Deep‐level transient spectroscopy (DLTS) and capacitance‐voltage (C‐V) measurements have been used to study the residual defects inn‐type AlGaAs co‐implanted with Be and either As or P. The co‐implant procedure is being studied as a method for limiting Be diffusion during the rapid thermal annealing process used to activate the Be. The only defect found in the upper half of the band gap by DLTS measurements between 10 and 450 K has an activation energy of 0.88 eV. This defect undergoes photocapacitance quenching, which together with its activation energy suggests that it is similar to the EL2 defect found in GaAs. In an analysis of high‐temperatureC‐Vdata, the defect manifests itself as a peak in the donor concentration. The donor concentration data were modeled by doing a double integration of Poisson’s equation assuming a Gaussian defect distribution and a uniform shallow donor distribution. This analysis indicates that the center of the Gaussian defect distribution is near the peak in the P or As distribution and that the peak concentrations are about 1×1017defects cm−3for the higher P and As fluences.