Manganese (Mn)-activated yttrium germanate phosphor thin films were prepared by conventional r.f. magnetron sputtering. The phosphor thin films prepared with a Y2O3 content of 33, 50 and 67 mol.% and postannealed above approximately 950 degreesC were crystallized and identified as Y2Ge2O7, Y2GeO5 and Y4GeO8, respectively, whereas all as-deposited thin films were found to be amorphous. Thin-film electroluminescent (TFEL) devices were fabricated using the postannealed. phosphor thin films. The obtainable electroluminescent (EL) characteristics were considerably affected by the Mn content and the postannealing temperature of the phosphor thin-film emitting layers. High luminances for yellow photoluminescence and EL emissions were obtained in TFEL devices using these ternary compound phosphors activated with Mn and postannealed at 1020-1045 degreesC. Maximum luminance of 2590, 3020 and 2500 cd/m(2) were obtained in 1 kHz-driven TFEL devices using Y2Ge2O7:Mn, Y2GeO5:Mn and Y4GeO8:Mn thin-film emitting layers, respectively.