We have fabricated GaInP/Er,O-codoped GaAs (GaAs:Er,O)/GaInP double heterostructure (DH) light-emitting diodes (LEDs) and successfully observed 1.5 mu m electroluminescence (EL) due to an Er-20 center under forward bias at room temperature. Er excitation cross section by current injection decreased with increasing GaAs:Er,O active layer thickness, implying reduced diffusion length of injected carriers in the active layer. Carrier dynamics in GaAs:Er,O have also been investigated by means of a pump and probe reflection technique. Time-resolved reflectivity of GaAs:Er,O exhibited a characteristic dip after a steep decrease to negative in less than 10 ps. The analysis of the characteristic dip revealed short lifetime in range of ps for photoexcited carriers. The extremely short lifetime is quite coincident with the reduced diffusion length of injected carriers, and suggests that a trap induced by Er and O codoping would play an important role in dynamics of nonequilibrium carriers in GaAs:Er,O.