氮化鋁鎵/氮化鎵高電子遷移率電晶體之射頻功率元件製作與分析

Abstract

氮化鎵材料因為其寬能隙、高載子遷移率與高載子濃度等特性，所以大量被應用在射頻與功率元件上。傳統會使用氮化鋁鎵/氮化鎵異質接面結構製作高電子遷移率電晶體，但隨著閘極不斷微縮，過厚的氮化鋁鎵障壁層將造成短通道效應發生，所以在之後的研究中能夠產生較強極化效應的氮化鋁銦逐漸取代氮化鋁鎵成為製作小線寬元件障壁層主要的材料。 在本篇論文中使用了氮化鋁鎵/氮化鎵與氮化鋁銦/氮化鎵兩種異質接面結構進行高電子遷移率電晶體的製作。首先，使用傳統氮化鋁鎵/氮化鎵異質接面結構製作出了閘極線約寬為 465 nm 的雙閘極元件，其在 VD= 6 V 的條件下擁有163.4 mS/mm 的最高轉導值與大約 103 的開關比，並且其次臨界擺幅約為 548.73 mV/dec。另外，透過散射參數量測與小訊號模型擬和結果得知當 VD= 6 V；VG= 0 V 時，元件之 fT/fmax可以達到 27.63/60.38 GHz，。最後，使用了 load-pull 量測系統針對 28 GHz 頻段進行 PAE 特性的量測，得出當在擁有最高轉導值的偏壓下，其 PAE 可以達到 13.01%。 最後也使用了氮化鋁銦/氮化鎵異質接面結構製作了閘極線約寬約為 400 nm的雙閘極元件，透過其直流特性觀察到閘極無法控制其通道關閉，推測為導通孔開在元件主動區域上所造成。因為量測金屬襯墊之鈦金屬在閘極導通孔處直接接觸元件表面，所以會使得閘極處蕭特基能障下降，造成元件閘極對於通道控制能力下降。因此，未來需修改光罩上導通孔之位置，以使元件能有正常的開關特性。

Gallium Nitride (GaN) materials, due to their wide energy bandgap, high carrier mobility, and high carrier concentration, have found widespread applications in RF and power devices. Traditionally, AlGaN/GaN heterojunction structures have been used to fabricate high electron mobility transistors (HEMTs). However, as gate lengths continue to shrink, thick AlGaN barriers have been observed to induce short-channel effects. Consequently, InAlN has gradually replaced AlGaN as the primary material for barrier layers in the fabrication of narrow-width devices, owing to its stronger polarization effect. In this thesis, we explore the fabrication of high electron mobility transistors using both AlGaN/GaN and InAlN/GaN heterojunction structures. Initially, we fabricated dual-gate devices with gate length of approximately 465 nm using the conventional AlGaN/GaN heterojunction structure. These devices demonstrated a maximum transconductance of 163.4 mS/mm and an on-off ratio of approximately 103 at VD = 6 V, with a subthreshold swing of about 548.73 mV/dec. Small-signal modeling and scattering parameter measurements at VD = 6 V; VG = 0 V indicated an fT/fmax of 27.63/60.38 GHz. Subsequently, we conducted PAE measurements at 28 GHz using a load-pull measurement system, achieving a maximum PAE of 13.01% under bias conditions with the highest transconductance. Furthermore, we fabricated dual-gate devices with gate length of approximately 400 nm using the InAlN/GaN heterojunction structure. DC characterization revealed that the gate was unable to control channel switching, likely due to via holes opening on the active region of the device. This issue was attributed to direct contact between the Ti used for DC and RF measurement on the gate and the device surface, leading to a reduction in the Schottky barrier height at the gate, thereby impairing the gate's ability to control the channel. Consequently, future work has to modify the positioning of via holes on the mask to restore normal switching characteristics to the devices.