1.5- and 1.3-μm multiquantum-well distributed-feedback lasers with semi-insulating current blocking layers are discussed. The LD structure is shown. The light output vs current characteristics at 20°C are given. The threshold current was 2.5 mA. Stable single-longitudinal-mode operation was maintained up to 10-mW output power. The measured carrier lifetime was 1.2 ns. For a 50-mA P-P modulation current, τd (the lasing delay time) is estimated to be 60 ps, which is about one tenth of that for a 20-mA threshold laser. τd of <100 ps makes possible the achievement of 5-Gb/s zero-bias modulation. Low parasitic capacitance in the SI-BH structure and high differential gain in the MQW-DFB structure are very effective for high-bit-rate modulation. Small signal modulation measurements showed that a resonance frequency of 5.4 GHz and a modulation bandwidth of 7.0 GHz were attained at a 20-mA bias current (3-mV output power). Responses at 2- and 5-Gb/s NRZ zero-bias random pulse modulation are shown. The pulse current was 50-mA P-P. The modulated eye patterns are clearly open. The 20-dB down spectral width was approximately 7 angstrom at 5-Gb/s modulation. These results are very promising for zero-bias modulation.