We present a superconductor-semiconductor materials system that is both scalable and monolithically integrated on a silicon substrate. It uses selective-area growth of Al-InAs hybrid structures on a planar III-V buffer layer, grown directly on a high-resistivity silicon substrate. We characterize the electrical properties of this materials system at millikelvin temperatures and observe a high average field-effect mobility of mu 3200 cm2/Vs for the InAs channel and a hard induced superconducting gap. Josephson junctions exhibit a high interface transmission, T 0.75, a gate-voltage-tunable switching current with a product of critical current and normal state resistance, ICRN 83 mu V, and signatures of multiple Andreev reflections. These results pave the way for scalable and high-coherence gate-voltage-tunable transmon devices and other superconductor-semiconductor hybrids fabricated directly on silicon.