The breakdown characteristics of two types of siliconp‐njunctions are studied. In the first type of junction which is commonly encountered, the breakdown occurs in many localized regions, the characteristic being determined by the aggregate effect of the localized regions; in the second type of junction the geometry is such that only one localized breakdown region occurs. Whereas the simple avalanche theory can explain the onset of breakdown for both types of junctions, the shape of theV‐Icurve in the breakdown region cannot be explained without the inclusion of another variable in the theory. Experimental evidence obtained by the use of pulse techniques indicates that this variable is the temperature rise due to the current flowing through the junction. It is found that this self‐heating is the most important single factor in determining the shape of theV‐Icurve and that almost the entire dynamic resistance in this region is due to this effect. A method is given for determining the temperature rise of the junctions, provided that theV‐Icharacteristic is known.