Fairchild Semiconductor of South Portland, Maine, announced today it has filed a patent infringement lawsuit against Taiwan-based Alpha & Omega Semiconductor Ltd. (AOS), countering a patent suit AOS filed against Fairchild Thursday in the US District Court for the Northern District of California. Fairchild said in a statement that AOS power transistor products infringe at least four of its US patents, and the company is seeking monetary damages as well as an injunction against the products being sold in the US. AOS said in its suit that Fairchild infringes two of its patents. Fairchild said it has attempted to negotiate a licensing deal with AOS since 2005.