U.S. stock prices have been heading up for so long, Wall Street-watchers are sounding the customary alarms that we are overdue for a "correction." The long bull market has lifted many tech-based companies, including the following four in suburban Philadelphia.

Universal Display (stock symbol: OLED): This Ewing Township firm, founded by the same Philadelphians who developed King of Prussia patent farm InterDigital, employs 200 Princeton University scientists and other scholars and salespeople. The team spent the past 20 years developing and licensing "organic" light-emitting diodes, the misnamed, molecule-thin films that Samsung, LG Display, and now Apple use in smartphones and video screens. Sales are approaching $300 million this year (up from $200 million last year), with after-tax profits expected to top $130 million.

So it's no surprise Universal shares have more than doubled, to an all-time high of $132, since I visited last year. Analysts at Morgan Stanley and Citic Securities say they are worth $200 or more. Others are more cautious, noting OLED already trades at a hot 76 times earnings (compared to 21 times for the S&P 500).

"About 30 percent of smartphones have OLED screens," CFO Sidney Rosenblatt told investors in a conference call Sept. 12. Rosenblatt predicted 50 percent growth in OLED demand from 2015-17 — and that happened, noted Deutsche Bank analyst Sidney Ho. Rosenblatt now says demand will jump at least that much over the next two years.

Incyte (INCY): Shares of the cancer- and pain-fighting drug developer, which moved into a new glass-walled headquarters on a hill overlooking Wilmington this year, doubled to the $120s in 2015, fell back to the $60-80 range last year, and are back around $120 after bouncing still-higher last spring. At that price, the company is worth $25 billion — showing the huge premium Wall Street still pays for pharma growth stories: This year's sales are expected to top $1.5 billion, double 2015's total.

Writing in STAT, veteran biotech columnist Adam Feuerstein warned earlier this month that investors are making "a huge bet" on Incyte drug uses not yet approved by the FDA (much of Incyte's income comes from Europe). Incyte employs around 1,000.

Tabula Rasa Health Care (TRHC): This Moorestown medication-software company, grown by scholars from Children's Hospital of Philadelphia and backed by locally based Rittenhouse Ventures, has doubled to around $24 a share since going public last September.

Wells Fargo analyst Peter Heinz Costa warned earlier this month the company was in danger of losing focus, and the stock dipped sharply — for a day. But Tabula Rasa founder Dr. Calvin Knowlton was able to persuade investors his $35 million-plus-sales-bonuses purchase of SinfoniaRX, an Arizona-based medication-software company, is "a natural extension," and the stock jumped back up to record highs. Sales are expected to top $125 million this year, up from $94 million last year, and yield a modest profit. The company employs around 250.

Teleflex (TFX): The Wayne-based medical-device company last week hit a record $235/share — almost double its value of two years ago — after announcing its $725 million (plus sales bonuses) purchase of the UroLift urinary-tract implant for prostate patients, the latest of the deals CEO Benson Smith and his team have cut at the rate of one every quarter for the past five years.

Teleflex now trades at 30 times earnings. In turning Teleflex into a premium-priced medical company, Smith and his lieutenants have completed a change in focus at the former mechanical-controls company and done their part to cement medical devices as one of the western Philly suburbs' signature industries.