A researcher experimenting with a new, more sensitive test for the AIDS virus said that the virus does not appear to be carried in sperm cells. That discovery could eventually mean that men with AIDS could father children by separating their sperm cells from infected semen and using those cells for artificial insemination. Semen, which is ejaculated, is made up of both sperm cells and seminal fluid. But Dr. Bernard Poisez, a researcher at the State University of New York Health Science Center at Syracuse, said it is a discovery that needs more study. The new technique of testing for the AIDS virus, developed by Cetus Corp. of Emeryville, Calif., uses a chemical reaction, called a polymerase chain reaction, to amplify genetic material--such as that from the AIDS virus--a million times, making it easier to detect.