本文提出一個可以有效縮短GaN-based HEMTs閘極長度的方法。在閘極的製程上，首先使用低階的多重波長汞燈曝光儀器製作出微米等級的閘極長度，再配合本文所提出的新式硝酸(HNO3)微縮閘極製程，藉此來獲得次微米閘極長度。而此硝酸微縮閘極製程所利用的原理是：GaN具有不易被酸鹼蝕刻的極佳化學穩定性、Ni/Au閘極金屬層的選擇性蝕刻。當元件的Ni/Au閘極金屬完成後，再將元件浸泡在硝酸數秒鐘，於是微米等級的閘極長度將被微縮成次微米等級。
本文成功的完成了一個低成本、簡單、快速且高效能的次微米閘極製程。次微米閘極有效的改善了元件的直流與微波特性：飽和電流密度IDSS0 (395.8mA/mm to 471.5mA/mm)、最大轉導值gm,max (121.5mS/mm to 189.8mS/mm)、夾止電壓Vpinch-off (–3.95V to –3.2V)、電流增益截止頻率fT (12.351GHz to 21.876GHz)、最大震盪頻率fmax (16.925GHz to 27.342GHz)、最小雜訊NFmin (1.7827dB to 0.7681dB)，並且維持了優異的高功率特性。透過新式的硝酸微縮閘極製程，使得GaN-based HEMTs在未來微波領域的應用扮演更重要的角色。

This thesis reports an effective method of shrinking gate length in GaN-based HEMTs. In the gate process, we first use low-end multi-wavelength Hg lamp to fabricate micro scale gate length and then utilize our novel nitric acid (HNO3) treatment to shrink gate length, so as to obtain sub-micro gate length. We employ the principle that GaN possesses excellent chemical stability so HNO3 will not etch GaN-based HEMTs. Besides, Ni metal is easily etched by HNO3 treatment. Contrary to Ni metal, Au metal is hardly etched by HNO3 treatment. Therefore, our devices were dipped into HNO3 for several seconds after Ni/Au gate schottky contact process. Thus the gate length will be shrunk from micro scale to sub-micro scale.
In this thesis, we have successfully accomplished a sub-micron gate process of low cost, simple, fast, and high efficiency. Sub-micron gate improved our devices' DC and microwave characteristics effectively: the saturation drain current density IDSS0 (395.8mA/mm to 471.5mA/mm), the maximum extrinsic transconductance gm,max (121.5mS/mm to 189.8mS/mm), the pinch-off voltage Vpinch-off (–3.95V to –3.2V), the unity current gain cut-off frequency fT (12.351GHz to 21.876GHz), the maximum oscillation frequency fmax (16.925GHz to 27.342GHz), and the minimum noise figure NFmin (1.7827dB to 0.7681dB). Besides, the excellent power characteristics were still maintained. Consequently, GaN-based HEMTs will play a very important role for microwave applications in the future through this novel HNO3 process of shrinking gate length.

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