17. Semiconductor Photon Detectors
Transcript of 17. Semiconductor Photon Detectors
17. Semiconductor Photon Detectors
Detector zoology0.26.0
AlN Direct gapIndirect gapIII-Nitrides
(c ~ 1.6 a0)
5.0E(eV)=1.24/ λ(㎛)AlN
Theory
p (e
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CdSe
CdTeSi
Theory
2.05.0
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3 0 3 5 4 0 4 5 5 0 5 5 6 0 6 52 5
Lattice Constant (Å)3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.52.5
Photon detection devices
Photons to thermal energy
(phototube)
Metal-Semicon. photoconductor(Schottky-barrier photodiode)( y p )
The External Photoeffect: Photoelectron EmissionPhotogenerated electrons escape from the material as free electrons photoelectronsPhotogenerated electrons escape from the material as free electrons. photoelectrons
< Ph t lti li t b (PM t b ) >
metal semiconductor
< Phototube > < Photomultiplier tube (PM tube) >
The Internal Photoeffect: PhotoconductivityExcited carriers remain within the material, serve to increase electrical conductivity.
Generation: Absorbed photons generate free carriers (electrons and holes)Generation: Absorbed photons generate free carriers (electrons and holes).Transport: An applied electric field induces these carriers to move,
which results in a circuit current.Amplification: large electric fields enhance the responsivity of the detector.
Here we will discuss three types of semiconductor photodetectors
PhotoconductorsPhotodiodes (PD)
Here we will discuss three types of semiconductor photodetectors
Quantum efficiencyResponsivity
Photon noisePhotoelectron noisePhotodiodes (PD)
Avalanche photodiodes (APD)Responsivity
Response time. Gain noise
Quantum efficiency of photodetectors
⎡ ⎤Number of Collected electronsInternal Quantum Efficiency
αη −⎡ ⎤⎣ ⎦= = −intNumber of Collected electrons 1
Number of Photons *Entering* detectorde
External Quantum Efficiency
( ) αζν
η −⎡ ⎤⎣ ⎦= = − − =ext/Number of Collected electrons
Number of Photons *Incident* on detector1 1
/Fph
o
d i qP
R eh
External Quantum Efficiency
Fresnel loss
S f bi ti ff t
Fraction absorbed in detection region
Surface recombination effect
Responsivity and Response time
Photo Current (Amps) hi q
Responsivity
ην
= = =Photo Current (Amps)Incident Optical Power (Watts) ext
ph
o
i qRP h
→ =: Photocurrent ph oi RP
Photoconductors
Photoconductors
Photodiodes
n
P +- ip
Two operation modes of PN photodiodes
Open circuit (photovotaic)Sh t i it ( h t d ti ) Open-circuit (photovotaic)operation of PDs
Short-circuit (photoconductive)operation of PDs
Open-circuit (photovotaic) operation of PDs
Photovoltage Vp across the device that increasesacross the device that increases with increasing photon flux. This mode of operation is used, for example, in solar cells
Short-circuit operation of PDs
Reverse-biased PDs
p-i-n Photodiodes (PIN PDs)
Heterojunction Photodiodes
Schottky-barrier Photodiodes(M t l i d t PD )(Metal-semiconductor PDs)
A thin semitransparent metallicA thin semitransparent metallicfilm is used in place of the p-type (or n-type) layer in the p-n junction photodiode.
•Simple to fabricate
•Quantum efficiency:MediumProblem: Shadowing of absorption region by contacts
•Capacitance: Low T i d•Capacitance: Low
•Bandwidth: HighCan be increased by thinning absorption layer and
To increase speed, decrease electrode spacing and absorption depth
backing with a non absorbing material. Electrodes must be moved closer to reduce transit time.
•Compatible with standard electronic processes
Absorptionlayer
•Compatible with standard electronic processesGaAs FETS and HEMTs InGaAs/InAlAs/InP HEMTs
Non absorbing substrate
Array Photodiodes : CCD & CMOS
CCD Sensor CMOS Sensor
Conventional Cameras use photographic films to record image.
Digital cameras use a solidDigital cameras use a solid-state device called an image sensor to record image in f f di i l i f i form of digital information.
CCD = Charge Coupled Device. CMOS = Complementary Metal Oxide Semiconductor
Comparison CCD/CMOS sensorsCMOS: low cost CCD : medium to high-end
Source: B. Diericks: CMOS image sensor concepts. Photonics West 2000 Short course (Web)
Charge-coupled devices (CCD)
charge transfer to next pixel cell
CCD (Charge coupled device)CCD (Charge coupled device)• Vertical charge transfer• Horizontal charge transferHorizontal charge transfer• Output capacitor reset
CCD
H i t l Shift R i t
Output capacitor Amp
Horizontal Shift Register
CCD IMAGERSCCD IMAGERS
QualitiesQualities■ Text book performance for all parameters
(QE, read noise, MTF, dark current, linearity, etc.).(QE, read noise, MTF, dark current, linearity, etc.).
Deficiencies■ Low high-energy radiation damage tolerance.
e.g. proton bulk damage and resultant CTE degradation.
■ Significant off-chip electronic support required.
■ Difficulty with high-speed readout (inherently a serial read out device).
CMOS image sensorsCMOS image sensors• Based on• Based on
standard production pprocess for CMOS chips, ll i t tiallows integration
with other componentscomponents.
CMOS IMAGERSCMOS IMAGERS
QualitiesQualities■ Very tolerant to high-energy radiation sources (long life time).
■ On- chip system integration (low power, low weight and compact designs).
■ Hi h d / l i ti■ High speed / low noise operation (inherently a parallel- random access readout device).
Deficiencies■ Currently lacks performance in most areas compared to the CCD
( h ti / ll ti / t f d / t)(charge generation/collection/ transfer and /measurement).
Avalanche Photodiodes (APD)
APD with only one type of carrier (e or h) is desirable.
• High resistivity p-doped layer increases electric field across absorbing region• High-energy electron-hole pairs ionize other sites to multiply the current• Leads to greater sensitivity
light absorptionintrinsic region
( li htl d d i )(very lightly doped p region)
High resistivity p region
larger charge density
APD with only one type of carrier (e or h) is desirable.
: ionization coefficients of e and h
Ionization ratio :
he h e …..
The ideal case of single-carrier multiplication is achieved whenThe ideal case of single-carrier multiplication is achieved when
APD gain