| 摘要: | 研究主題以高阻矽基板作為基底成長氮化鋁銦鎵/氮化鎵異質結構磊晶片，製作毫米波電晶體並分析其特性。研究內容包括改善歐姆接觸預處理製程、Γ型閘極結構設計、閘極寬度設計，在評估元件的穩定性與良率及權衡射頻特性之間的相互影響後，最終將閘極長度定義在150 nm，並採用Γ型閘極結構與常見的T型閘極進行比較，藉由測量分析元件之小訊號特性及大訊號特性線性度結果評估元件表現，輔以暫態量測並分析特性劣化之來源，探討具有不同的閘極結構對於高頻元件特性的影響，以利改進元件特性。
為達成高穩定性與高良率的Γ型閘極，本研究以TDUR-P015/Dilute ZEP-A7雙層光阻結構，進行閘極足部與頭部的曝光，並透過曝光記號控制Γ型頭部偏移量，以閘極長度150 nm的Γ型閘極，作為提升功率輸出特性與線性度的方法。
本研究中製作的高電子遷移率電晶體在直流特性方面，Γ型與T型閘極元件的電流密度分別為797 mA/mm與755 mA/mm，Γ-gate結構設計有助於平整化(smooth)2DEG通道上電場的峰值，使其有效的分散閘極邊緣的電場，降低閘極漏電流，具體結果是Γ-gate比起T-gate結構，臨界電壓從-3.5V偏移至-3V，崩潰電壓從80.4 V提升至85.8 V。以去嵌化小訊號量測Γ-gate與T-gate元件之小訊號特性，在閘極長度150 nm時fT/fmax分別為90/142 GHz與83/130 GHz，由於Γ型結構使得元件閘極電容(Cgs)明顯下降，所以有較高的功率增益截止頻率。元件之大訊號特性，是在量測頻率為28 GHz下，將元件操作在Class-AB的條件下進行，Γ-gate與T-gate的元件飽和輸出功率為21.02/20.34 dBm、功率增益為10.22/9.73 dB、PAE為11.87/11.78 %和功率密度為1.26/1.08 W/mm ; 而在Class-B的條件下，Γ-gate與T-gate的元件飽和輸出功率為18.8/17.6 dBm、功率增益為10.01/10 dB、PAE為13.5/12.6 %和功率密度為0.75/0.58 W/mm，Γ-gate元件因為其更高的轉導值，有助於功率特性的提升。在線性度量測結果中，在頻率為28 GHz中，以Class-AB的條件，得到Γ-gate/T-gate元件的輸出三階截斷點(OIP3)分別為27.5/26.1 dBm ;而在Class-B下，Γ-gate/T-gate元件的OIP3為19.6/18.02 dBm，說明在更動元件操作偏壓點之後，Γ-gate元件因為其閘極操作範圍增加，轉導值在較寬的偏壓範圍內保持平坦，因此Γ型結構相較於Γ-gate的線性度較T-gate元件優異。記憶效應是在class B的條件下，Γ-gate/T-gate元件的ΔIM3為12.35/11.8 dBc，因為Γ-gate能更有效抑制3階訊號，造成upper和lower訊號不對稱，說明Γ-gate受記憶效應影響的情況更明顯。未來在鈍化層厚度增厚能降低元件的表面電流以及改善元件截止狀態之漏電流後，元件的崩潰電壓能再提高，也能將元件的操作區間擴大，有機會再提升大訊號特性。
;This thesis deals with the fabrication and characterization of millimeter-wave transistors based on AlInGaN/GaN heterostructure grown on a high-resistivity silicon substrate. The research includes the improvement of ohmic contact pre-processing, the design of Γ-gate structure, and the design of gate width. After evaluating the stability, yield, and trade-offs between RF characteristics, the gate length is finally defined as 150 nm. The Γ-gate structure is compared with the conventional T-gate structure to evaluate the influence of different gate structures on DC/RF characteristics by the small-signal and large-signal measurement. Additionally, transient characteristics are also measured to analyze the source of characteristic degradation and provide a reference for device reliability.
To achieve high stability and high yield for the Γ-gate structure, a double-layer photoresist structure of TDUR-P015/Dilute ZEP-A7 is used to expose the gate foot and head, and the Γ-gate head offset is controlled by the exposure mark. A Γ-gate structure with a gate length of 150 nm is used to improve the output power characteristics and linearity.
The DC characteristics of the high electron mobility transistors (HEMTs) fabricated in this paper showed that the current density of the Γ-gate and T-gate devices is 797.2 mA/mm and 755 mA/mm, respectively. The Γ-gate structure helped to smooth the peak electric field on the 2DEG channel, which more effectively dispersed the electric field at the edge of the gate to reduce leakage current. Specifically, the threshold voltage of Γ-gate structure shifts from -3.5 V to -3 V and increases the breakdown voltage from 80.4 V to 85.8 V compared to the T-gate structure. The small-signal characteristics with a gate length of 150 nm shows the fT/fmax of the Γ-gate and T-gate devices are 90/142 GHz and 83/130 GHz, respectively. This is attributed to the reduced gate capacitance in the Γ-gate structure. The large-signal characteristics of the devices were measured at 28 GHz with the devices biased in Class-AB condition. The saturated output power of Γ-gate and T-gate devices was 21.02/20.34 dBm, power gain was 10.22/9.73 dB, PAE was 11.87/11.78 %, and power density was 1.26/1.08 W/mm, respectively. Under Class-B operation, the saturated output power of Γ-gate and T-gate devices was 18.8/17.6 dBm, power gain was 10.01/10 dB, PAE was 13.5/12.6 %, and power density was 0.75/0.58 W/mm, respectively. These results show that the Γ-gate structure improves the power characteristics due to their higher transconductance. As to the linearity, the output third-order intercept point (OIP3) of Γ-gate and T-gate devices was 27.5/26.1 dBm at 28 GHz with Class-AB operation. Under Class-B operation, the OIP3 of Γ-gate and T-gate devices was 19.6/18.02 dBm, respectively. These results show that the Γ-gate device has better linearity than the T-gate device, because the Γ-gate device has a wider gate operation range and a flatter transconductance over a wider bias range. In the memory effect measurement results, the variation on IM3 of the Γ-gate and T-gate devices was 12.35 dBc and 11.8 dBc, respectively, under Class-B bias condition. This is because the Γ-gate device is more effective in suppressing the third-order signals, which causes the upper and lower signals to be asymmetrical. This indicates that the T-gate device exhibits more pronounced memory effect under Class-B operation. In the future, the large-signal characteristics could be further improved by increasing the thickness of the passivation layer to reduce the surface current and off-state leakage current, and increase the breakdown voltage of the devices. |
