This paper proposes a wideband CMOS power amplifier (PA) with flat gain using a pole-controlled transformer in the sub-THz band for wireless chip-to-chip communication. An analysis of the transformer-based interstage matching network is presented, which indicates a zero in-band ripple condition. For verification, a pole-controlled PA (with an area of 0.33 mm² including PADs) is fabricated using a 40-nm CMOS process and measured. The proposed PA achieves a measured 3-dB bandwidth of 38.5 GHz from 96.5 GHz-to-135 GHz, small-signal gain of 15.8 dB, and saturated output power of 14.6 dBm with a peak power-added efficiency (PAEMAX) of 9.44%. The proposed PA shows state-of-the-art performance with respect to the bandwidth and other figures of merit.