Photodetectors Lecturer: Mauro Mosca ( University of Palermo –DEIM A.A. 2014-15.

PhotodetectorsPhotodetectors

Lecturer: Mauro MoscaLecturer: Mauro Mosca

(www.dieet.unipa.it/tfl)(www.dieet.unipa.it/tfl)

University of Palermo –DEIM

A.A. 2014-15A.A. 2014-15

PhotodetectorPhotodetector

ThermalThermal ThermoelectricThermoelectric BolometersBolometers PyroelectricPyroelectric

PhotonicPhotonic PhotomultipliersPhotomultipliers PhotoconductorsPhotoconductors PhotovoltaicsPhotovoltaics

Thermoelectric detectorsThermoelectric detectors

- Principle of thermocouples

large electricalconductivities

thermopile

minimize Jouleheating effects

small thermalconductivities

minimize heatconduction losses

BolometersBolometers

current flowing SMALL Why?...current must notraise temperaturetoo much

Pt ~ Ni =

= 0.005 K-1

Sensingelement

Pyroelectric detectorsPyroelectric detectors

ferromagnetic material

molecules with a permanent electrical dipole

- lead zirconate- lithium tantalate

Dispositivi emissivi: catodiDispositivi emissivi: catodi

# emitted electron

# absorbed photons= quantum yield

lowest value for e : Caesium (2.1 eV)

NaKCsSb (S20)

Negative Electron Affinity (NEA)Negative Electron Affinity (NEA)

PhotomultipliersPhotomultipliers

Dynodes (~ 100 V)

+

++

+++

++++

++++++++++++

Photoconductive detectorsPhotoconductive detectors

VBB

RPC

Photoconductive detectors: Photoconductive detectors: application circuitsapplication circuits

se si è interessatisolo alle variazionidi intensità radiante

PCL

LBBO RR

RVV

lumP

lumL

LBBO

P

kR

RVV

lumLPCL P

kRRR Se

lumLBB P

k

RV

segnale d’uscita piccolo! (RL piccola)

lumLPCL P

kRRR Se

BBV

scarsa sensibilità! (RPC piccola)

Photoconductive detectors : Photoconductive detectors : application circuitsapplication circuits

Photoconductive detectors : Photoconductive detectors : application circuitsapplication circuits

The most common method used to extract thesignal is to modulate the incident radiation at a specific frequency by placing a mechanical chopper in front of the sensor or by electrically modulating the radiation source

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either

The signal due to radiation is now an AC signal while the dark current is a DC signal.The AC signal can be separated from the DC background signal using an AC amplifier

Photoconductive detectors: gainPhotoconductive detectors: gainI0

x

=

=

Photoconductive detectors: gainPhotoconductive detectors: gainI0

x

photoconductive gain G = ratio of the rate of flow of electrons per secondto the rate of generation of e--h+ pairs within thedevice

Photoconductive detectors: gainPhotoconductive detectors: gain

Photoconductive detectors: gainPhotoconductive detectors: gain

Se consideriamo che:

Ev he )( L

Vhe )(

v

Lt tr

tr

c

tG

Photoconductive detectors: Photoconductive detectors: responseresponse

rgtr

c

tG

WDLreGi g WDLDh

IeG

0

tr

c

th

Pe

Fotoresistors (LDR)Fotoresistors (LDR)

Large surface

Close electrodes

tr

c

th

Pei

response sensitivity

Photoconductive detectors:Photoconductive detectors:pros and conspros and cons

c

ttr

high

low f

Multiple-quantum well (MQW) Multiple-quantum well (MQW) detectorsdetectors

p-np-n junction detector (photodiode) junction detector (photodiode)

- photovoltaic mode- photoconductive mode

i

I

Silicon photodiodeSilicon photodiode

oppure…

Silicon photodiodeSilicon photodiode

Silicon photodiode: responsivitySilicon photodiode: responsivity

Photodiode materialsPhotodiode materials

• Ge G = 1.88 m

• InxGa1-xAs (x = 0.53) G = 1.68 m

lattice matched to InP

Response time of photodiodesResponse time of photodiodes

• transit time accross the depletion region

• junction capacitance effects

is minimized…

Response time of photodiodesResponse time of photodiodes• carrier diffusion

Noise in photodiodesNoise in photodiodes

Schottky photodiodesSchottky photodiodes

fotoeccitazione elettroni metallo più lunghe

Metal-semiconductor-metal (MSM) Metal-semiconductor-metal (MSM) photodetectorphotodetector

capacità più piccole dispositivi più veloci

Avalanche photodetectors (APD)Avalanche photodetectors (APD)

The guard ring structure is a low doping region where depletion regionextends an appreciable distance into it

In the vicinity of guard ring the total depletion layer is greater (hence themaximum electric field is lower) than in the central region

reduced breakdown no current leakage fron the edge

PhototransistorPhototransistor

VCB < 0

IB

IE = IB

Maggiore sensibilità (mA)

ma…

Minore velocità (s contro i ns dei fotodiodi)

Charge-Coupled DevicesCharge-Coupled Devices(CCD)(CCD)

Charge-Coupled DevicesCharge-Coupled Devices(CCD)(CCD)

Charge-Coupled DevicesCharge-Coupled Devices(CCD)(CCD)

CCD: read-out mechanismsCCD: read-out mechanisms

CCD: read-out mechanismsCCD: read-out mechanisms