Ford Motor Co. yesterday opened a $33 million computer chip factory the company hopes will capture a piece of the Reagan administration's proposed "Star Wars" space defense program, as well as other defense electronics projects.

The Colorado Springs, Colo., factory, operated by Ford Microelectronics Inc., is intended to become the nation's first high-volume producer of gallium arsenide (GaAs) microchips -- a new material that many microelectronics experts expect will replace silicon as the major medium for integrated circuits.

Ford, the nation's second-largest auto maker, established Ford Microelectronics in 1982 to develop custom integrated circuits for automotive uses and for Ford Aerospace, one of the nation's biggest defense contractors. Ford Aerospace, now working on several missile defense systems, did $1.5 billion in business last year.

Currently, the $26 billion worldwide semiconductor industry uses silicon to etch the computer and memory chips that are found in personal computers, microwave ovens, automobile engines and a range of other electronic devices.

Electrical engineers contend that GaAs chips are intrinsically better than silicon wafers because GaAs facilitates faster calculations, withstands wider temperature ranges, and is 1,000 times more resistant to radiation.

"GaAs clearly is the most practical high-speed replacement for silicon," said Larry Weis, spokesman for Ford Microelectronics. "The very nature of the compound makes gallium suitable for satellite and military applications," he said.

But GaAs chips also are more difficult to produce than silicon, which means that they are more expensive. For example, a single GaAs wafer today costs $200, "fifteen times more than the cost of a silicon chip," Weis said.

As a result, GaAs chips are too expensive to put into automobiles, which means their production by Ford will be geared primarily toward space defense and other specialized computer and satellite communications operations, Weis said.

"GaAs would be an advantage where high-speed electronic devices are needed. You don't need that kind of high-speed electronics in a car. So, they've got to be thinking about military applications," said Thomas Hinkelman, president of the San Jose-based Semiconductor Industry Association.

GaAs chip production is so specialized and so new that sales of those chips last year accounted for "less than $200 million" of worldwide semiconductor sales of $26 billion, said Hinkelman, whose association represents the nation's largest semiconductor producers.

Ford Microelectronics researchers estimated the current total GaAs market at $75.4 million. Other forecasters expect GaAs chips to represent 5 to 7 percent of the total semiconductor market by 1990.

Other companies exploring GaAs production include Harris Microwave, a subsidiary of the Melbourne, Fla.-based Harris Corp., and California-based Gigabit Logic.