有機與氧化物半導體之垂直載子遷移率及高效能金屬基極電晶體---總計畫及子計畫一---有機半導體之垂直載子遷移率及其餘金屬基極電晶體之應用(I)

Abstract

電晶體是半導體電子元件中不可或缺的一個重要組件。以矽製成之場效應電晶體 已經是過去數十年中改變人類日常生活型態並且進展快速的半導體工業中的基礎元 件。以矽製成之電晶體可以非常快的處理資料，但是此元件因為價格昂貴且需要高製 程溫度，因而不適合做為大面積中等操作速度的應用，包含了超薄軟性顯示器、人造 皮膚、射頻標籤。在過去二十年中，已經有非常大量的研究工作入在使用有機半導體 應用在上述之應用中。主要的工作多著墨於類似操作原理之有機場效應電晶體。然而， 目前對於有機電晶體之基礎研究與實際應用之差距仍然很大。相對的，有機發光二極 體卻已經進入商業化。主要的因素之一是有機場效應電晶體高度對介面的缺陷敏感且 高分子具有低載子遷移率，因此機場效應電晶體並非最佳的有機電子元件。 為了將有機電子元件之基礎研究與應用之差距縮小，在此計畫中，我們將研究兩 種形式的垂直有機電晶體。此種垂直有機電晶體的操作原理並非是場效的。事實上， 垂直有機電晶體更像於有機二極體，其電流是垂直於基板並且是在有機材料塊材中流 動的，並非限制於表面之通道。有機發光二極體是一種兩端元件，然而垂直有機電晶 體具有額外一端是製作於半導體中去控制源極與汲極間的垂直載子流。在熱載子電晶 體中，金屬基極的厚度是薄於載子的平均自由路徑，因此熱載子可以彈道式的穿越基 極而不會有碰撞發生。基極電壓可以調變射極電流而改變汲極電流。在空間電荷限制 電晶體中，金屬基極的厚度是大於載子平均自由路徑的，但是金屬基極上製作有許多 孔洞。因此，可以穿過基極上的孔洞的電流是可以被射極與基極之間的偏壓調變的。 因為射極與汲極之間的距離只有數十奈米，因此即使用低載子遷移率的高分子材料仍 然可以達到高電流輸出以及高速操作。因而此種元件可以解決有機場效應電晶體之低 電流輸出、操作速度以及穩定性的問題。此外，垂直有機電晶體之製作過程更簡化了， 且不需要微影蝕刻之製程。 有機熱載子電晶體以及有機空間電荷限制電晶體是由我們首先提出，許多原創性 的文章已經被發表在著名的國際期刊中以顯示此種元件的可行性。許多良好的特性已 經在過去三年中逐一的被證實。此種電子元件相較於一般場效應電晶體更是需要具有 高垂直遷移率之材料。進一步的突破可以藉由物理學家以及有經驗的化學家的合作來 達到。許多我們先前對於有機發光二極體的經驗可以應用在此一元件上。來自國科會 的補助對與此計畫是很重要的，尤其是國科會的補助將可以維持此計畫在國際上的名 聲與領先地位。

Transistor is the essential device for the entire electronic technology. Silicon field-effect transistor has been the foundation of the rapid advancement of the semiconductor industry which changed our daily life in the past decades. Silicon transistors are able to process data at very high speed, but they are not suitable for large-area moderate-speed applications including ultra-thin flexible display, artificial skin, and radio-frequency tags due to high cost and high processing temperature. In the past twenty years there is a tremendous amount of research effort to use organic semiconductors in such applications. The majority of the works are done for organic field-effect transistors with similar operation principle to the successful silicon transistor. However so far the gap between basic research and real applications based on organic transistors remains wide. On the contrary, organic light emitting diode has entered the era of commercialization. One of the key reasons is that field-effect transistor seems not to be the optimal device principle for organic materials due to the intrinsic low carrier mobility and high sensitivity to the interface impurities and disorder. In order to close this gap in this proposal we will study two vertical types of organic transistors whose operation principle is not field-effect. In fact it is more similar to the successful organic diode which is a sandwich structure with vertical current in the bulk instead of confined near the interface. OLED is a two terminal device while the vertical transistor has the third metal base electrode inserted into the semiconductor to control the vertical carrier flow between the emitter and the collector. In hot carrier transistor the metal is thinner than the carrier mean free path so the carriers move ballistically, i.e. hot carriers, through the metal base into the collector without collisions. The base voltage modulates the emitter current and subsequently the collector current. In the space-charge-limited transistor the metal base is thicker than the mean free path but contains many small openings. The diode current through the opening can be modulated as the carriers are expelled back to the emitter as the base-emitted is reversely biased. Because of the emitter-collector distance is only tens of nanometers high current output and high speed can be achieved even for low carrier mobility typical for OLED, thus solving the problems of organic field-effect transistors for current output, speed, and stability. The fabrication is even simpler then field-effect transistor as no photo-lithography is needed. The ideas of organic hot carrier transistor and space-charge-limited transistors were first proposed by us and we published the original papers to demonstrate their feasibility. Promising results have been realized in the past three years. Materials with high vertical mobility are highly desired for this purpose, in contrast to most of the organic electronics research focused on the horizontal mobility. Major further breakthrough can be done only by the combination of physicist and experienced chemists as in this collaboration. Many of our previous experiences on OLED can be applied to this topic because the vertical transistor is rather like an extension of a diode. The support from NSC to this vertical transistor project is crucial for us to maintain the momentum and leadership which will result in both international prestige and technological leadership in the near future.