- Does your sensor convert the stimulus directly into electric output?
The best energy resolution in modern radiation detectors can be achieved in semiconductor
materials, where a comparatively large number of carriers for a given incident
radiation event occurs. In these materials, the basic information carriers are electron–
hole pairs created along the path taken by the charged particle through the detector.
The charged particle can be either primary radiation or a secondary particle. The
electron–hole pairs in some respects are analogous to the ion pairs produced in the
gas-filled detectors. When an external electric field is applied to the semiconductive
material, the created carriers form a measurable electric current. The detectors operating
on this principle are called a solid-state or semiconductor diode detectors. The
operating principle of these radiation detectors is the same as that of the semiconductor
light detectors. It is based on the transition of electrons from one energy level
to another when they gain or lose energy.
- What steps are required to transform the signal into electric output?
To fabricate a solid-state detector, at least two contacts must be formed across
a semiconductor material. For detection, the contacts are connected to the voltage
source, which enables carrier movement. The use of a homogeneous Ge or Si, however,
would be totally impractical. The reason for that is in an excessively high leakage
current caused by the material’s relatively low resistivity (50 kcm for silicon).When
applied to the terminals of such a detector, the external voltage may cause a current
which is three to five orders of magnitude greater than a minute radiation-induced
electric current. Thus, the detectors are fabricated with the blocking junctions, which
are reverse biased to dramatically reduce leakage current. In effect, the detector is
a semiconductor diode which readily conducts (has low resistivity) when its anode
(p side of a junction) is connected to a positive terminal of a voltage source and the
cathode (an n side of the junction) to the negative. The diode conducts very little (it
has very high resistivity) when the connection is reversed; thus, the name reverse
biasing is implied. If the reverse bias is made very large (in excess of the manufacturer
specified limit), the reverse leakage current abruptly increases (the breakdown
effect), which often may lead to a catastrophic deterioration of detecting properties
or to the device destruction.
- What types of physical effects and energies are involved in the sensing and in transformation?
- How the sensing and transformation can be modeled mathematically or physically?