Diode detector(PIN photo diode detector)

Hyun Suk Kim

goldsuk89@gmail.com

7th of Jan, 2014

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