3/2003 Rev 1 II.3.5 – slide 1 of 23 IAEA Post Graduate Educational Course Radiation Protection and Safe Use of Radiation Sources Session II.3.5 Part IIQuantities and Measurements Module 3Principles of Radiation Detection and Measurement Session 5Semiconductor Detectors

3/2003 Rev 1 II.3.5 – slide 2 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:  N-type  P-type  Intrinsic/Depletion region  Resolution  Efficiency

3/2003 Rev 1 II.3.5 – slide 3 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

3/2003 Rev 1 II.3.5 – slide 4 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

3/2003 Rev 1 II.3.5 – slide 5 of 23 e ee e e e e e e ee e e e e e e ee e e e e e e ee e e e e e e ee e e e e e e ee e e e e e Semiconductor Detectors

3/2003 Rev 1 II.3.5 – slide 6 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 7 of 23 e ee e e e e e e ee e e e e e e ee e e e e e e ee e e e e e e ee e e e e e e ee e e e e e e ExtraElectron N-Type Si Containing Group V Donor Impurity

3/2003 Rev 1 II.3.5 – slide 8 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 9 of 23 e ee e e + e e e ee e e e e e e ee e e e e e e ee e e e e e e ee e e e e e e ee e e e e e P-Type Si Containing Group III Acceptor Impurity PositiveHole

3/2003 Rev 1 II.3.5 – slide 10 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 11 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 12 of 23 Reverse Bias Intrinsic/Depletion Region Cathode (-) Anode (+) Semiconductor Detectors

3/2003 Rev 1 II.3.5 – slide 13 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 14 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 15 of 23 Semiconductor Detectors Gamma rays transfer energy to electrons (principally by compton scattering) and these electrons traverse the intrinsic region of the detector e (+)(+)(+)(+) (-)(-)(-)(-)

3/2003 Rev 1 II.3.5 – slide 16 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 17 of 23 Semiconductor Detectors e ee e e + e e e ee e e e e e e ee e e e e e e ee e e e e e e

3/2003 Rev 1 II.3.5 – slide 18 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 19 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 20 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 21 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 22 of 23 Ge(Li) and Si(Li) Detectors  For gamma ray detection, the detector efficiency for the photoelectric effect is proportional to Z 5, 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 23 of 23 Where to Get More Information  Cember, H., Introduction to Health Physics, 3 rd Edition, McGraw-Hill, New York (2000)  Firestone, R.B., Baglin, C.M., Frank-Chu, S.Y., Eds., Table of Isotopes (8 th 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)