As shown in Figure 2 below, the depletion region increases in the direction
of the weakest field (the area of highest resistivity) as reverse voltage is
applied. This expansion continues as the reverse voltage is increased
until the stress on the field reaches the breakdown threshold.
Epitaxially-grown layers have very abrupt differences in resistivity layers while
deep-diffused devices are less abrupt. This difference creates a variation
in the depletion-region movement; that is, deep diffusion results in depletion-
region movement in the N+ and P+ direction. The abrupt junctions in
epitaxially-grown layers result in depletion-region expansion primarily toward
the N+ layer.
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