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Semiconductor detector
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The usefulness of semiconductors for radiation detection measurements stems from the special properties created at a junction where n and p-type semiconductors are brought into good thermodynamic contact. To understand gamma-ray detectors it is important to understand p-n junctions.


  • n-type material has an excess of electrons; and 

  • p-type material an excess of holes. 

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.

Have a go Use the animation opposite to demonstrate this.

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...


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