Optical signaling without a high voltage driver for electric-optic modulation is in high demand to reduce power consumption, packaging complexity, and cost. In this work, we propose and experimentally demonstrate a silicon mode-loop Mach–Zehnder modulator (ML-MZM) with record-high modulation efficiency. We used a mode-loop structure to recycle light twice in the phase shifter. With an L-shaped PN junction, a comparably large overlap between the PN junction and optical modes of both TE0 and TE1 was achieved to lower the driving voltage or decrease the photonic device size. Proof-of-concept high-efficiency modulation with low VπL of 0.37V·cm was obtained. Subvoltage Vπ can be realized with a millimeter’s length phase shifter by this scheme, which makes the realization of CMOS-compatible driverless modulation highly possible. 40 Gb/s signaling with a bit error rate below the 7% forward-error-correction threshold was then demonstrated with the fabricated ML-MZM, indicating great potential for high-speed optical communication.