optical transducers 08

7/30/2019 Optical Transducers 08

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Optical transducers

AKA photodetectors!

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Photodetectors


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Atoms in Solids

Atoms form a lattice structure

The lattice affects the structure of the energy levels of eachatom we now have joint levels for the entire structure

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Band Theory

Three bands of energy levels form

Valence Band most of the electrons are

here Conduction Band electrons here give the

material electrical conductivity

Forbidden Band electrons must jump thisband to get from the valence to theconduction band


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Lattice Bands

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Conduction

In order for an electron to become free andparticipate in current flow, it must gainenough energy to jump over the forbiddenband

For semiconductors at room temperature,there is not enough energy to conduct.

As temperature increases more electronshave the energy to jump the forbidden band

Resistivity decreases

This is the opposite behavior of conductors


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Resistivity

R

T

Conductor

Semiconductor

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Semiconductors When an electron becomes free, it

creates a hole in the lattice structure

A hole is effectively a positive charge


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Electron and Hole Movement

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Intrinsic Semiconductor Elemental or pure semiconductors have

equal numbers of holes and electrons

Depends on temperature, type, and size.

Compound Semiconductors can be formedfrom two (or more) elements (e.g., GaAs)


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Extrinsic Semiconductors

A pure semiconductors where a smallamount of another element is added toreplace atoms in the lattice (doping).

The aim is to produce an excess of eitherelectrons (n-type) or holes (p-type)

Typical doping concentrations are one partin ten million

Doping must be uniform throughout thelattice so that charges do not accumulate

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N-Type and P-Type

One valence electron too many (n-type) Arsenic, antimony, bismuth, phosphorus

One valence electron too few (p-type) Aluminum, indium, gallium, boron


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The PN Junction DiodeStart with a P and N type material.Note that there is excess negativesin the n-type and excess positivesin the p-type

Merge the two some of the negativesmigrate over to the p-type, filling in theholes. The yellow region is called thedepletion zone.

More positivethan rest of N

More negativethan rest of P

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Biasing the Junction

Apply a voltage as indicated. The freecharge carriers (negative charges in the Nmaterial and positive charges in the Pmaterial) are attracted to the ends of thecrystal. No charge flows across the

junction and the depletion zone grows.This is called reverse bias.

Switch polarity. Now the negativecharges are driven toward the junction inthe N material and the positive chargesalso are driven toward the junction in theP material. The depletion zone shrinksand will disappear if the voltage exceedsa threshold. This is called forward bias.


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Photoelectric effect and band gaps

Photoelectric effect

Photons knock electrons into conduction band

Band gap and work function

E = h (h is Plancks constant, is light frequency)

Kmax= h - ( = Work Function, Kmax is max kineticenergy of freed electron)

PN Junction Diode

A diode is formedby interfacing ann-typesemiconductor

with a p-typesemiconductor.

A pn junction isthe interfacebetween n and pregions.

Diode symbol


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Photo Diodes and

PhotodetectorsIf depletion region ofpnjunction diode is illuminated with

light with sufficiently high frequency, photons can provide

enough energy to cause electrons to jump the semiconductor

bandgap to generate electron-hole pairs:

GE

hch

PE ==

h = Plancks constant = 6.626 x 10-34 J-s

= frequency of optical illumination

= wavelength of optical illumination

c = velocity of light = 3 x 108 m/s

Photon-generated current can be used in photodetectorcircuits to generate output voltage

Diode is reverse-biased to enhance depletion-region width

and electric field.

Riov PH=

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Photoresistors

CdS (Cadmium Sulfide) and CdSe (Cadmium Selenide) cells are common

( I ) Directly beneath the conduction band of the CdS crystal is a donor level and there is an acceptor level above the

valence band. In darkness, the electrons and holes in each level are almost crammed in place in the crystal and the

photoconductor is at high resistance.

( II ) When light illuminates the CdS crystal and is absorbed by the crystal, the electrons in the valence band are excited

into the conduction band. This creates pairs of free holes in the valence band and free electrons in the conduction band,

increasing the conductance.

( III ) Furthermore, near the valence band is a separate acceptor level that can capture free electrons only with difficulty,

but captures free holes easily. This lowers the recombination probability of the electrons and holes and increases the number

for electrons in the conduction band for N-type conductance

Goes from M to Ohms

CdS tends to like Yellow...

Condition like FSRs (voltage divider, transimpedance amp, etc.)

Photons knock electrons into conduction band

1 photon can release 900 electrons

Acceptor band keeps electron lifetime high

-> Lower Resistance with increasing light

Slow response...


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Photodiodes

Photons interacting in the depletion region produce electron-hole

pairs Electrons diffuse through depletion region, driven by the E-field, to

arrive at the N layer and electrode, producing current.

Depletion region bigger (more reverse bias)

More efficient (higher probability of photon interaction)

Faster (charge doesnt have to diffuse across longer lengths before it hits E-field, hence less charge stored, hence smaller capacitance)

PIN diodes increase the collection area - faster response

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apac ance an

Response

Silicon has more than 2x response in IR

Reverse bias lowers capacitance (makes device faster, extends sensitivity)


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uan um

Efficiency

Quantum efficiency (QE) is a figure given for a photosensitive device(charge-coupled device (CCD), for example) which is the percentage ofphotons hitting the photoreactivesurface that will produce an electron-holepair. It is an accurate measurement of the device's sensitivity.

Signal-to-noise ratio is also important!!

Thinned silicon

- More efficient,

esp. at short

- Rear illumination

better, as the

pads/traces in front

get in the way

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Photoconductive and photovoltaic

operation

Photovotaic - produces voltage withillumination

Typical photodiode (.2 mm2) produces 30 Ain sunlight and 30 pA on a clear moonlessnight)

Photoconductive - increasing current withillumination

Reverse bias voltage applied

Much faster, more sensitive

Dark current threshold at zero illumination


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o o o e

ICs

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The Phototransistor

Like diodes, all transistors are light-sensitive. Phototransistors aredesigned specifically to take advantage of this fact. The most-common

variant is an NPN bipolar transistor with an exposed base region. Here,light striking the base replaces what would ordinarily be voltage applied tothe base -- so, a phototransistor amplifies variations in the light striking it.Note that phototransistors may or may not have a base lead (if they do, thebase lead allows you to bias the phototransistor's light response.

Phototransistors run in the photoconductive mode

Theyre pretty slow, on average (e.g., Khz response)

But give a fair amount of gain and are very easy to use.

Generally ground emitter and provide a collector resistor to setgain

Photodarlingtons give more gain, but can be slower

http://encyclobeamia.solarbotics.net/articles/phototransistor.html


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Avalanche Photodiode (APD)

High voltage across detector produces

avalanche gain (e.g., factor 300) Very Sensitive and Fast response (e.g., ns)

Voltage tends to be large to get enough voltage

100-400 volts are standard

Newer devices run at about 30 volts or so...

Photoelectrons

generated in E1 and

doping in region E2

generates avalanche

gain from collisions

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Vacuum Photodiodes

Accelerating, focusedelectric field acts aslens and providesavalanche gain atdiode


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Photomultiplier Tubes

Most sensitive photodetector

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Use a Lens to increase sensitivity

Lens collects light over increased area

Effective gain at optical wavelengths isroughly k = 0.92 (A/a)

A is area of lens, a is area of detector


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Lateral Effect Photodiodes

Also called photopots - photoelectrons

create current that divides in proportion to

position

- Made by UDT, Hammamatsu, etc.

- Somewhat expensive...

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Imagers

CCD = Charge Coupled Device

CMOS = Complementary Metal Oxide Semi-

Conductor

CCD = specialized production plant and process

CMOS = standard silicon production line

Large-arrayphotodetectors suitable for creating images


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Basic Mechanism of CCD

Image Sensors

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Light

Charge

(Electrons)

Photo Sensor(b)(Light-sensitive Region)

Vertical CCD (c)

Horizontal CCD (d)Ampl ifi er(x)

CCD Image Sensor

Pixel (a)

Output


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Charge Transfer from Photo

Sensor to Vertical CCD

Like Water Draining from a Dam

Vertical CCD

Light

Photo Sensor

Gate

Charge

(Electrons)Gate Opens

Charge

(Electrons)

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Charge Transfer by CCD in a Bucket-brigade Fashion

CCD CCDCCDCCD

Charge Charge Charge Charge


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Gate Gate

Output

Gate Gate

Horizontal CCD

Floating Diffusion (FD)

Ampl if ier of CCD Image Sensor

Charge

Micro Wire

Voltage

Generated on

Surface of FD

Ampli fier

Output

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Basic Mechanism of CMOS

Image Sensors


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Photo Sensor(b)(Light-sensitive Region)

Light

Charge

Signal

Ampli fier(y)Pixel (a)

Column Signal Wire (f)(Micro Wire)

Pixel-select Switch (e)

ONON ON

Output

ON

CMOS Image Sensor

Column

Circuit (h)Row Signal Wire (i)(Micro Wire)

Column-select Swi tch (g)

Pixel Row (j)

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Voltage Generated on Surface of Photo Sensor

Like the Ris ing Water Level of a Bucket

High

0 V

Voltage

Surface

Voltage

Photo Sensor

Light

Surface Voltage

to Amplifier

High

0 V

Voltage

Surface

Voltage

Photo Sensor

Surface Voltage

to Amplifier

When Charge is NOT Accumulated

in Photo Sensor

When Charge is Accu mulated

in Photo Sensor

Fig. A Fig. B

Charge


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Pros and Cons of CCD and

CMOS Image Sensors

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Pros and Cons

nonedepends poor to excellentVertical Smear

lowhighPower

higherModerate to highSpeed

ModerateHighDynamic Range

low to ModerateHighUniformity

slightly betterModerateResponse

HigherLowerRelative R&D Cost

HighLowSensor Complexity

Higher thanexpectedHighSystem Complexity

ModerateLowSystem Noise

Bits (digital)Voltage (analog)Signal out of Chip

VoltageElectron PacketSignal out of Pixel

CMOSCCDFeature


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Fill Factor = sensitivity

CCD CMOS

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(a) General block diagram of

an optical instrument.

(b) Highest efficiency is

obtained by using an

intense lamp, lenses to

gather and focus the light

on the sample in the

cuvette, and a sensitive

detector.

(c) Solid-state lamps and

detectors may simplify thesystem.

Optical instrumentation


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Light sources and detectors

Sources

Incandescent bulb

Light emitting diode

(LED)

Gas and solid state

lasers Arc lamp

Fluorescent source

Detectors

Thermal detector

(pyroelectric)

Photodiode

Phototransistor

Charge-coupleddevice (CCD)

Photoconductive cell

Photomultiplier tube

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Block diagram of a single beam spectrophotometer. Theprism serves as the dispersing device while themonochromator refers to the dispersing device (prism),entrance slit, and exit slit. The exit slit is moveable in thevertical direction so that those portions of the powerspectrum produced by the power source (light source)that are to be used can be selected.

Spectrophotometer

optical transducers 08

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