Evolution of electrical magnitudes in gradual p-n junctions with deep levels during the emission of majority carriers

A real (nonabrupt) p-n junction has been theoretically analyzed. It consists of a graded profile of shallow dopant atoms and a uniform profile of deep impurities, their relative concentrations varying along the structure (i.e., there are regions where deep impurity concentration is highest and others where dopant concentration dominates). This type of structure was excited by majority-carrier pulses, which allowed us to describe and explain new components in the charge distribution through the junction. The change in the interpretation of results from the application of capacitance techniques to these samples is quite remarkable. The validity of the theory is verified by comparison with experimental results obtained for silicon p-n junctions highly doped with platinum. The detailed analysis of the electrical model of a gradual junction with two deep levels, located in both halves of the band gap has allowed us to explain the following: (a) the disappearance of peaks in deep level transient spectroscopy (DLTS), (b) the existence of both positive and negative signals in a majority-DLTS spectrum, and (c) decreasing capacitance and voltage transients due to the emission of majority carriers or transients in which rising and falling sections are combined. The last two points cannot be explained by using the extended model of p⁺-n or n⁺-p junctions even if the deep-level concentration N_T is assumed to be of the same order as the free carrier concentration.