A discussion of some known physical models for second breakdown

The problems about second breakdown can be discussed in three parts: 1) causes of localized high temperatures in the crystal, 2) mechanism of voltage drop, 3) physical state of the crystal after the voltage drop, i.e., the state of the second breakdown, when it is stable and reversible. About point 1) today very much is known. Point 2) cannot be discussed independent of point 3). In regard to point 3) three models have been published: 1) English and Power (1953). A small stable molten zone exists in the crystal. Visual observations (mesoplasmas) have led to this theory. Approximations show, that such a state is possible without destroying the transistor or the diode. 2) Melchior and Strutt (1964). High carrier densities occur at high temperatures and reduce the space charge in the junction in two steps with respect to the different impurity densities on both sides of the junction. This reduction causes the voltage drop. These temperatures are far below the melting point. 3) Weitzsch (1965). A small molten channel exists between the collector and the emitter in transistors or between the low-resistivity zone and the metal in diodes. More accurate calculations with rotational elliptic coordinates show the possibility of realizing such a state. There are sufficient calculations for model 3), which is an extension of model 1) only. Model 2) cannot be discussed in a closed form without knowledge of the ionization coefficients in the Townsend equation at high temperatures. There are two other models with possible low voltage state, but probably being not specific for second breakdown.