Data: FactSet; Chart: Matt Phillips/AxiosSemiconductor stocks have been enjoying their greatest rally ever. Why it matters: It's another proofpoint driving investors and analysts alike to an inescapable conclusion: The AI build-out is almost certainly the biggest investment boom in American history. Driving the news: Computer chip stocks just recorded their best quarterly increase on record, with the Philadelphia Stock Exchange Semiconductor index rising 87.8% for the three months that ended Tuesday. That was the biggest quarterly gain in records stretching back to 1994, trouncing even the eye-watering gains of the dot-com era. So far, the Philly semi index — often known as the SOX — has more than doubled in 2026, and we're only halfway home. Between the lines: Of course, this is all about AI, which has supercharged demand for certain chips — in particular makers of memory chips like DRAM and NAND — that have a relatively fixed supply due to the cost and time it takes to build new chip fabs. The result? Gonzo price increases that are supercharging profits. Case in point: Take Micron Technology, long a relatively sleepy U.S. producer of what were seen as commodity memory chips. Last year, in its fiscal Q3 — which ends in May — it earned $1.91 a share.In its recently reported fiscal Q3 2026, the company earned $25.11 a share.That's a 1,215% year-over-year increase in earnings. How it works: Since profits are a critical driver of stock prices, shares of Micron have exploded.They're up 836.5% over the last 12 months, creating more than $1.1 trillion in market value.And there are others. State of play: Intel was up 216.4% in the second quarter. Advanced Micro Devices rose more than 185.6%. Applied Materials, KLA and Lam Research — all makers of equipment to produce chips — more than doubled during the quarter.Even chip giants Nvidia and Broadcom, big winners in earlier phases of the AI boom, had a strong quarter, rising 14.7% and 22%, respectively.Reality check: As incredible as the rally has been, the run-up clearly has some traders and investors alert to the risk of a bust, which has been a feature of some other giant investment booms of the past.Volatility for the chip stocks surged in June, with the sector at the heart of a "bloodbath" early in the month.Even big winners like Micron have seen giant day-to-day swings, tumbling 13% last week only to turn around and surge 16% a couple days later after reporting earnings.Caveat: Though it's incredibly difficult to predict market crashes, there has been some legitimate work on it, with scholars finding that increases in stock issuance, volatility and accelerating market gains — as well as outperformance of newer, rather than older, companies — can help identify booms that subsequently collapse.