American Telephone & Telegraph Co. yesterday announced it had begun sample production of a one-megabit chip -- a memory chip capable of storing more than one million pieces of information, enough to fill about 100 typewritten pages.

AT&T says it is the first American company to begin production of the chip and that it will begin full-scale volume production early next year. However, several Japanese semiconductor companies -- including Hitachi Ltd. and Toshiba -- also are manufacturing sample quantities of megabit chips and scheduling full production for next year.

Historically, Japanese companies have dominated the computer-memory market. Recent Semiconductor Industry Association figures indicate that Japan has a 90 percent share of the market for 256K random-access memory chips. The megabit chip, a fourfold increase in capacity, would be the successor to that product, which represents the current state of the art.

AT&T executives said that the company would compete directly against Japanese companies in the semiconductor market -- a market that recently has been in the grip of its deepest recession in 20 years.

"We have very aggressive plans for the megabit chip," said Don J. Liedberg, manager of sales for AT&T's components and electronic systems.

He estimated that AT&T could sell from 10 million to 50 million megabit chips next year. He added that the company also would aggressively market its 256K chip, now that it has added manufacturing capacity.

In the past, AT&T has kept the chips it produces through its Western Electric subsidiary for its own internally developed computer systems and telecommunications switches. In 1983, AT&T was one of the first companies to introduce a 256K chip. It said then that it would market the chips to other companies. However, a surge in internal demand effectively prevented that.

"We did not find it pleasant to pull in our horns in on the 256K," said Liedberg. "The reality was that the capacity was not available. It is now."

The AT&T announcement, in the middle of a general semiconductor slump, reflects that the company, unlike most of America's semiconductor manufacturers, is a multibillion-dollar company with diverse electronics lines.

"We do not have all our chips in the memory basket, no pun intended," said Liedberg.