NEW YORK ( TheStreet ) -- Shares of Integrated Silicon Solution ( ISSI ) were gaining 7.5% to $20.14 Thursday after the chipmaker received a letter from Cypress Semiconductor ( CY ) offering to buy the company. In its letter to Integrated Silicon, Cypress offered to buy the company for $19.75 a share, representing a 16% premium over the chipmaker's closing price on Tuesday, according to the Wall Street Journal. Cypress said its offer is superior to the $19.25 a share bid for the company by a group led by Summitview Capital in March. "We would have preferred to participate in your sale process, but were not contacted," Cypress said in its letter. About 2.2 million shares of Integrated Silicon were traded by 12:09 p.m. Thursday, well above the company's average trading volume of about 406,000 shares a day. TheStreet Ratings team rates INTEGRATED SILICON SOLUTION as a Hold with a ratings score of C+.