Download Session II305 Semiconductors - International Atomic Energy Agency

Session II.3.5
Part II Quantities and Measurements
Module 3 Principles of Radiation
Detection and Measurement
Session 5 Semiconductor Detectors
3/2003 Rev 1
IAEA Post Graduate Educational Course
Radiation Protection and Safe Use of Radiation Sources
II.3.5 – slide 1 of 23
Semiconductor Detectors
 Upon completion of this section the student will be
able to explain the process and characteristics of
semiconductor detectors including the concepts:
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N-type
P-type
Intrinsic/Depletion region
Resolution
Efficiency
II.3.5 – slide 2 of 23
Semiconductor Diodes
 Semiconductors are typically made of silicon or
germanium
 For portable detectors, silicon is typically used
because the band gap is greater which results in
less thermally generated “noise”
 To reduce this noise in germanium detectors it is
necessary to cool the detectors using liquid nitrogen
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II.3.5 – slide 3 of 23
Semiconductor Detectors
 Silicon forms a crystal that has a diamond
shaped lattice
 Each silicon atom has four covalent bonds
 In the diagram in the next slide, each
covalent bond is represented by a pair of
valence band electrons
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II.3.5 – slide 4 of 23
Semiconductor Detectors
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II.3.5 – slide 5 of 23
Semiconductor Detectors
 There are two types of silicon and
germanium semiconductor detectors, N-type
and P-type
 N-type detectors have an excess of donor
impurities, usually group V elements
 An extra electron is donated at the site of the
impurity resulting in an extra negative
charge
3/2003 Rev 1
II.3.5 – slide 6 of 23
N-Type Si Containing
Group V Donor Impurity
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Extra
Electron
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II.3.5 – slide 7 of 23
Semiconductor Detectors
 P-type detectors have an excess of acceptor
impurities, usually group III elements
 A hole is created at the site of the acceptor
impurity, this results in a positive charge at
the site of the impurity
3/2003 Rev 1
II.3.5 – slide 8 of 23
P-Type Si Containing
Group III Acceptor Impurity
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Positive
Hole
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II.3.5 – slide 9 of 23
Semiconductor Detectors
 The sensitive volume of a diode detector is referred
to as the depletion or intrinsic region
 This is the region of relative purity at a junction of
n-type and p-type semiconductor material
 At this junction, the electrons from the n-type silicon
migrate across the junction and fill the holes in the
p-type silicon to create the p-n junction where there
is no excess of holes or electrons
3/2003 Rev 1
II.3.5 – slide 10 of 23
Semiconductor Detectors
 When a positive voltage is applied to the n-type
material and negative voltage to the p-type material,
the electrons are pulled further away from this
region creating a much thicker depletion region
 The depletion region acts as the sensitive volume of
the detector
 Ionizing radiation entering this region will create
holes and excess electrons which migrate and
cause an electrical pulse
3/2003 Rev 1
II.3.5 – slide 11 of 23
Semiconductor Detectors
Reverse Bias
Anode (+)
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Cathode (-)
Intrinsic/Depletion Region
3/2003 Rev 1
II.3.5 – slide 12 of 23
Semiconductor Detectors
 Diode detectors are often referred to as
“PIN” detectors or diodes. “PIN” is from
P-type, Intrinsic region, N-type
 The intrinsic region is several hundred
micrometers thick
 The intrinsic efficiency (ignoring attenuation
from the housing) is 100% for 10 keV
photons
3/2003 Rev 1
II.3.5 – slide 13 of 23
Semiconductor Detectors
 The efficiency is reduced to approximately
1% for 150 keV photons and remains more or
less constant above this energy
 Above 60 keV, the interactions involve
Compton scattering almost exclusively
3/2003 Rev 1
II.3.5 – slide 14 of 23
Semiconductor Detectors
Gamma rays transfer
energy to electrons
(principally by
compton scattering)
and these electrons
traverse the intrinsic
region of the detector
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II.3.5 – slide 15 of 23
Semiconductor Detectors
 When a charged particle traverses the
intrinsic (depletion) region, electrons are
promoted from the valence band to the
conduction band
 This results in a hole in the valence band
 Once in the conduction band, the electron is
mobile and it moves to the anode while the
positive hole moves to the cathode (actually
it is displaced by electrons moving to the
anode)
3/2003 Rev 1
II.3.5 – slide 16 of 23
Semiconductor Detectors
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II.3.5 – slide 17 of 23
Semiconductor Detectors
 The average energy needed to create an
electron-hole pair in silicon is about 3.6 eV
 The average needed to create an ion pair in
gas is about 34 eV, so for the same energy
deposited, we get about 34/3.6 or about
9 times more charged pairs
3/2003 Rev 1
II.3.5 – slide 18 of 23
Energy Resolution
 The energy resolution in a detector is E/E, which is
proportional to N where N is the number of charged
pairs
 Using a semiconductor detector, we receive about
9, or 3 times the resolution of a gas ionization
detector system
 Compared to a scintillation detector which requires
about 1000 eV to create one photoelectron at the PM
tube, the resolution is about 17 times better
3/2003 Rev 1
II.3.5 – slide 19 of 23
Germanium vs Silicon Detectors
 Germanium (Ge) requires only 2.9 eV to
create an electron-hole pair vs. 3.6 eV for
silicon, so the energy resolution is
(3.6/2.9) = 1.1 times that of silicon
 The problem with Ge is that thermal
excitation creates electron-hole pairs. For
this reason liquid nitrogen is used to cool
the electronics of germanium systems
3/2003 Rev 1
II.3.5 – slide 20 of 23
Ge(Li) and Si(Li) Detectors
 Germanium with lithium ions used to create
the depletion zone form what is known as a
Ge(Li) “jelly” detector
 Silicon with lithium ions used to create the
depletion zone comprise what is known as a
Si(Li) “silly” detector
3/2003 Rev 1
II.3.5 – slide 21 of 23
Ge(Li) and Si(Li) Detectors
 For gamma ray detection, the detector
efficiency for the photoelectric effect is
proportional to Z5, where Z is the atomic
number of the detector material
 Since for Ge, Z=32, and the Z of Si is 14, Ge
detectors are about 62 times more efficient
than Si detectors
3/2003 Rev 1
II.3.5 – slide 22 of 23
Where to Get More Information
 Cember, H., Introduction to Health Physics, 3rd
Edition, McGraw-Hill, New York (2000)
 Firestone, R.B., Baglin, C.M., Frank-Chu, S.Y., Eds.,
Table of Isotopes (8th Edition, 1999 update), Wiley,
New York (1999)
 International Atomic Energy Agency, The Safe Use
of Radiation Sources, Training Course Series No. 6,
IAEA, Vienna (1995)
3/2003 Rev 1
II.3.5 – slide 23 of 23