One of the current challenges in photonics is developing highspeed, power-efficient, chip-integrated optical communications devices to address the interconnects bottleneck in high-speed computing systems(1). Silicon photonics has emerged as a leading architecture, in part because of the promise that many components, such as waveguides, couplers, interferometers and modulators(2), could be directly integrated on silicon-based processors. However, light sources and photodetectors present ongoing challenges(3,4). Common approaches for light sources include one or few off-chip or wafer-bonded lasers based on III-V materials, but recent system architecture studies showadvantages for the use of many directly modulated light sources positioned at the transmitter location(5). The most advanced photodetectors in the silicon photonic process are based on germanium, but this requires additional germanium growth, which increases the system cost(6). The emerging twodimensional transition-metal dichalcogenides (TMDs) offer a path for optical interconnect components that can be integrated with silicon photonics and complementary metal-oxidesemiconductors (CMOS) processing by back-end-of-the-line steps(7-9). Here, we demonstrate a silicon waveguide-integrated light source and photodetector based on a p-n junction of bilayer MoTe2, a TMD semiconductor with an infrared bandgap(10). This state-of-the-art fabrication technology provides new opportunities for integrated optoelectronic systems.