This paper presents and investigates an ultra-low-power wireless transmitter (TX), fabricated in a standard 130nm BiCMOS technology for energy-harvesting sensor network applications, and demonstrates its operation when powered by a thermoelectric generator (TEG). The proposed ultra-low-power transmitter enables battery-less short-range communication while maintaining a compact feature size. To achieve low-power operation, the transmitter employs injection-locking and frequency-multiplication techniques for ultra-low-power frequency synthesis, combined with on-off keying (OOK) modulation and an integrated duty-cycling mechanism for further power reduction. Operating in the 433MHz ISM band and supplied by dual voltages of 0.25V and 0.9V, the TX delivers -21dBm output power with a continuous-mode power consumption of approximately 100μW. Under aggressive duty-cycling, the average power is reduced to the nanowatt range (≈ 150nW), enabling operation from a compact thermoelectric generator driven by a small body-to-ambient temperature difference (ΔT< 10°C). The fabricated chip occupies an active area of 0.019mm². These results demonstrate a significant advancement toward fully integrated, energy-autonomous wireless transmitters for wearable and industrial Internet-of-Things (IoT) applications.