A silicon dual-ring modulator consisting of two serially cascaded rings with embedded PN junctions is driven by a differential signal pair. We show by simulation and experiment that the device has advantages over the single-ring modulator in terms of optical bandwidth, 3-dB modulation bandwidth and bit rate, at the expense of a 1.7-dB increase in the transmission penalty and a twofold increase of the RF power consumption. Driven by differential pseudo random binary sequence (PRBS) signals of 0.5-V peak-to-peak voltage (Vpp), the dual-ring modulator exhibits optical bandwidths of 66 pm and 40 pm at 12.5 Gb/s and 20 Gb/s, respectively. In contrast, the single-ring modulator has an optical bandwidth of 26 pm under a single-end PRBS signal of 0.5 Vpp at 12.5 Gb/s, and its eye diagram closes if the bit rate rises to 20 Gb/s.