The　emission　mechanism　of　scandia-doped　dispenser　(SDD)　cathodes　is　characterized　based　on　systematic　investigation　of　microstructure,　surface　behavior,　evaporation,　and　emission.　High　current　density　SDD　cathodes　have　been　developed　in　recent　years　to　meet　the　requirements　of　advanced　vacuum　electron　devices.　Space　charge　limited　current　densities　of　over　100　A/cm(2)　have　been　reliably　reached　at　a　temperature　of　950　degrees　C-b.　SDD　cathodes　are　composed　of　a　tungsten　matrix　with　submicron　structure　and　uniformly　dispersed　materials　containing　Sc,　Ba,　Ca,　Al,　and　O　as　nanosized　particulates　in　the　matrix.　The　authors　conclude　that　the　copious　electron　emission　from　this　cathode　type　results　from　a　layer　of　Ba-Sc-O　on　W　formed　after　activation　on　top　of　the　matrix.　This　layer　is　about　100　nm　thick　and　maintains　a　stable　composition　over　its　lifespan.　Its　emission　can　be　explained　in　terms　of　a　semiconductor　layer　with　characteristics　that　differ　from　a　metal-like　emission　model.　The　Ba-Sc-O　layer　is　formed　by　codiffusion　of　Sc,　Ba,　and　O　(generated　inside　the　W-matrix)　to　the　surface　of　the　W-matrix　during　activation.　Furthermore,　this　layer　guarantees　a　low　evaporation　rate　of　emission-relevant　materials　and　thus　enables　a　longer　lifespan.　(C)　2011　American　Vacuum　Society.　[DOI:　10.1116/1.3609251]