Motorola Inc., which already dominates China's pager market, is proposing to call up a further $720 million in investment in the country, this time for a semiconductor plant.

That project will bring the Schaumburg, Ill., company's aggregate investment in the northeastern port city of Tianjin to $1 billion. Earlier undertakings are a pager factory and cellular phone workshop.Motorola (China) Electronics Ltd., the local firm, also said profits earned in China will be reinvested within the country.

The new investment, designed to enhance existing Chinese capacity in producing semiconductors, will finance a high-technology wafer plant. Motorola said it will introduce manufacturing technology for its submicron eight-inch chip, the most advanced yet made in China.

Semiconductors are widely used in a huge array of modern gadgetry, including vehicles, computers and telecommunications.

Richard Younts, a Motorola vice president, said construction of the plant, covering more than 3 million square feet, is due to begin in November. Operations are targeted for early 1998 with an unspecified proportion of the 3,000 wafers produced each week to be exported.

In addition, Motorola intends to expand capacity for radio-pager and cellular-phone production in Tianjin. Plans are being discussed for a joint venture with Nanjing Panda Electronics Group Corp. to produce personal computers.

The company signed a letter of intent earlier this year for a joint-venture manufacturing plant for digital cellular infrastructure and subscriber equipment with China Posts & Telecommunications Industry Corp. and its subsidiary, Hangzhou Telecomm Equipment Co.

Motorola China Electronics, based in Tianjin's economic and technology development zone, began life in 1992 with investment of $120 million. The company boosted that by $160 million last year.

Jeff Dryjanski, vice president and general manager of the company's Greater China paging subscriber division, said Motorola plans to invest between $1 billion and $1.5 billion in China over the next five years.

Motorola China won two awards last year - in the top 10 foreign-funded firms in exports and in sales - the only electronics company to score twice in the ranking by the China Foreign Investment Association.