Understanding Neutron Radiography Post Exam Reading VIII-Part 2a of 2A

Detector
The diffused p-n junction detector (Fig. 21a) gets its name from its
manufacturing process. A slice of p type (electron depleted) silicon or
germanium crystal, with a layer of n type (electron rich) impurity (usually
phosphorus) deposited on the surface, is heated to form a p-n junction just
below the surface. The phosphorus may also be painted onto the silicon and
made to diffuse into it by applying heat. Because the n type material has an
excess of electrons and the p type has an excess of holes (holes may be
thought of as unit positive charges), the natural action of the combined
materials tends to align the electrons on one side of the junction and the
holes on the other. Thus a difference of potential is built up across the
junction. By applying an external voltage to the crystal of such polarity as to
oppose the natural movement of electrons and holes (reverse bias), the
potential barrier across the junction is increased and a depletion region is
produced.
Charlie Chong/ Fion Zhang