氮化鎵材料擁有寬能隙、高電子飽和速度、高臨界電場與低導通電阻等優點，因此近年來，以氮化鎵材料為主的元件在高功率與高頻應用上相當受到重視。在不同的基板下，又以矽基板氮化鎵元件最受關注，他具有成本效益，矽基板氮化鎵(GaN-on-Silicon)的研究被視為未來的科技主流之一。
一般常見的氮化鎵高電子遷移率電晶體是利用金(Au)來做歐姆接觸以及閘極電極，然而金的成本相對較高，因此成為了大規模生產的障礙之一。並且一般的歐姆接觸退火都在800℃以上以形成穿刺(spike)現象，使金屬往下鑽並且有效碰觸到二維電子氣通道，從而降低歐姆接觸電阻。但是在高溫的退火下，其金屬表面會變得非常粗糙且金在高溫下可能會穿過阻擋層並擴散到半導體基板中而導致閘極漏電流的增加。本實驗將採用無金製程並且搭配低溫退火的方式來取代傳統使用金的製程，從而降低元件製程成本與提高元件的可靠度。
在本次論文中，我們採用相對簡單的Ti/Al的歐姆接觸取代傳統的Ti/Al/Ni/Au，透過蝕刻將二維電子氣通道吃穿，改以掘入的側壁與金屬形成直接接觸，並且搭配低溫550℃來進行退火，獲得最佳的接觸電阻(R_c)為0.28 Ω-mm，此方法能使金屬表面較為平坦，並且能使接觸電阻不會對掘入深度太過敏感。而我們測得的最大外部轉導為232 mS/mm，截止頻率為43.6 GHz，最大震盪頻率為66.7 GHz。

Gallium Nitride (GaN)-based materials have the advantages of wide band gap, high electron saturation velocity, high critical electric field and low on-resistance. Therefore, GaN-based devices have received considerable attention in high power and high frequency applications. Among the of GaN-based devices grown on different substrates, the GaN-on-Silicon devices have attracted most attentions, which are very cost effective compared with the potential of large-scale wafer size. The GaN-on-silicon is considered as one of the mainstream technologies in the future.
Au is usually used as one of the ohmic and gate metals in the conventional GaN HEMT. However, Au is with relatively high cost, which is one obstacle for mass production. Also, the typical ohmic contacts need annealing at above 800℃ to form metal spikes, so that metal drills down and effectively touches the two-dimensional electron gas channel, thereby reducing the ohmic contact resistance. However, at high temperature, the metal surface will become very rough and Au may pass through the barrier layer and diffuse into the semiconductor substrate causing increased leakage current. In this experiment, an Au-free process and a low-temperature annealing method will be used to replace the traditional process using Au, thereby reducing the cost of the device process and improving the reliability of the device.
In this work, we replace the traditional Ti/Al/Ni/Au metal stack with a relatively simple Ti/Al ohmic contact. It is shown that the best contact resistance of 0.28 Ω-mm can be achieved with recess beyond the AlGaN Schottky barrier where the ohmic contacts are formed on the sidewall of the recess and anneal through a low temperature of only 550 C. This method can make metal surface relatively smooth and make the process relatively insensitive to the exact recess depth. The measured maximum extrinsic transconductance g_(m.max), current gain cutoff frequency f_T, and maximum oscillation frequency f_max are 232 mS/mm, 43.6 GHz, and 66.7 GHz, respectively.

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