Cymer Inc., a supplier of excimer light sources used in semiconductor manufacturing, announced that Young K. Sohn and Edward H. Braun have joined its board of directors and that Richard P. Abraham has retired from the board, which has been expanded from seven to eight members.   . . .    Fairchild Semiconductor has named Hans Wildenberg to the new position of COO. Matt Towse has been promoted to senior vice president and CFO. Fairchild's assistant general counsel, Paul Delva, has been promoted to vice president and general counsel, and Pete Groth has been promoted to vice president and treasurer.   . . .   Mitsubishi Electric & Electronic USA Inc. announced it will establish a new Semiconductor Division, effective today, to provide compound semiconductors in discrete and module form for optoelectronic and microwave/radio frequency customers based in North America. The new division will be based in California and will provide marketing, sales and application engineering support. Regional sales offices and third-party sales and distribution channels will also be located throughout the US and Canada.