An Extremely High Frequency (EHF) dual-mode PA with a power combiner is designed using 40-nm bulk CMOS technology. One of the unit PAs can be switched off for the low power applications. In the design, circuit level optimization and trade-off are performed to ensure the good performance in both modes. The PA achieves a PSAT of 17.4 dBm with 29.3% PAE in high power mode and a PSAT of 12.6 dBm with 19.6% PAE in low power mode. The reliability measurements are also conducted and a lifetime of 80613 hours is estimated based on a commonly used empirical model. The excellent performance (e.g., highest reported PAE) achieved in this design further confirms the scaling of CMOS technology will continue to benefit the mm-wave transceiver design.