Low-temperature measurements of impurity effects have been conducted in $n$-type GaAs with impurity concentrations of the order of (1-3) × ${10}^{15}$ ${\mathrm{cm}}^{-3\mathrm{}}$. The behavior of these materials in strong magnetic fields is well described by a recently developed theory which characterizes the impurity level with a broadened energy distribution and the conduction band with a Gaussian-like tail. The reduced impurity ionization energies in strongly doped materials are explained in terms of this model. Electric field ionization of these impurity levels in strong magnetic fields was found to be consistent with the model and recent theories of impact-ionization phenomena.

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