Impact ionization effects have been observed in near-intrinsic silicon and germanium structures which exhibit an abrupt transition from a high-resistance state to a low-resistance state through a negative resistance region. Measurements on high-resistivity n- and p-type silicon samples with symmetric Ohmic or blocking contacts show that the threshold voltage for this transition is proportional to sample thickness which is indicative of bulk breakdown. On the other hand, the sustaining voltage in the low-resistance state is thickness independent, indicating the formation of an avalanche breakdown layer. The results agree generally with Gunn's theory of avalanche injection.

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