Formation of a discharge in solid dielectrics

In recent decades theoreticians have unanimously favored theories involving impact ionization by electrons, but experimental evidence in support of this viewpoint appeared much later. The most convincing evidence that electrical breakdown of alkali halide crystals (AHC) in a uniform field is due to impact ionization by electrons is provided, in our opinion, by two experimental facts: i) the breakdown voltage is a minimum at a thickness of about 1-2 ~m [4, 5], which corresponds to the minimum on the Paschen curve for gas breakdown; 2) hot electrons (up to hundreds of electronvolts) are emitted from a crystal situated in a strong electric field [6]. This relates to the case of breakdown of AHC when the cathode has no microprojections with small radius of curvature, which happens when an alcoholic electrolyte is used as an electrode. In [7, 8], however, the possibility of EBSD due to impact ionization by electrons was called in question. The basis for this, according to [7], is that the conduction band of AHC consists of narrow allowed bands (2-3 eV) separated by forbidden gaps [9, i0]. It was inferred from this [7] that an electron cannot be accelerated to the energy of about i0 eV required for impact ionization and, hence, another breakdown mechanism was proposed. It was suggested [7] that strong field domains are formed in a dielectric, as in semiconductors, and that current instability in these domains causes breakdown of the dielectric. According to calculations [7], current electronic instability in AHC is observed at lower fields than those which give rise to impact ionization.