A novel full D-band (110-170 GHz) multi-mode transconductance down-converter mixer is realized in a 250-nm indium-phosphide double heterojunction bipolar transistor technology. A single-balanced topology is chosen and an active power combiner for the RF and the local oscillator (LO) signals' combination is used. The designed mixer is feasible to work at x 1, x 2, x 3, x 4 subharmonically LO-pumped mixing modes with relatively low LO powers of 0, -1, 5, and 6 dBm, respectively. The measured conversion gain achieves typical values of -3, -1, -5, and -4 dB over the full D-band while the best noise figures of 12, 13.5, 18.5, and 19 dB are obtained, respectively. Through the multi-mode operation in terms of subharmonic LO-pump-frequency, the designer can make a tradeoff between LO frequency, LO power, and noise figure.