Walter H. Brattain, 85, a coinventor of the transistor and who was one of three scientists named cowinners of the 1956 Nobel Prize in physics, died Oct. 13 at a nursing home in Seattle. He had Alzheimer's disease.

Dr. Brattain shared his prize with John Bardeen and William Shockley. All three had been research scientists at Bell Telephone Laboratories at Murray Hill, N.J., where they did their research in semiconductors and, in 1948, discovered the "transistor effect" and for inventing the transistor itself.

The transistor, a tiny, inexpensive substitute for the radio vacuum tube. Both devices amplify and oscillate electric impulses. However, the transistor is cheaper, and operates with greater efficiency and power. Unlike the vacuum, it has no heated cathode -- it operates instantaneously and "cold."

Dr. Brattain's and Bardeen's original "point contact" transistor was rapidly supplanted by later models. First came J.N. Shine's "photo" transistor in 1950 and Shockley's new "junction" transistor a year later, which was followed in 1952 by the "tetrode" transistor of R.L. Wallace Jr.

The transistor entered commercial use in 1953. By the time Dr. Brattain was awarded the Nobel Prize, the transistor had almost entirely replaced the vacuum tube. The transistor was first used by Bell Telephone in its "card translator," used in the rapid selection of routes for long-distance calls. The transistor also found almost immediate use in new and improved communications, such as hearing aids, that required smallness and small power consumption.

Along the way, work Dr. Brattain and his colleagues pioneered in the transistor and in semiconductor research gave birth to modern electronics. It revolutionized the technology used in space exploration, satellite communications and especially in computers. It led to the transistor radio, better televisions and better telephones.

Dr. Brattain was born to American parents in Amoy, China. He grew up in Washington state. He was a 1924 graduate of Whitman College in Walla Walla, earned a master's degree at the University of Oregon and a doctorate at the University of Minnesota.

He was a member of the technical staff of the National Bureau of Standards here from 1928 to 1929. He then joined Bell Labs, where he worked until retiring in 1967. During World War II, he worked for the war research division at Columbia University, where he did research on the magnetic detection of submarines.

Dr. Brattain was a 1952 recipient of the Stuart Ballantine Medal and was a corecipient of the John Scott Medal in 1955. He was a fellow of the American Academy of Arts and Sciences, the American Physical Society, and the American Association for the Advancement of Science. In 1974, he was elected to the National Inventors Hall of Fame.

His first wife, the physical chemist Keren Gillmore, whom he married in 1935, died in 1957. He married Emma Jane Miller in 1958. He had a son by his first marriage, William G. Brattain.