In this letter, we report the fabrication of Uni-travelling-carrier photodiode (UTC-PDs) for mm-wave applications. For the fabricated PDs, the area of the p electrode is smaller than that of the absorption area. Though this leads to a response roll off when the frequency is below 3.5 GHz, a higher impedance of the PD can be obtained, which leads to a higher RF power output for a <inline-formula> <tex-math notation="LaTeX">50~\Omega </tex-math></inline-formula> load resistance. For the PD having <inline-formula> <tex-math notation="LaTeX">50\times 4~\mu </tex-math></inline-formula>m2 absorption area and <inline-formula> <tex-math notation="LaTeX">46\times 2.5~\mu </tex-math></inline-formula>m2 p electrode area, the measured RF output power at 6 mA photocurrent, −2V bias and 15 GHz is 0.2 dBm. The ideal RF output power for a PD at this current is 0.35 dBm, which is only 0.15 dB higher than the measured RF power, indicating a high RF power output efficiency.