7th of Jan, 2014
Diode detector
(PIN photo diode detector)
Hyun Suk Kim
goldsuk89@gmail.com
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
direction, and high (ideally infinite) resistance in the
other
Use of Diode detector in SNUH
① Diode
② Photo-diode
SFD diode detector(P-type)
CT detector array
Mapcheck2(N-type)
EPID
(Hydrogenated amorphous silicon
(a-Si:H) PIN photodiode)
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”
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” = 𝜺)
Prevents further diffusion of the majority carriers
Diode detector is made by
P-N junction principle
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
Diode detector is made by
P-N junction principle
Mapcheck2
N-type diode
SFD
P-type diode
Need no bias voltage in diode
detector
• The typical width of the “depletion region” is less than several μm
• “built-in potential” is less than 1 volt, the electric field across the
pn junction is very high (greater than 103 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
RECEIVING EXTERNAL BEAM RADIATION THERAPY)
How can we measure dose
using 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
RECEIVING EXTERNAL BEAM RADIATION THERAPY)
3. Photodiode detector
(PIN-Photodiode)
What is PIN-Photodiode
• Made of a PIN structure
PIN diode is a diode with intrinsic semiconductor region
between p-type semiconductor and n-type semiconductor
Intrinsic semiconductor
– In concentration, hole = electron
– Intrinsic semiconductor = pure 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
EPID(Electronic Potal Imaging Devices)
CT detector array
Hydrogenated amorphous silicon
(a-Si:H) P-I-N photodiode
Varian Image Acquisition System 3
Reference guide, 2011
Courtesy of Prof. Kim J-H
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
Visible ray
linearity in low dose range.
Takahiko Aoyama et al, Med. Phys 29(7), 2012
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