High Quality Content by WIKIPEDIA articles! The Samsung Galaxy S II (GT-I9100) is a smartphone running under the Android operating system that was announced by Samsung on February 13, 2011 at the Mobile World Congress. It is the successor to the Samsung Galaxy S, with a different appearance and significantly improved hardware. The Galaxy S II was one of the slimmest smartphones of the time, mostly 8.49 mm thin, except for two small bulges which take the total thickness of the phone to 9.91 mm.