Why Is PHLX Semiconductor Index Down 4.98% Today, August 18? Bond Yields Hit AI Chips

The PHLX Semiconductor Index (SOX) slid 4.98% as long-dated U.S. Treasury yields rose to multi-year highs, pressuring valuations of long-duration AI and memory equities after a crowded year-long rally. Broad-based declines across major constituents signaled de-risking rather than company-specific fundamentals. Near-term focus is on whether yields stabilize and semiconductor breadth improves, as rate volatility can continue to drive index-level positioning and risk appetite.