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

Surface Barrier Detectors
The operation of surface barrier and lithium drifted detectors is the same as
for the p-n junction: a depletion region is produced, in which there exists an
electric field. The means of producing the depletion region (as well as its
dimension and location within the crystal) vary from one type of detector to
another. The operation of a surface barrier detector (Fig. 21b) depends on the
surface conditions of the silicon or germanium. At the surface of a piece of
pure crystal, an electric field exists such that both holes and electrons are
excluded from a thin region near the surface. For n type crystals, the field
repels free electrons. If a metal is joined to the crystal, the free electrons are
still repelled but a concentration of holes is produced directly under the
surface. If a reverse bias is then applied, a depletion region is produced.
Surface barrier detectors give better resolution for particle spectroscopy than
p-n junctions but wider depletion regions are possible with the latter. (The
wider the depletion region, the higher the energy of particles can be analyzed
because a particle must expend all its energy in a depletion region.)
Charlie Chong/ Fion Zhang