Why Micron Stock Exploded 40% Higher in September

Even before Micron's (MU 2.28%) earnings report on Sept. 23, shares of the memory chip manufacturer had already logged an impressive month-to-date gain. Strong results and guidance ultimately led the stock even higher to close out September. Micron stock gained 40.6% last month, according to data provided by S&P Global Market Intelligence, largely thanks to booming demand for artificial intelligence data centers. Image source: Getty Images. Scrambling for AI computing capacity Two things related to AI are happening within the memory chip market. First, demand for high-bandwidth memory, a special type of dynamic random-access memory critical for AI accelerators, is exploding. Mega-deals involving OpenAI, Oracle, Nvidia, and other tech giants to build massive AI data centers will require equally massive quantities of HBM chips. Micron sold $2 billion worth of HBM chips in the fourth quarter of fiscal 2025, which ended on Aug. 28, and it's working on bringing its next-generation HBM4 chips to market. Nearly all of its HBM3 chip supply for calendar 2026 is spoken for, and the company is talking to customers about HBM4 commitments. For the time being, Micron is easily selling every bit of HBM that it can make. The second development is related to standard DRAM memory chips. While AI data centers also need commodity server DRAM chips, manufacturers including Micron are aggressively prioritizing HBM production. Even with somewhat weak demand for PCs and smartphones, which both require DRAM chips, overall supply is now tight. This situation has pushed up prices, boosting Micron's bottom line further. Together, these trends pushed up Micron's revenue by 46% year over year in the fourth quarter to $11.3 billion. Non-GAAP gross margin expanded by more than 9 percentage points to 45.7%, and adjusted earnings per share more than doubled. Micron expects both trends to continue into fiscal 2026. The company expects to generate around $12.5 billion in revenue during the first quarter, along with a non-GAAP gross margin of roughly 51.5%. That gross margin is historically high for Micron. Micron stock looks cheap, but be careful Based on the average analyst estimate for fiscal 2026 adjusted earnings per share, Micron trades at a price-to-earnings ratio of just above 11. That may look incredibly inexpensive for a company growing so quickly and benefiting so greatly from the AI boom, but investors need to be careful. The memory chip market is cyclical, and pricing is largely determined by supply and demand. Every single boom, marked by demand outpacing supply, has been followed by a bust. Micron makes a lot of money during booms, but the bottom line can plunge deep into negative territory during severe downturns. Amazon founder Jeff Bezos called AI a bubble on Friday, joining a chorus of high-profile voices warning of overexuberance. If AI infrastructure is overbuilt, which looks likely given the massive investments being made, demand for memory chips could fall off a cliff once the reckoning arrives. That's the big risk with Micron stock. Despite how amazing things look right now, a downturn is always coming.