Contents
1. Basic Physics of Diode as a Radiation Detector
2. Diode detector (P-type, N-type)
3. Photodiode detector (PIN-Photodiode)
1. Basic Physics of Diode as a Radiation Detector
What is Diode
• Made of silicon or germanium
• Low (ideally zero) resistance to current flow in one direc-tion, and high (ideally infinite) resistance in the other
Use of Diode detector in SNUH
EPID(Hydrogenated amorphous silicon
(a-Si:H) PIN photodiode)
SFD diode detector(P-type)
Mapcheck2(N-type)
① Diode ② Photo-diode
CT detector array
History of Diode detector in Radiation Oncology
• Silicon diode detectors have been used in high-energy photon and electron beam dosimetry for many years (from Jones 1963)
• Diodes have gained in popularity since the 1980s
• Now, one of the most useful dosimeters
∴ medical physics understanding is required for users to accurately and effectively use
What is semiconductor• Semiconductor
– Material which has electrical conductivity between conductor and insulator
– Ability to change conductivity by the addition of impurities ("doping")
– Require only a few eV to generate a charge carrier
Radiation detection & measurement, 4/E, Knoll
① N-type semiconductor
• N-type semiconductor
– doped with impurities of 5 valance electrons element P(phosphorous) called a “donor”
– “donor” contribute a free electron to the silicon
– Electrons(-) are the majority carriers– Holes(+) are the minority carriers
Silicon has 4 valence electrons, each making a covalent bond with a neighbor
② P-type semiconductor
• P-type semiconductor
– doped with impurities of 3 valance electrons element B(boron) called an “acceptor”
– “acceptor” contribute a free electron to the silicon
– Holes(+) are the majority carriers – electrons(-) are the minority carriers
P-N junction
• Made of p-type and n-type semiconductor in direct contact• Also called the “depletion region”
Diode detector is made by P-N junction principle
In p-n junction
Majority carriers from each side diffuse to the opposite side(electron in n-side → p-side)
concentration equilibrium
opposite charge ions establish an electric field (“built-in potential” = )
Prevents further diffusion of the majority carriers
2. Diode detector(P-type & N-type)
P-type & N-type diode detector
• N-type diode is formed by doping “acceptor” (3 valance electrons element) into N-type semiconductor
• P-type diode is formed by doping “donor” (5 valance electrons element) into P-type semiconductor
N-type diode P-type diode
SFDMapcheck2
Diode detector is made by P-N junction principle
Need no bias voltage in diode detector
• The typical width of the “depletion region” is less than several m
• “built-in potential” is less than 1 volt, the electric field across the pn junction is very high (greater than V/cm)
∴ The high electric field across the pn junction makes charge collection possible for the diode without external bias
TG-62, 2005 (DIODE IN VIVO DOSIMETRY FOR PATIENTS RE-CEIVING EXTERNAL BEAM RADIATION THERAPY)
How can we measure dose us-ing diode?
Incident ionizing radiation
electron-hole pairs
The minority carriers (electrons on the p-side and holes on the n-side) diffuse toward the opposite side
Measured by the electrometer
+
-
TG-62, 2005 (DIODE IN VIVO DOSIMETRY FOR PATIENTS RE-CEIVING EXTERNAL BEAM RADIATION THERAPY)
3. Photodiode detector(PIN-Photodiode)
What is PIN-Photodiode
• Made of a PIN structure
Intrinsic semiconductor
– In concentration, hole = electron– Intrinsic semiconductor = pure semiconductor
PIN diode is a diode with intrinsic semiconductor region between p-type semiconductor and n-type semiconductor
Diode Operation(Bias)
• Forward bias : – As electrons and holes are pushed toward the junction– “Depletion region” is reduced – Act like “conductor”
• Reverse bias :– As electrons and holes are pulled away from the junction– “Depletion region” widen– Act like “opened circuit” (no leakage current)
→ detect only photo-generated carrier
∴ Reverse bias can be used as Photodiode detector
Physical Principles of PIN photo diode
Reverse bias ↓
radiation greater than the band-gap energy of the material (1.12 eV for Si)
↓Excite a valance band electron
into the conduction band↓
causing a current to flow in an external circuit
Fundamentals of Photonics, 2nd Edition, Saleh
Use of Photodiode detector in Radiation oncology
Hydrogenated amorphous silicon (a-Si:H) P-I-N photodiode
Varian Image Acquisition System 3 Reference guide, 2011
CT detector array
Courtesy of Prof. Kim J-H
EPID(Electronic Potal Imaging Devices)
Advantage of PIN-photodiode
• Sensitive detectors susceptible to visible and infrared radiation(R. Nowotny et al, Nuc. instr. and meth. 147, 1977)
• Used with x- and gamma-ray spectroscopy with a high energy resolution(Takahiko Aoyama et al, Med. Phys 29(7), 2012)
• PIN improve response time and higher efficiency than PN structure
Takahiko Aoyama et al, Med. Phys 29(7), 2012
Visible raylinearity in low dose range.
What can we do?
• Apply the photodiodes in MV energy region– Feasibility study was performed
(Benigno et al, Med. Phys. 41(1), 2014)
• Using commercially available PIN-photodiodes for measuring organ dose in kV energy region– Immediately readout
Discussion & Question
Thank you for your attention
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