Epitaxy tools (MOCVD and MBE) that grow the compound-semiconductor layers used in laser diodes and high-frequency chips. They sit upstream of the compound-semiconductor wafer foundries as tooling. Supply is set by a small group of toolmakers, so the balance tightens when demand outruns what they can deliver.
A sustained tight read means demand for compound-semiconductor epitaxy tools is running ahead of what a small group of toolmakers can build and deliver, so laser and high-frequency wafer foundries wait for deposition systems before adding layer-growth capacity. The read is toolmaker cohort revenue, a proxy: it tracks how hard those suppliers are being worked, not delivered tool count. Relief arrives only as tool output and installation catch up, over quarters rather than a single order cycle.