考慮溫度效應的閘極場射三極體之數值模擬

Abstract

本論文提出一個新的數值模擬工具, 用來模擬考慮溫度效應的閘極場射三 極體.在本論文中所用的模擬格子是非常具有彈性的, 靠近尖端部分大約 只有10-8公分, 而在陽極附近有 10-4 公分左右. 這樣的模擬格子可以正 確地估計尖端電場而無需用到修正值(field enhancement factor)6,而且 可以節省電腦計算的時間. 除此之外, 我們還討論並証明模擬格子的數目 對獲得正確模擬結果的重要性.大部分發表的論文都觀察到尖端幾何形狀 對閘極場射三極體發射電流能力有非常大的影響. 然而, 我們還証明了調 整閘極寬度將能改善閘極和陽極之間的漏電流.我們利用包含空間電荷的 模擬來解釋為什麼閘極場射三極體的發射面積會被低估. 並且可以解釋關 於飽和電壓的爭議. 另外, 在高電場時, 我們也觀察到空間電荷降低尖端 發射電流的現象.最後, 我們討論到閘極場射三極體的熱穩定性. 假如考 慮到那汀漢( Nottingham)效應, 那麼認為艾菲爾鐵塔(Eiffel-Tower) 形 狀的尖端是最穩定結構的觀點就有問題了. 我們的結果顯示那汀漢( Nottingham)效應是加熱尖端的主要效應, 而非一般所說的焦耳加熱效 應. 此外, 我們也討論到尖端半角以及功函數對閘極場射三極體的影響. Presented here is a new numerical siumlation tool for gated field emission triode(GFET) considering space charge effect and temperature effect. The mesh structure used in our simulation tool is very flexible, about 1 A near the tip and 1 um near anode. This kind of mesh structure can be used to accurately estimat the field at tip peak without using field enhancement factor and the CPU time is greatly reduced. In addition, the imporant of the number of meshes in obtaining correct simulation result is discussed and verified. The strong geometrical dependence of tip emission for GFET is usually observed in most simulation programs. However, we have shown that gate thickness can be optimized to reduce the leakage current between gate and cathode. Simulation including space charge effect has resulted in the speculation of the underestimation of area of GFET array. This can be used to explain the controversy of the value of the on-set voltage. And the space charge effect lowering the tip emitting current is observed, too. Finally, the thermal stability of GFET is discussed. If Nottingham effect is considered, the knowledge of the Eiffel-Tower shape tip is the most stable structure is questioned. Our results show that Nottingham effect is the dominant factor to heat tip other than Joule heating effect as some papers suggested. In addition, the influence of tip half angle and work function on thermal characteristics of GFET is discussed.