Simplify the following text for a Grade 8 native English-speaking person.
Original: Bardeen , together with William Shockley and Walter Brattain , invented the transistor on 23 December 1947 .
Simplified: Brattain refused to work with Shockley further and was assigned to another group . Neither Bardeen nor Brattain had much to do with the development of the transistor beyond the first year after its invention .