Pre-secondary breakdown behaviour of silicon epitaxial diodes

Diodes built in epitaxial layers can exhibit two distinct modes of -behaviour in the avalanche region, prior to the development of secondary breakdown. The mode depends on whether the diode is of the punch-through type when avalanching, i.e. the depletion layer extends to the nn ⁺ substrate-layer interface, or not. A series of epitaxial diodes of the two kinds were made and a method developed for estimation of current density uniformity from light emission of the avalanching diode up to the point of secondary breakdown. In the first case the current density remains uniform across the diode area, including sidewalls, up to the secondary breakdown point, while in the second case it starts to concentrate in the bottom area of the junction, usually at some weak spot, long before secondary breakdown happens. In order to find the reason for this the depletion region was divided into three zones to facilitate analysis, the results of which are used to explain the two modes of behaviour and contribute to the understanding of secondary breakdown in transistors in the cut-off region.