For low junction capacitance C j, low doping, that is, lightly doped substrate, is required.Epitaxial Structuresįrom the above discussion, we can consider the following requirements : Thus for a small value of series resistance a low-resistivity substrate is needed. For a p-n diffused junction the series resistance due to the p diffused layer will be negligible compared to the resistance due to the n-layer of the junction. The bulk series resistance of a p-n junction is due to the finite resistivity of the p-type and n-type regions of the junction, outside the depletion region. The junction depth should also not be too small especially for the cases in which very high breakdown voltages, typically more than 100 V, are required. Therefore, for a high breakdown voltage a light doping is required. At heavier doings, however, the influence of junction curvature on breakdown voltage becomes less. The breakdown voltage for planar junctions will be somewhat lower than the values obtained from the above equation, especially at the lighter doping levels, where the breakdown voltage can be very substantially less, this is due to the effect of the junction curvature in the region underneath the edges of the oxide window, which results in an increase in the electric field intensity. Notice that in the latter case the depletion region is almost entirely on the lightly doped (substrate) side of the junction and extends very little into the diffused layer side.įor a one sided junction, the breakdown voltage will be a function principal of the doping level on the more lightly doped side of the junction. ![]() ![]() The figure (a) corresponds to a general case of p-n junction, while figure (b) corresponds to a one-sided p-n junction. For almost all diffused p-n junctions the doping on the diffused layer side of the junction will be very much heavier than the doping on the other (substrate) side of the junction, so that most diffused junctions can be considered to be one-sided junctions. The depletion or space-charge region is the region adjacent to the p-n junction that is essentially depleted (or devoid) of all mobile charges (that is, free electrons and holes), so that it acts like an insulator. Before describing the fabrication sequences for ICs, it will be useful to provide insight into the use of epitaxial structures for devices.Ī reverse-biased p-n junction can be considered to be a parallel-plate capacitor with the depletion region being the insulator or dielectric as shown in the figure below.
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