Who is suppose to write software design document?

Who is suppose to write software design document? the developer and document writer are not suppose to be same person?

Development of a systyem is team work. Everyone has its own responsibility.Software Design document is responsiblity of architect/designer ( or designing team) that develop design of system based on requirements identified and analyzed during requirement analysis phase. Everyone in development team is considered as a developer but each one has further defined responsibility. Once design is finalized, it can be written by some technical writer. A developer can be design writer if he is part of designing team.