摘要:本計畫的主要目的在研製由分子束磊晶成長(MBE)或金屬有機化學汽相沉積(MOCVD)成長之發光電晶體(Light-emitting transistor, LET)和電晶體雷射(Transistor laser),一種全新的半導體光電元件,同時是一異質接面雙極性電晶體(HBT),又具有載子復合放光的特性,能夠同時輸出「電」和「光」的調變訊號(modulated signal)。我們將著重在磷化銦鎵/砷化鎵(InGaP/GaAs)材料系統的發光電晶體之研究和製造,透過改變電晶體基極中的量子井結構,來提高元件的發光和電流增益(current gain)的直流特性,進而改善發光電晶體的高頻調變速率,同時開發在光電積體電路(OptoElectronic Integrated Circuit)的應用潛力。發光電晶體中最重要的部分在於電晶體的基極(Base),傳統上載子由電晶體的射極(Emitter)進入到基極,形成「少數載子」(Minority Carrier)分佈,這些少數載子有機會在基極被復合,形成基極電流(IB),剩餘沒有被復合的載子,在極短的傳輸距離(時間)內(基極的厚度通常在1000Å左右)透過擴散的方式跑到集極(Collector),被基極-集極的逆向偏壓所影響,形成集極電流(IC)輸出。發光電晶體的原理,就是透過在基極至入未參雜的量子井,來增強載子在基極的「放光性」復合效率,使得電晶體形成一個同時具有電訊號輸入(IB),電訊號輸出(IC),光訊號輸出(Light)的三端光電元件。我們希望藉由改變不同量子井的設計(能階差異,數量,寬度,在基極的位置),來改變載子在基極內的復合放光效率,藉此「控制」載子的復合生命週期(carrier recombination lifetime)。在過去五十年,所有的相關研究都將載子的生命週期認定在1 ~ 2 ns,而發光二極體(LED)的自發性放光(spontaneous emission)的調變速度始終在幾百個MHz附近,所有的光通訊光源都只能透過二極體雷射(diode laser),然而雷射的調變響應,也因為載子的復合生命週期太長,造成元件有很大的共振頻率響應(resonance response),造成光訊號的失真和傳輸頻寬的下降。因此,發光電晶體本身因為結構的不同,吾人第一次發現載子生命週期可輕易達到小於100 ps,且達到世界紀錄的7 GHz自發性放光的調變頻寬。我們希望藉由此研究計畫,進一步觀察到更多的基礎物理現象,以及突破目前的LED調變世界記錄,達到10 GHz的頻寬,開發具有潛力的光積體電路元件,和光通訊系統中所需要的更快速、穩定、便宜的光源。
Abstract: The proposed research topic is to develop and characterize a novel semiconductor optoelectronic device grown by MBE and MOCVD, the heterojunction bipolar light-emitting transistor (HBLET), which is a three-port device with simultaneous electrical and optical output. The HBLET, inherited from the heterojunction bipolar transistor (HBT), not only functions as a transistor with a family curve (I-V), Gummel-plot, and electrical current gain (=IC/IB), but also emits significant optical output controlled by the base recombination current IB to form a three-port optoelectronic device. To control the base recombination (i.e. light output) is therefore the most important part to study the LET performance. Considered in a conventional n-p-n transistor, carriers (here are electrons) injected from emitter side (IE) will transport through base region and be collected by collector and from the collector current (IC) due to the reverse bias between base-collector junction. Part of the carriers (electrons) within the base transit time (in ps range) will recombine with holes in the base region and form the base current (IB), but most of the carriers will transport to collector side (IC) resulting in a current gain much greater than one (=IC/IB ~100). Here the essential concept of a light-emitting transistor is to enhance base recombination “radiatively” so that we can utilize the optical signal as an additional channel for communication and interconnect. We employ an undoped quantum-well (QW) inside the heavily-doped base region since a QW is considered an effective recombination center. In this way, we will have two collectors in our LET system: one is the “electrical” collector providing electrical output (IC), the other one is the “optical” collector providing optical output (L). The “optical” collector only collects “fast-recombining” carriers because the base transit time is only in ps range, leading to a fast recombination lifetime, which is considered about ~ 1 ns for the past 50 years in the light-emitting diodes and diode lasers. In this proposal different QW structures including energy bandgap, size, number, position in the base region will be designed and grown by MBE or MOCVD in the InGaP/GaAs system to study the effect of QW on the LET dc characteristics, such as the complementary relationship between the optical and electrical output, the energy conservation of optical and electrical gain, and the L-IB optical efficiency. In addition to that, rf characteristics is another interesting figure-of-merit we would like to investigate, especially comparing to light-emitting diode. With different QW designs, we can “tailor” the effective recombination lifetime, B, to sub-100 ps readily. LET has been demonstrated a world-record spontaneous modulation bandwidth of 7 GHz with a corresponding recombination lifetime of 23 ps. We believe that the device modulation speed is still parasitic-limited and can be much improved once we have a fully understanding of QW effects on the LET in the nearly future. We hope through this proposed investigation, we can break the world record into “double-digit,” say 10 GHz spontaneous modulation bandwidth, and observe some new physics phenomenon due to fast carrier recombination lifetime in our transistor-based light-emitting system.