Millimeter-wave (mm-wave) radar has become a key technology in wireless sensor net-works (WSNs) due to its high spatial resolution and penetration capability, enabling ap-plications such as smart traffic control and non-contact health monitoring. Achieving fine range resolution necessitates wide signal bandwidth, which places stringent demands on power amplifier (PA) performance in terms of bandwidth, efficiency, and output power. Therefore the design of the power amplifier for WSN poses significant challenges. This paper presents a broadband mm-wave PA implemented in a 40-nm CMOS process, utiliz-ing transformer-based power combining to enhance efficiency and bandwidth simultane-ously, which can adequately meet the requirements of WSN systems. The PA achieves a 3-dB flat power bandwidth up to 62% from 13 to 24.8 GHz. At 19 GHz, it delivers a satu-rated output power (Psat) of 12.3 dBm, a 1-dB compression point (P1dB) of 10.15 dBm, and exhibits a peak power-added efficiency (PAE) of 23%, with 17.2% PAE at P1dB. The PA consumes 43 mW from a 1.1 V supply and occupies an active area of only 0.06 mm2. These results validate the effectiveness of transformer-based combining for achieving compact, high-performance broadband PAs in CMOS, and demonstrate its suitability for mm-wave radar systems requiring high range resolution. The amplifier provides a high stability, with output return losses better than −10 dB.