1. Diode detector (PIN photo diode detector)
7th of Jan, 2014
Diode detector (PIN photo diode detector)
Hyun Suk Kim

2. Contents Basic Physics of Diode as a Radiation Detector
2. Diode detector (P-type, N-type)
3. Photodiode detector (PIN-Photodiode)

3. 1. Basic Physics of Diode as a Radiation Detector

4. 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

5. Use of Diode detector in SNUH
② Photo-diode
SFD diode detector(P-type)
CT detector array
EPID
(Hydrogenated amorphous silicon
(a-Si:H) PIN photodiode)
Mapcheck2(N-type)

6. 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

7. 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

8. ① 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

9. ② 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

10. 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

11. 2. Diode detector (P-type & N-type)

12. 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
N-type diode
P-type diode
SFD
Mapcheck2

13. 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 V/cm)
∴ The high electric field across the pn junction makes charge collection possible for the diode without external bias
TG-62, (DIODE IN VIVO DOSIMETRY FOR PATIENTS RECEIVING EXTERNAL BEAM RADIATION THERAPY)

14. 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, (DIODE IN VIVO DOSIMETRY FOR PATIENTS RECEIVING EXTERNAL BEAM RADIATION THERAPY)

15. 3. Photodiode detector (PIN-Photodiode)

16. 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

17. 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

18. 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

19. 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

20. 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
linearity in low dose range.
Visible ray
Takahiko Aoyama et al, Med. Phys 29(7), 2012
S , HAMAMATSU(760~1100 nm)

21. 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

22. Discussion & Question

23. Thank you for your attention