A novel backside-illuminated mesa-structure dual-drifting layer (DDL) uni-traveling-carrier photodiode (UTC-PD) is reported to demonstrate high-power performance at sub-THz frequencies. The DDL structure consists of a velocity overshoot layer and a velocity saturation layer, formed by inserting a 20 nm p-type cliff layer into the thick depletion region. In the overshoot layer, photo-generated electrons drift at overshoot velocity under the carefully designed electric field profile, thus resulting in a short electron transit time. The saturation layer serves as a voltage sacrificing layer to enable high bias voltage operation, which leads to alleviated load voltage swing effect, as well as improved saturation performance. Our DDL UTC-PD exhibits a 3-dB bandwidth of 106 GHz with a responsivity of 0.17 A/W under a wide bias voltage range from 4 to 8 V. The photocurrent reaches up to 28 mA, corresponding to an output power of 7.3 dBm at 105 GHz.