The $1 Trillion Chipmaker That Couldn't Save the Broader Market: SK Hynix's Record Day in a Fractured KOSPI

South Korea's KOSPI hits all-time high driven by SK Hynix's 9.3% surge and new leveraged ETFs, but only 75 of 918 stocks advanced, highlighting extreme concentration risk.

South Korea’s benchmark KOSPI index scored a new all-time high on Wednesday, but the feat was built on an extraordinarily narrow foundation. Of the 918 regularly traded stocks on the main board, only 75 advanced while 826 declined. At the heart of that lopsided gain was SK Hynix, whose 9.3% surge—enough to trigger a trading halt and push its market capitalization past $1 trillion—highlighted both the promise and the peril of a market increasingly dependent on a single theme.

Shares of the memory chip giant closed at 2,243,000 won, giving it a market value of $1.12 trillion. That made SK Hynix only the second Korean company, after archrival Samsung Electronics, and the third in Asia after Taiwan Semiconductor Manufacturing Co., to join the trillion-dollar club. The stock touched an intraday high of 2,358,000 won, briefly crossing the $1 trillion threshold for the first time. Its year-to-date return now stands at 231%.

Two powerful catalysts converged to produce the day’s move. The first was the launch of South Korea’s first single-stock leveraged exchange-traded funds, which triggered a massive rotation by retail investors. They sold a net 1.08 trillion won worth of SK Hynix shares and poured 1.35 trillion won into the new leveraged products. That surge of activity pushed the country’s entire ETF market above 500 trillion won for the first time. The second catalyst came a day earlier, when SK Hynix unveiled its new “iHBM” thermal management solution, embedding silicon-based, thermally conductive and electrically non-conductive cooling elements directly into the interposer layer between HBM memory and graphics processors. The company claims the technology reduces thermal resistance by 30% and plans to deploy it in the next-generation HBM5 stack.

The flood of algorithmic and retail orders was so intense that the Korea Exchange activated a sidecar circuit breaker at 9:06 a.m. local time, temporarily halting computer-driven trading. It was the 19th time such a measure has been triggered this year.

The KOSPI closed up 2.3% at 8,229.70 points, having hit a record intraday high of 8,457.09 points—a gain of 5.1% from the previous day’s close. But the technology-heavy KOSDAQ index, which lacks a similar concentration of AI-related heavyweights, fell 1.5%. Samsung Electronics and SK Hynix now account for roughly half of the entire KOSPI market capitalization.

Behind the rally is an explosion in demand for high-bandwidth memory used in Nvidia’s AI chips. HBM prices doubled in the first quarter and are expected to climb another 63% in the current quarter. Analysts forecast that supply will lag demand until at least 2028, redefining the memory sector from a cyclical, low-margin business into a critical piece of AI infrastructure. That structural revaluation was underscored earlier this week when U.S. rival Micron Technology also crossed the $1 trillion mark.

Yet the concentration risk is hard to ignore. The KOSPI’s record rests on two stocks, and the broader market is showing signs of strain. The question hanging over the rally is whether AI-driven demand can sustain current pricing levels—and whether Korea’s equity market, dangerously dependent on a pair of chipmakers, can withstand a shift in sentiment.