The Si doped n-Al0. 45 Ga0. 55N ep ilayers have been grown on sapphire ( 0001) substrate by metal organic chemical vapor deposition. The transmittance measurement indicates that the materials absorb ultraviolet radiation a-bove 280nm notablely and the rocking curve of the AlGaN ep itaxial material shows that there are abundant disloca-tions. Photoconductive detectors are fabricated on the material and the mechanism of photoconductor in AlGaN is discussed.