We present two major points in this paper: (1) Statistics derived from the negative-U property of the DX center are not consistent with existing Hall experiments of Si-doped ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As. (2) The discrepancy between the negative-U model and Hall experiments can be improved if there exist two different donors SD and DX with comparable concentrations $N_{\mathrm{SD}}$ and $N_{\mathrm{DX}}$, $N_{\mathrm{SD}}$/$N_{\mathrm{DX}}$ increases with increasing $N_{\mathrm{Si}}$. DX is a negative-U center binding two electrons and SD is a shallow donor. SD centers provide electrons and the electrons are captured by DX centers, and therefore the Fermi energy is no longer pinned to the DX energy level. Many experiments are reinterpreted in this view.

• Received 30 January 1989

DOI:https://doi.org/10.1103/PhysRevB.40.1430