An experimental study is made of filamentary impact ionization breakdown observed in germanium doped with both group III and group V elements at liquid helium temperature.The approximate size of the filament at various current values after ionization is estimated from the measurement of voltage‐current characteristics of small cross section bars with high current pulses. Measurements are also made with cylindrical ``tecnetron'' structures in which the conduction channel can be reduced to very small areas by reverse bias of a ring‐shapedp‐njunction alloyed around a small cylindrical bar. Results of the two experiments are consistent. Near the lowest current value at which ionization can be sustained (about 40 &mgr;a), the approximate filament diameter is 10−3cm.