| 摘要: | 氮化鎵高電子遷移率電晶體，具備高濃度二維電子氣與高電子遷移率，在高頻功率應用中具有顯著優勢。然而，為提升高頻操作需求而進行之線寬微縮，會降低崩潰電壓，限制輸出功率，漸變式通道（Graded channel）結構可同時改善高頻與崩潰特性，為具潛力之磊晶結構設計。
本研究利用Silvaco TCAD對漸變式通道GaN HEMT進行系統性模擬分析，結果顯示，漸變層厚度主要影響電子在通道內的空間分佈，而鋁含量梯度則透過改變極化強度，調控通道電荷分佈與載子濃度。相較於傳統結構，漸變式通道HEMT，具有較負之臨界電壓、更高汲極電流以及更平坦之轉導特性，有效降低轉導二階導數之峰值，提升線性度表現，同時使通道電場峰值降低最高達29%。然而，霍爾量測結果顯示，通道厚度與鋁含量梯度的增加，皆會加劇合金散射效應，進而劣化載子傳輸特性。
為驗證模擬結果，本研究製作通道厚度為12 nm、鋁含量梯度分別為 8%（Sample A）與 18%（Sample B）之元件。實驗結果顯示，高鋁梯度樣品，具有較平坦之轉導曲線與較寬之閘極電壓擺幅，顯示其線性操作區域較佳。高頻特性方面，Sample A之fT與fMAX分別為62.7 GHz與106.9 GHz，優於Sample B之54 GHz與90.3 GHz。此差異主要歸因於Sample A具有較低的串聯電阻與較高的轉導。元件於不同閘極偏壓下之高頻操作範圍亦為線性度的重要指標，Sample A的fT與fMAX對應之GVS約為2.5與2.3 V；Sample B分別為3 V與2.8 V。證實較高鋁含量梯度所帶來的轉導平坦化，可延伸至高頻操作區域，能在較寬的閘極電壓範圍內維持穩定增益。
綜合模擬與實驗結果，本研究指出通道厚度為12 nm、鋁含量梯度介於13% 至18% 時，可在高線性度、高頻性能與載子傳輸特性間取得良好平衡，對未來高效率與高線性度GaN射頻元件設計具有參考價值。
;Gallium nitride (GaN) high-electron-mobility transistors (HEMTs), characterized by a high-density two-dimensional electron gas (2DEG) and high electron mobility, offer significant advantages for high-frequency power applications. However, the lateral scaling required to meet high-frequency operation degrades the breakdown voltage, thereby limiting output power. Graded-channel structures provide a promising epitaxial design approach by simultaneously improving high-frequency performance and breakdown characteristics.
In this study, systematic TCAD simulations of graded-channel GaN HEMTs were performed using Silvaco TCAD. The results indicate that the thickness of the graded layer primarily affects the spatial distribution of electrons within the channel, while the aluminum composition gradient modulates the polarization strength, thereby controlling the channel charge distribution and carrier concentration. Compared with conventional structures, graded-channel HEMTs exhibit a more negative threshold voltage, higher drain current, and a flatter transconductance profile. This flattening effectively reduces the peak value of the second derivative of gm, leading to improved linearity performance, while simultaneously reducing the peak channel electric field by up to 29%. However, Hall measurement results reveal that increasing both the channel thickness and aluminum composition gradient enhances alloy scattering effects, which in turn degrade carrier transport properties.
To validate the simulation results, devices with a channel thickness of 12 nm and aluminum composition gradients of 8% (Sample A) and 18% (Sample B) were fabricated. Experimental results show that the high aluminum-gradient sample exhibits a flatter transconductance curve and a wider gate voltage swing, indicating an improved linear operating region. In terms of high-frequency performance, Sample A demonstrates cutoff frequency (fT) and maximum oscillation frequency (fMAX) values of 62.7 GHz and 106.9 GHz, respectively, compared to 54 GHz and 90.3 GHz for Sample B. This performance difference is primarily attributed to the lower series resistance and higher transconductance of Sample A.
The high-frequency operating range under varying gate bias is also a critical indicator of linearity. The gate voltage swing (GVS) corresponding to fT and fMAX for Sample A is approximately 2.5 V and 2.3 V, respectively, while Sample B exhibits wider ranges of 3.0 V and 2.8 V. These results confirm that the transconductance flattening induced by a higher aluminum composition gradient extends into the high frequency operating region, enabling stable gain over a broader gate voltage range.
Based on both simulation and experimental results, this study concludes that a channel thickness of 12 nm combined with an aluminum composition gradient in the range of 13% to 18% achieves a favorable balance among high linearity, high-frequency performance, and carrier transport properties. These findings provide valuable insights for the design of future high efficiency, high linearity GaN RF devices. |
