A 220-GHz power amplifier (PA) with high gain, high output power, and high efficiency for a 220-GHz transceiver system is presented in this letter. At terahertz (THz) and sub-terahertz (sub-THz) bands, the maximum available gain/maximum stable gain (MAG/MSG) of the device degrades rapidly, which increases the difficulty of high-gain and high-efficiency PA design. In this letter, the gm-boosting technique is analyzed and used in differential cascode (CC) topology to boost the gain of the single stage. Metal–oxide–metal (MOM) capacitors are used to reduce the insertion losses of the matching networks, therefore enhancing the gain and efficiency. The proposed PA exhibits a measured gain of 22.5 dB at 225 GHz with a 3-dB bandwidth of 35 GHz from 204 to 239 GHz. The measured OP1dB of 6.0 dBm and the peak power added efficiency (PAE) of 2.96% are achieved at 215 GHz. The measured $P_{\mathrm {sat}}$ is larger than 9 dBm from 200 to 216 GHz. The PA occupies a small area of 0.162 mm2 with a compact core area of 0.069 mm2. The power consumption of the chip is 245 mW. From the aspect of overall performances, including gain, output power, efficiency, and chip area, this work exhibits one of the best performances in silicon-based PAs at sub-THz bands. Besides, to the best of our knowledge, the proposed PA exhibits the highest efficiency in silicon-based implementations beyond 210 GHz.