統矽雙載子電晶體的電流增益β在0.028%的外加雙軸壓縮力和張力機械力則分別有4.9%和–5.0%的改變。元件操作電流的改變是由於外加機械力改變了元件的遷移率和本質載子濃度。另外，對於矽鍺異質接面雙載子電晶體而言，元件本身就有因矽和鍺晶格常數不匹配所造成的壓縮力應變力，因此外加機械力是和原本有的這個壓縮力應變力互相競爭，進而影響元件的電流增益β。
並列摘要
In this dissertation, the SiGe optoelectronics device and circuit applications with SiGe heterojunction phototransistor (HPT) are studied. The digital and analog circuit with MOSFET and the characteristics of bipolar device under external mechanical strain are also comprehensively investigated. First, the integration of the photodetector is essential for optical communication chips. However, to utilize the SiGe HPT as a photodetector, the accurate device model should be chosen. The Gummel-Pool model is not capable to model the SiGe HPT due to the simplicity of the model and inaccuracy on high current operation and poor frequency response predicability. Three candidates for the compact bipolar device model such as VBIC, Hicum, and Mextram, are studied and compared. The Mextram model has the accuracy on high current operation and frequency response characteristics with less model parameters than other two models. Therefore, we choose Mextram model in our further device modeling on SiGe HPT under optical illumination. The SiGe HPT is integrable with SiGe heterojunction bipolar transistor (HBT) process and can be modeled by a modified Mextram model for the circuit simulation. The impact ionization to obtain an extra gain for the optoelectronic conversion and the “Early voltage reduction” under constant illumination are well-modeled in a modified model. The nkT recombination current and the substrate contact to enhance the HPT speed are incorporated in ac model. It shows a good agreement between measurement and simulation. The transimpedance amplifier (TIA) circuit with 10GHz bandwidth is designed and fabricated. The ~500Ω transimpedance gain and the ~10GHz bandwidth are achieved. The eye-diagram with 10GHz 215-1 non-return-to-zero (NRZ) pseudo random binary sequence (PRBS) data input shows a good eye opening result, indicating an appropriate application in OC192 standard for 10GHz optical communication system. Furthermore, the integrated SiGe HPT with TIA circuit is fabricated. The impulse response shows a ~0.5GHz bandwidth under 850 nm optical illumination. The slow speed of the integrated SiGe HPT with TIA circuit is limited by the HPT without additional multiple quantum well absorption. Then, the strained-Si technology is reviewed and studied. The appropriate external stress can enhance device and circuit performance. The external mechanical strain has the flexibility to provide strain on various directions and can be applied after device fabrication. The 7.4% speed enhancement is achieved for the 250 nm node ring oscillator under uniaxial tensile strain for mutually perpendicular layout of the NFET and the PFET. The speed enhancement is less than 1.5% for the conventional parallel layout of the NFET and the PFET. A 180 nm node transimpedance amplifier has ~5% bandwidth enhancement using biaxial tensile strain or uniaxial tensile strain parallel to NFET channel to tune the peaking frequency of active inductor in the circuit. The package strain can provide an extra useful parameter for the future digital and analog circuit design. Finally, the current gain ( β = IC/IB ) variations of the mechanically strained SiGe HBT and Si bipolar junction transistor (BJT) devices are investigated experimentally and theoretically. The β change of HBT is found to be 4.2% and –7.8% under the biaxial compressive and tensile mechanical strain of 0.028%, respectively. For comparison, there are 4.9% and –5.0% β variations for BJT under the biaxial compressive and tensile mechanical strain of 0.028%, respectively. The current change due to externally mechanical stress is the combinational effects of the dependence of the mobility and the intrinsic carrier concentration on strain. In SiGe HBT, the mechanical stress is also competing with the compressive strain of SiGe base, inherited from the lattice misfit between SiGe and Si.