Millimeter-wave (mmWave) wireless communication is moving to 5G and 6G to increase its data rate and channel capacity. In a wireless transmitter, power amplifier (PA) is widely considered to be the most critical building block, as it serves as the final stage and often dominates the overall transmitter efficiency and linearity. As a result, mmWave 5G and 6G require spectrum efficient modulation signals such as quadrature amplitude modulation (QAM), orthogonal frequency-division multiplexing (OFDM) together with carrier aggregation (CA) with a large peak-to-average power ratio (PAPR). Such high PAPR signals seldom operate at their peak power level. While a classic linear PA, such as a Class-AB PA, achieves its highest efficiency at the peak power, its average efficiency can drop significantly when handling these modulated signals with high PAPR. Therefore, it is necessary to develop PAs with efficiency enhancement techniques for back-off power while maintaining high linearity. Such advancements could significantly improve the average efficiency of PAs, especially when amplifying spectrum efficient modulated signals, making them better suited to meet the demands of modern wireless communication systems. Among various back-off efficiency enhancement techniques, Doherty PA architecture is one of the most popular approaches at mmWave frequency band due to its minimum digital computation overhead. Although significant research has been conducted on Doherty PA in the literature, it remains a challenge to design Doherty PAs with high power, high linearity, low loss, deep back-off efficiency enhancement, and over-GHz modulation bandwidth (BW).

In this doctoral thesis, I will first give a brief introduction of PA design challenges at mmWave and some existing back-off efficiency enhancement techniques in chapter 1. Then, in chapter 2, I will present a review of the Doherty PA architecture, from its original invention to its modern CMOS implementations. I will also introduce a few uncommon interpretations of the Doherty architecture from different perspectives to facilitate a more intuitive understanding of its operation. Following this, in chapter 3-5, I will introduce three Doherty PA prototypes designed during my PhD study, aiming to address the power, efficiency, and bandwidth challenges for mmWave PAs.

Finally, chapter 6 concludes this thesis and discusses some potential future research directions. Chapter 7 summarizes the publications and experiences of the author during the PhD.