石墨烯熱電子電晶體效益增進及高頻特性之研究

Abstract

雙極性電晶體（Bipolar Transistors，BJTs）是高頻的重要元件之一，但它的截止頻率受限於base的厚度，因此二維材料是一個極佳的選擇作為base的材料，由於二維材料原子層級的厚度。垂直式二維材料熱電子電晶體（Vertical 2D material Hot Electron Transistors）被視為非常具有前瞻性的元件，應用於高頻領域，在理論上已經可以達到兆赫茲（Terahertz）等級的程度[1]。這是一個和雙極性電晶體（Bipolar Transistors，BJTs）極為相像的電子元件，其主要有三個部分，射極（Emitter）、基極（Base）以及集極（Collector）。其特別的之處還有在基極以及射極中間成長氧化層（Base-emitter Insulator），作為穿隧氧化層（Tunneling Oxide）；集極以及基極中間也成長氧化層（Collector-base Insulator），作為過濾氧化層（Filtering Oxide），使載子以穿隧（Tunneling）的方式從射極穿隧道基極，再從基極穿隧至集極，使用高動能、高速的熱電子（Hot Electron）作為載子傳輸。 在這項研究實驗中，使用高參雜n型矽基板（Highly Doped N-type Silicon Substrate，n++ Si）作為射極，並且上方使矽基板自然生長出約三奈米厚的二氧化矽 (SiO2)的原生氧化層（Native Oxide），作為穿隧氧化層，石墨烯（Graphene）作為該元件的基極，十奈米厚二氧化鈦（TiO2）以及三十奈米厚二氧化鉿（HfO2）作為雙層的過濾氧化層（Double Layer Filtering Oxide），以鈦（Ti）金屬作為集極。 目前的文獻尚無製作出有高頻響應的垂直式二維材料熱電子電晶體，其主要原因為元件的表現、效益不佳，其主要參考的指標有共基極電流增益（Common-base Current Gain，α）、共射極電流增益（Common-emitter Current Gain，β）以及電流密度（Current Density，JC），而在這項研究中，成功利用上述之結構製作出高效益的之石墨烯熱電子電晶體，其共基極電流增益α達0.96；共射極電流增益β達18；電流密度JC達7 A/cm2，且元件之截止頻率（Cut-off Frequency，fT）可達5.5 GHz。

One of the most important high frequency device is bipolar transistors (BJTs). The cut-off (fT) is limited for the thickness of base region so 2D materials is a great candidate as a material for base region for its ultra-thin thickness. Theoretically, vertical 2D material Hot Electron Transistors is seen to be a high potential device for potential Terahertz applications. It is like a bipolar transistor and there are regions, emitter, base and collector, too. Furthermore, there are two potential barriers. One is between base and emitter, tunneling oxide, and the other one is between collector and base, filtering oxide. The carriers can tunnel from emitter to base and then tunnel from base to collector. The carrier we use is hot electron which has high kinetic energy and high speed. In this experiment, we use highly doped n-type silicon substrate as emitter, about 3 nm thickness native silicon dioxide (SiO2) as tunneling oxide, graphene as base, TiO2 (10 nm) and HfO2 (30 nm) as double layer filtering oxide and Titanium (Ti) as collector. We are the first team to successfully demonstrate a graphene-based hot electron transistor with high frequency response. The device we fabricated remains high common-base current gain (α~0.96), high common-emitter current gain (β~18), high current density JC (~7 A/cm2) and the cut-off frequency (fT) achieves ~5.5 GHz.