The Philadelphia Semiconductor Index (SOX) and Why Asia Watches It

“The SOX fell overnight” refers to the PHLX Semiconductor Sector Index, ticker SOX. Nasdaq calculates it, and its stated aim is to measure the performance of the 30 largest US-listed semiconductor companies. It is a weighted basket of chip designers, chipmakers and equipment makers, with caps on the largest weights. It matters in Asia for a simple reason of timing: the US session ends before dawn in Seoul, Taipei and Tokyo, so the SOX close is the most recent chip-stock reading when Asia opens.

Why is it called the Philadelphia index?

PHLX is the Philadelphia Stock Exchange, which Nasdaq describes as the oldest stock exchange in the United States, founded in 1790. Nasdaq OMX purchased it in 2008, and the index carries the PHLX name. According to Nasdaq’s index page, the SOX began on December 1, 1993 at a base value of 200.00, was split two-for-one on July 24, 1995, and has had listed options since September 7, 1994.

What does a company need to get in?

Nasdaq’s methodology sets the rules. A security must:

• be listed on a US exchange that Nasdaq treats as eligible;
• be classified in the Semiconductors or Production Technology Equipment subsector of the Industry Classification Benchmark (ICB);
• have a market value of at least US$100 million;
• have traded at least 1.5 million shares in each of the six calendar months up to the reference date, and have traded for at least three months before it;
• have listed options in the US, or be eligible for them.

Common stocks, ordinary shares and American depositary receipts (ADRs) all qualify, and there is no rule about the company’s home country. One security per issuer is allowed. Once a year Nasdaq takes the 30 largest eligible securities by market value.

One rule matters for Asian companies. An ADR counts only for the shares held on deposit at its US depositary bank, adjusted by the ADR ratio, so a non-US company “may be included in the Index at less than its full global market capitalization,” the methodology says. TSMC enters the SOX at the size of its US-listed ADS pool, not at the size of the whole company.

How is it weighted?

The SOX is a modified market-cap-weighted index. Each member starts with its share of the group’s total market value. Then caps apply at each quarterly rebalance:

Rank by market value	Maximum weight
Largest	12%
Second	10%
Third	8%
All others	4%

Any excess is spread over the smaller members in steps until every weight fits. Nasdaq adopted these caps on April 22, 2024. Before that, the five largest members were capped at 8% each and the rest at 4%.

Between rebalances, weights move with prices. Nasdaq’s fact sheet lists the ten largest weights on September 30, 2026:

Ticker	Company	Weight
NVDA	Nvidia	11.35%
AVGO	Broadcom	8.67%
MU	Micron Technology	8.13%
INTC	Intel	4.91%
AMD	Advanced Micro Devices	4.75%
MRVL	Marvell Technology	4.57%
AMAT	Applied Materials	4.08%
KLAC	KLA	4.06%
TSM	Taiwan Semiconductor Manufacturing Co.	4.02%
ADI	Analog Devices	4.01%

Because the excess above each cap is passed down the list, the smaller members carry more weight than their market value alone would give them. The caps are applied at each quarterly rebalance; in between, weights move with prices, and on September 30 several sat a little above 4%.

When does the list change?

Nasdaq reconstitutes the SOX once a year. It ranks companies on the last trading day of July, and changes take effect at the open on the first trading day after the third Friday in September. Weights are reset quarterly: the reference dates are the last trading days of February, May, August and November, and the new weights take effect after the third Friday of March, June, September and December. Both are announced after the close six trading days before they take effect.

In 2026 the reconstitution took effect on Monday, September 21. Comparing Nasdaq’s lists for September 18 and September 21, Lattice Semiconductor (LSCC), Qnity Electronics (Q), SiTime (SITM) and Tower Semiconductor (TSEM) joined, and Nova (NVMI), Qorvo (QRVO), Rambus (RMBS) and Skyworks Solutions (SWKS) left. The next weight reset falls after the third Friday of December 2026, December 18.

Which Asian and other non-US companies are in it?

On October 19, 2026, the 30 members included four listings of companies based outside the US, according to Nasdaq’s list and the addresses in their SEC filings:

Ticker	Company	Based in	What trades in the US
TSM	Taiwan Semiconductor Manufacturing Co.	Hsinchu, Taiwan	ADSs on the NYSE
ASML	ASML Holding	Veldhoven, Netherlands	New York registry shares on Nasdaq
ARM	Arm Holdings	Cambridge, UK	ADSs on Nasdaq
TSEM	Tower Semiconductor	Migdal HaEmek, Israel	Ordinary shares on Nasdaq

TSMC is the Asian name. Each TSM ADS represents TSMC common shares held on deposit; our TSM vs 2330 explainer covers the ratio and why the US price runs above Taipei’s.

No Korean or Japanese company was in the October 19 list: Samsung Electronics, SK hynix and Kioxia Holdings do not appear, and neither does SanDisk. SK hynix’s ADSs began trading on Nasdaq on July 10, 2026, three weeks before the 2026 reference date, while the rules ask for three months of trading. Among memory makers, Micron is in the index.

Which funds track the SOX?

Nasdaq’s index page lists the products linked to it, in the US and in Asia:

• Invesco PHLX Semiconductor ETF (SOXQ), on Nasdaq. Its summary prospectus names the PHLX Semiconductor Index, compiled by Nasdaq, as the underlying index.
• TIGER US PHLX Semiconductor Nasdaq (381180) in South Korea, from Mirae Asset Global Investments, and a version with two times the index’s daily move (423920).
• Cathay U.S. PHLX Semiconductor Sector ETF (00830) in Taiwan, from Cathay Securities Investment Trust.
• Global X Semiconductor ETF (2243) in Japan, from Global X Japan.

The home markets come from Nasdaq’s April 2024 research note on the index.

SOXX is a different case. BlackRock’s page for the iShares Semiconductor ETF (SOXX), which began in 2001, lists its benchmark as the NYSE Semiconductor Index. SOXX does not track the SOX.

When does the SOX close in Asian time?

Nasdaq publishes the SOX once a second from 09:30 to 17:16 New York time, and its closing value can be corrected until 17:15. The US regular session ends at 16:00 New York time. Until US clocks change on November 1, 2026, that is 05:00 the next morning in Seoul and Tokyo and 04:00 in Taipei and Singapore. From November 2 it is one hour later. So the SOX close for a Monday in New York arrives before Tuesday’s open in Asia. For the full hand-off between the four markets, see One Chip Day, Four Closes.

Does Asia follow the SOX the next day?

Sometimes it does, sometimes it does not. Here are three recent US sessions, each next to the following Asian session (closing levels: SOX from Nasdaq, KOSPI from the Korea Exchange, TAIEX from the Taiwan Stock Exchange, Nikkei 225 from Nikkei Inc.):

US session	SOX	Next Asian session	KOSPI	TAIEX	Nikkei 225
Wed, Oct 14	−4.30%	Thu, Oct 15	−7.57%	−3.41%	−4.90%
Fri, Oct 16	−3.83%	Mon, Oct 19	+1.76%	−0.61%	−0.62%
Mon, Oct 19	+1.61%	Tue, Oct 20	−3.73%	−1.13%	−1.30%

To redo the first row: the SOX closed at 11,684.38 on October 14 and 12,208.79 on October 13, a change of −4.30%. On October 15 the KOSPI closed at 5,554.22 against 6,008.81 the day before. On October 19 the SOX closed at 11,316.34, up from 11,137.29 on October 16; on October 20 the KOSPI closed at 5,552.74, down from 5,768.01.

Over a longer stretch, we paired each SOX daily change from April 16 to October 19, 2026 with the next session in each Asian market, keeping only pairs with exactly one US session between two Asian sessions (holidays on either side drop some days):

Index	Pairs	Same direction as the SOX	Opposite
TAIEX	120	88	32
KOSPI	112	72	40
Nikkei 225	113	72	41

These counts describe timing only. They do not show that the SOX causes the next Asian move, and on October 20 all three Asian indexes fell after the SOX rose. The Chips map shows how each chip stock in Korea, Taiwan, Japan and the US closed, market by market.