The DX center, the lowest energy state of the donor in AlGaAs with x<0.22, is responsible for the reduced conductivity as well as the persistent photoconductivity observed in this material at low temperature. Extensive studies of the properties of this deep level in Si-doped AlGaAs are reviewed here. Data are presented showing that the characteristics of the DX center remain essentially unchanged when it is resonant with the conduction band (x < 0.22) and that, independent of other compensation mechanisms, the DX center therefore limits the free carrier concentration in Si-doped GaAs to a maximum of about 2 x 1019 cm-3. Recent measurements suggesting that the lattice relaxation involves the motion of the Si atom from the substitutional site toward an interstitial site are also presented. Evidence for the negative U model, that the DX level is the two electron state of the substitutional donor, is discussed. © 1989, Taylor & Francis Group, LLC. All rights reserved.