Shares in US chipmakers Qualcomm and AMD have fallen sharply, pressuring the global tech sector as investors reassessed smartphone demand and AI-related spending.

Qualcomm shares dropped more than 8% in after-hours trading after the company issued a weaker-than-expected revenue forecast for its fiscal second quarter, despite delivering a modest earnings beat for the December period.

The world’s largest supplier of smartphone processors said it expects second‑quarter revenue of between US$10.2 billion and US$11 billion (A$14.7 billion–A$15.8 billion), below expectations of around US$11.2 billion. Qualcomm guided earnings per share to a midpoint of US$2.55, compared with forecasts closer to US$2.90.

For the first quarter, Qualcomm reported revenue of US$12.3 billion, up 5% year on year and slightly ahead of expectations. Earnings per share came in at US$2.78, below some analyst forecasts.

Qualcomm chief executive Cristiano Amon (pictured) said near-term handset sales were being constrained by industry-wide memory supply shortages, as data centre operators absorb a growing share of global memory production.

However, Amon said demand for premium and high-tier smartphones remained solid, helping offset pressure at the lower end of the market.

Meanwhile, AMD shares closed down more than 17% in regular trading, marking their steepest one-day fall in years, even after the chipmaker beat expectations for the December quarter.

AMD reported earnings per share of US$1.53 on revenue of US$10.3 billion, well ahead of Bloomberg consensus estimates. The company also issued a stronger-than-expected outlook for the March quarter, forecasting revenue of between US$9.5 billion and US$10.1 billion.

Despite the upbeat results, investors appeared to lock in profits following a strong rally over the past year, with analysts noting that expectations for AI-related growth had become “sky high”.

The declines in Qualcomm and AMD added to broader weakness across US tech stocks, as concerns grow over supply constraints, valuations and lofty AI expectations.