Semiconductors are electronic devices that are used in many applications. Two types of semiconductors have found use in radiation dosimetry: silicon diodes and metal oxide semiconductors - field effect transistors (MOSFET).
Principles of silicon diodes
Pure silicon crystal is a poor conductor due to strong covalent bonds between adjoining silicon atoms in the lattice. Silicon is doped with another element to produce:
- N-type semiconductors, which contain electron donors (elements with an excess of electrons, to the right on the periodic table)
- P-type semiconductors, which contain electron acceptors (elements with too few electrons, to the left of the periodic table)
P-type semiconductors are said to have 'holes', which is actually the lack of an electron. Like electrons, 'holes' are capable of moving throughout the semiconductor.
A silicon diode contains N-type and P-type semiconductors, divided by a thin band of pure silicon that is non conducting. If ionising radiation strikes the diode, charged particles will be released, allowing current to flow through the dosimeter. Silicon diodes require attachment to a power supply to determine dose.
Uses of silicon diodes
Silicon diodes are small and relatively tough, making them useful in a variety of applications. They are particularly suited to in vivo dosimetry due to their small size. They can also be used for relative dosimetry in other scenarios, using slab or anthropomorphic phantoms.
Principles of MOSFETs
MOSFETS also use P-type and N-type semiconducting silicon, but in a different arrangement. A MOSFET contains two terminals (source and drain) attached to p-type silicon. These terminals are separated from a N-type silicon body which forms the largest part of the transistor. A gate terminal, separate from the source/drain terminals by a insulating layer of silicon dioxide.
For current to flow through the MOSFET (from source to drain) a voltage needs to be applied to the gate terminal. This is known as the threshold voltage.
When the MOSFET is irradiated, charged particles migrate to the margins between the p-type and n-type silicon. This reduces the threshold voltage needed to allow current to flow through the MOSFET. The reduction in voltage is proportional to the amount of radiation the MOSFET was exposed to.
Because the MOSFET is permanently altered by exposure to radiation, it can only absorb so much dose before it is rendered unreadable. This limits the use of MOSFETs to 'single use'.
Uses for MOSFETs
MOSFETs are useful tools for in vivo dosimetry due to their size and portability. Their cost and limited lifespan have limited their role in dosimetry.