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

V)

0.3

0 4

4.0

th(㎛

)

GaN

ZnSZincblendGaN

Ban

dga 0.4

0.50 6

3.0

2 0 avel

engt

AlP

GaP AlAs

ZnSeCdS ZnTe

InN

GaN

0.60.7

1.0

2.0

1 0

Wa

InNGaP

GaAs InPAlSb

CdSe

CdTeSi

Theory

2.05.0

0.0

1.0

6H-S

iC

ZnO GaSb

InSb

GeInAsA

l 2O3

3C-S

iC

Al 2O

3

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