O 3 2 6H-SiCZnOAl 3C-SiCAl O 3 2

17. Semiconductor Photon Detectors

Detector zoology

6.0 0.2

AlN Direct gap

Indirect gap

III-Nitrides (c ~ 1.6 a₀)

5.0 _AlN E(eV)=1.24/ λ(㎛)

Theory

p (eV)

0.3

0 4

4.0

th(㎛)

GaN

Zincblend ZnS GaN

Bandga

0.4 0.5 0 6

3.0

2 0 avelengt

AlP

GaP AlAs ZnSe

CdS ZnTe InN

GaN

0.60.7

1.0 2.0

1 0

Wa

InN GaP

GaAs InP

CdSeAlSb CdTe Si

Theory

2.0 0.0 5.0

1.0

6H-SiC ZnO GaSb

InSb Ge

AlO23 InAs

3C-SiC

Al2O3

3 0 3 5 4 0 4 5 5 0 5 5 6 0 6 5

2 5

Lattice Constant (Å)

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

2.5

Photon detection devices

Photons to thermal energy

(phototube)

Metal-Semicon. photoconductor (Schottky-barrier photodiode)

( y p )

The External Photoeffect: Photoelectron Emission

Æ Photogenerated electrons escape from the material as free electrons Æ photoelectrons Æ Photogenerated electrons escape from the material as free electrons. Æ photoelectrons

< Ph t lti li t b (PM t b ) >

metal semiconductor

< Phototube > < Photomultiplier tube (PM tube) >

The Internal Photoeffect: Photoconductivity

Æ Excited 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 Photoconductors

Photodiodes (PD)

Here we will discuss three types of semiconductor photodetectors

Quantum efficiency Responsivity

Photon noise Photoelectron noise

Photodiodes (PD)

Avalanche photodiodes (APD)

Responsivity

Response time. Gain noise

Quantum efficiency of photodetectors

⎡ ⎤

Number of Collected electrons Internal Quantum Efficiency

η_int = Number of Collected electrons = −^⎡_⎣ ⁻α ^⎤_⎦

Number of Photons *Entering* detector 1

e d

External Quantum Efficiency

( )

η_ext = Number of Collected electrons = − ^⎡_⎣ − ^{− ⎤}_⎦ = / Number of Photons *Incident* on detector 1 1

F /

ph o

d i q

R e P

h 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) i h q Responsivity

η ν

= Photo Current (Amps) = = Incident Optical Power (Watts) ^ext

ph o

i q

R P h

→Photocurrent : i_ph = RP_o

Photoconductors

Photoconductors

Photodiodes

n

P +

- i

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 increases across 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 metallic A thin semitransparent metallic

film is used in place of the p-type (or n-type) layer in the p-n junction photodiode.

•Simple to fabricate

•Quantum efficiency:

Medium

Problem: Shadowing of absorption region by contacts

•Capacitance: Low T i d

•Capacitance: Low

•Bandwidth: High

Can 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

Absorption layer

•Compatible with standard electronic processes GaAs 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 solid Digital 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 sensors

CMOS: 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 transfer Horizontal charge transfer

• Output capacitor reset

CCD

H i t l Shift R i t

Output

capacitor Amp Horizontal Shift Register

CCD IMAGERS CCD IMAGERS Qualities

Qualities

■ 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 sensors CMOS image sensors

• Based on

• Based on standard production p

process for CMOS chips,

ll i t ti allows integration with other

components

components.

CMOS IMAGERS CMOS IMAGERS Qualities

Qualities

■ 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 absorption intrinsic 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 :

Æ h Æ Æ e Æ h Æ e Æ …..

The ideal case of single-carrier multiplication is achieved when The ideal case of single-carrier multiplication is achieved when

APD gain