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Remember:
As these excess carriers diffuse under thermal
influences, electrons stray from the n side to the p side, and holes
from the p side to the n side. If an excess electron and an excess hole
collide they will combine together and annihilate. The result will be a region
around the physical junction of the n and p type materials, where the
excess charge carriers have cancelled each other out. This is called the
depletion region.
Use the animation opposite to demonstrate this.
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Each conduction electron that is removed from the n-type
region leaves behind a static ionised positive donor impurity. Similarly
each
hole that is removed from the p-type side leaves behind a static
acceptor site with an extra electron. The combined effect is to build up
a net positive space charge on the n side and a net negative space
charge on the p side. This gives rise to a voltage across the junction called the
contact voltage (0.4 V for germanium). This potential difference is such
that it opposes any further thermal diffusion of electrons and holes.
When a positive voltage is connected to the negative type
semiconductor this is called a reversed bias junction. The width of
the depletion layer can be adjusted by varying this bias voltage.
The depletion region acts like a
high resistivity parallel plate ionisation chamber, making it feasible
to use it for radiation detection. An electrical signal is produced from
interactions taking part in the depletion region of the detector.
See here for
equations...
See here for
semiconductor materials used for gamma-ray detection...
