We report an experimental measurement of nonlinear dc current-voltage (I-V) characteristics in p-type Ge extrinsic photoconductors at liquid He temperatures which exhibit a region of voltage-controlled negative differential resistance (NDR) above the threshold for impurity breakdown, accompanied by a spontaneous current oscillation of frequency fs ∼ 5 to 10 kHz. The dc I-V characteristics are well explained in terms of a simple rate equation model and, it is argued, the spontaneous current oscillations are due to moving space charge waves which arise from a negative differential rate of impurity impact ionization. © 1986.