the geometry around trimethyl around N atom in (ch3)3N is pyramidal due to sp3 hybridisation and due to lone pair – bond pair repulsion.
In case of (Sih3)3 N, the atom is sp2 hybridised and the lone pair is present in the 2p orbitaland it is transferred to empty d orbital of Si atomforming dpi – p pi bond. Thus the involvement of lone pair in d pi – p pi bond makes (Sih3)3N a weaker base than (ch3)3 N.