  We present a superconductor-semiconductor material 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 characterized
the electrical properties of this material system at millikelvin temperatures
and observed a high average field-effect mobility of $\mu \approx
3200\,\mathrm{cm^2/Vs}$ for the InAs channel, and a hard induced
superconducting gap. Josephson junctions exhibited a high interface
transmission, $\mathcal{T} \approx 0.75 $, gate voltage tunable switching
current with a product of critical current and normal state resistance,
$I_{\mathrm{C}}R_{\mathrm{N}} \approx 83\,\mathrm{\mu V}$, and signatures of
multiple Andreev reflections. These results pave the way for scalable and high
coherent gate voltage tunable transmon devices and other
superconductor-semiconductor hybrids fabricated directly on silicon.
