應用於第六代行動通訊系統行動裝置中頻率範圍三高效率功率放大器之砷化鎵異質接面雙極性電晶體

Abstract

隨著資料傳輸需求的持續增加，第六代（6G）行動通訊系統頻率範圍三(FR3) 頻段（7–15 GHz）兼具頻寬及覆蓋率的優勢，而具備高輸出功率（30–35 dBm）、高功率附加效率（PAE）與低操作壓（5-8V）的行動裝置中功率放大器（PA）被視為推進該系統商用化的關鍵元件。傳統的砷化鎵異質接面雙極性電晶體（GaAs HBT）技術，在過去數十年中廣泛應用於行動裝置中頻率範圍一(FR1)的功率放大器，但在 FR3 頻段時增益（Gain）有限，導致PAE偏低；此外，應用於Ｘ頻段（8–12 GHz）雷達的 GaAs HBT PA，雖然能實現 5–12 W 的高輸出功率與 30%–50% 的 PAE，但操作電壓在 10 V 以上，尚須降低操作電壓才能滿足行動裝置應用的需求。如何在較低操作電壓下提升PAE，是將 GaAs HBT 技術應用於 FR3 行動裝置PA時的主要挑戰，也是目前相關研究主要聚焦於高電子遷移率電晶體（HEMT）的原因。 本論文提出一套涵蓋針對6G FR3 PA的GaAs HBT元件設計、GaAs HBT 功率陣列單元的PAE優化方法，以及最終的PA設計特性展示。（1）針對GaAs HBT元件，透過技術電腦輔助設計軟體（TCAD），調整元件的磊晶結構以及版圖設計，提高元件的安全操作電壓至6.5V及最高震盪頻率（fmax）至124.5GHz。（2）基於前述GaAs HBT元件來建立功率陣列單元，透過熱流模擬軟體ANSYS，建立熱模擬模型，以精確預測 HBT 器件的接面溫度，並藉此優化版圖設計與電路佈局，使該功率陣列單元特性達成輸出功率為 32.6 dBm、增益 12.4 dB、PAE 為 57.14%。（3）基於前述之功率陣列單元，實作出一個應用於12 GHz ，且包含兩級增益放大的高PAE高功率 PA，並可透過並聯兩個功率單元，更進一步提升該 PA的峰值輸出功率。整體而言，本研究針對6G FR3 行動裝置PA的電路需求，進行從GaAs HBT元件、功率陣列單元，乃至電路特性的全面優化，完成實測輸出功率為 34.4 dBm、增益 24 dB、PAE 為 50%的PA。在相同頻率、輸出功率與行動終端電壓條件下，其效能實屬 GaAs HBT 放大器中，就作者知識所及之最高功率附加效率PA。

With the continuous increase in data transmission demand, the 6G FR3-band (7–15 GHz) offers the advantages of both wide bandwidth and extended coverage. Power amplifiers (PAs) with high output power (30–35 dBm), high power-added efficiency (PAE), and low operating voltage (5–8 V) for mobile devices are regarded as key components to enable the commercialization of 6G FR3 systems. Conventional gallium arsenide heterojunction bipolar transistor (GaAs HBT) technologies have been widely used in mobile handset FR1 PAs over the past decades due to their low standby power, proven reliability, and high linearity. However, their limited gain at FR3 frequencies leads to degraded PAE performance. On the other hand, GaAs HBT PAs developed for X-band (8–12 GHz) radar applications can achieve output power of 5–12 W and PAE of 30%–50%, but typically require operating voltages above 10 V, which are incompatible with handset power supplies. Therefore, improving PAE under low operating voltage condition remains the major challenge for applying GaAs HBT technology to FR3 mobile PAs, and this challenge has driven most current studies to focus on high electron mobility transistor (HEMT) technologies instead. This work presents a comprehensive design methodology encompassing GaAs HBT device development, power stage PAE optimization, and hybrid microwave integrated circuit (HMIC) implementation for 6G FR3 PAs. (1) At the device level, technology computer-aided design (TCAD) simulations were employed to optimize the epitaxial structure and layout of the HBT, achieving a safe operating voltage of 6.5 V and a maximum oscillation frequency (fmax) of 124.5 GHz. (2) Based on the optimized HBT device, a power cell array was developed and analyzed using ANSYS thermal simulations to accurately predict the junction temperature. The optimized thermal and electrical layout yielded a power cell achieving output power of 32.6 dBm, gain of 12.4 dB, and PAE of 57.14%. (3) Building upon this power cell, a two-stage PA operating at 12 GHz was implemented, and by combining two identical PA units, the peak output power was further enhanced. Overall, the proposed HMIC achieved a measured output power of 34.4 dBm, gain of 24 dB, and PAE of 50% at 12 GHz under a operating voltage of 6.5 V. To the best of the author’s knowledge, this represents the highest reported PAE among GaAs HBT power amplifiers operating at similar frequency and output power under mobile terminal voltage conditions.