TransistorsTransistors• History• Transistor Types

• BJT: A bipolar (junction) transistor is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications • FET :The field-effect transistor (FET) relies on an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material • Power transistors

What is a Transistor?

A Transistor is an electronic devicecomposed of layers of a semiconductormaterial which regulates current orvoltage flow and acts as a switch or gatefor electronic circuit.

History of the TransistorJohn Pierce –supervised the Bell John Pierce –supervised the Bell Labs team which built the first Labs team which built the first transistor (1947)transistor (1947)First Solid State Transistor – (1951) Gordon K. Teal (left) and Morgan Sparks at Bell Laboratories, 1951

Akio Morita, who founded a new Akio Morita, who founded a new company named Sony Electronics company named Sony Electronics that mass-produced tiny that mass-produced tiny transistorized radios (1961)transistorized radios (1961)

Processor development followed Moore’s Law

Applications

SwitchingAmplificationOscillating CircuitsSensors

Composed of N and P-type Semiconductors

• N-type Semiconductor has an excess of electrons– Doped with impurity with more valence electrons than silicon

P-type Semiconductor has a deficit of electrons (Holes)– Doped with impurity with less valence electrons than silicon

P-N Junction (Basic diode):

- Bringing P and N Semiconductors in contact

P Type N Type- Creation of a Depletion Zone

Reverse Biased => No Current

Applying –Voltage to Anode increasesBarrier Voltage & Inhibits Current Flow

• Applying Voltage to Cathode

• Barrier Voltage to Anode allows current flow

Forward Biased => Current

Types Of TransistorsTypes Of TransistorsNPN: transistor where the majority current carriers are electrons

The majority current carriers in the PNP transistor are holes

Transistor Operation Transistor Operation

California Test QuestionsCalifornia Test Questions

A transistor circuit is used as an amplifier. When a signal is applied to the input of the transistor, the output signal is A a smaller amplitude. B an equal amplitude. C a larger amplitude. D zero amplitude.

C: The collector and emitter will amplify the output signal from the Bias input.

California Test QuestionsCalifornia Test Questions

A transistor is classified as a semiconductor because: A the transistor conducts electricity. B the transistor increases the amplitude. C the transistor increases the frequency. D intentionally introducing impurities into an

extremely pure silicon or germanium. silicon or germanium. D intentionally introducing impurities into an extremely pure silicon or germanium.silicon or germanium.

SummarySummary

Transistors are composed of three parts – a Transistors are composed of three parts – a base, a collector, and an emitter .base, a collector, and an emitter .Semi-conductive materials are what make the Semi-conductive materials are what make the transistor possible .transistor possible .There are two main types of transistors-junction There are two main types of transistors-junction transistors and field effect transistors.transistors and field effect transistors.Field effect transistor has only two layers of Field effect transistor has only two layers of semiconductor material, one on top of the other semiconductor material, one on top of the other

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Application of photodiodes

A brief overview

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Diode devices• Check valve behavior

– Diffusion at the PN junction of P into N and N into P causes a depleted non-conductive region

– Depletion is enhanced by reverse bias

– Depletion is broken down by forward bias

• When forward biased– High current flow junction

voltage• When reverse biased

– Very low current flow unless above peak inverse voltage (PIV) (damaging to rectifying diodes, OK for zeners)

D1

cathode-

anode+

+ -

Depletion region

1N412

Diode

Schematic Symbol

SemiconductorElements

TypicalComponentAppearance

P -doped

N -doped

Breakdownvoltage(PIV)

V

I

JunctionVoltage

0.7 - silicon0.3 - germanium

Forwardbias

current

Reversebias

current

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Quantum devices

• Absorption of a photon of sufficient energy elevates an electron into the conduction band and leaves a hole in the valence band.

• Conductivity of semi-conductor is increased.• Current flow in the semi-conductor is induced.

Conduction band

Energy gap

Valence band

Energylevel

+

-

Photon(hv)

Hole

Electron

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Photodiode structure

n- region

p+ Active AreaInsulation

Depletion region

Back Metalizationn+ Back Diffusion

FrontContact

RearContact

Incident light

Absorbtion in the depletion layer causses current to flow across the photodiode and if the diode is reverse biased considerable current flow will be induced

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Photodiode fundamentals

• Based on PN or PIN junction diode– photon absorption in the depletion

region induces current flow– Depletion layer must be exposed

optically to source light and thick enough to interact with the light

• Spectral sensitivity

Material Band gap (eV)

Spectral sensitivity

silicon (Si) 1.12 250 to 1100 nm

indium arsenide (InGaAs) ~0.35 1000 to 2200 nm

Germanium (Ge) .67 900 to 1600 nm

I

P

N

+

-

h

RLIL

electron

hole

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Photodiode characteristics

• Circuit model– I0 Dark current (thermal)

– Ip Photon flux related current

• Noise characterization– Shot noise (signal current related)

– q = 1.602 x 10–19 coulombs– I = bias (or signal) current (A)– is = noise current (A rms)

– Johnson noise (Temperature related)– k = Boltzman’s constant = 1.38 x 10–23 J/K– T = temperature (°K)– B = noise bandwidth (Hz)– R = feedback resistor (W)– eOUT = noise voltage (Vrms)

qiis 2

kTBReout 4

Ip Rj Cj

Rs

I0

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Photodiode current/voltage characteristics

Isc (light level dependent)

CurrentV

olta

ge

Increasing Light level

Dark current

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Trans-impedance amplifier function

• Current to voltage converter (amplifier)• Does not bias the photodiode with a voltage as

current flows from the photodiode (V1 = 0)• Circuit analysis

sf II

0oI

01 V

sffff IRIRV +

-

+

-

IsVout

Vf

Io

+

IfV1

sffout IRVV

–Note: current to voltage conversion

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Diode operating modes

• Photovoltaic mode– Photodiode has no bias voltage– Lower noise– Lower bandwidth– Logarithmic output with light intensity

• Photoconductive mode– Higher bandwidth– Higher noise– Linear output with light intensity

+

-

+

-

Vout

+

-

+

-

VoutVs-

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For the photovoltaic mode

• I = thermal component + photon flux related current

• where I = photodiode currentV = photodiode voltageI0 = reverse saturation current of diode

e = electron chargek = Boltzman's constantT = temperature (K) = frequency of lighth = Plank’s constantP = optical power = probability that hv will elevate an electron across the band gap

hePeII kT

eV

10

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Circuit Optimization

• Burr-Brown recommendations (TI)• Photodiode capacitance should be as low as possible.• Photodiode active area should be as small as possible so that

CJ is small and RJ is high.• Photodiode shunt resistance (RJ ) should be as high as

possible.• For highest sensitivity use the photodiode in a “photovoltaic

mode”.• Use as large a feedback resistor as possible (consistent with

bandwidth requirements) to minimize noise.• Shield the photodetector circuit in a metal housing.• A small capacitor across Rf is frequently required to suppress

oscillation or gain peaking.• A low bias current op amp is needed to achieve highest

sensitivity

THE LIGHT EMITTING THE LIGHT EMITTING DIODEDIODE

Chapter 6Chapter 6

CB

VB

When the electron When the electron falls down from falls down from conduction band and conduction band and fills in a hole in fills in a hole in valence band, there is valence band, there is an obvious loss of an obvious loss of energy.energy.

The question is; The question is; where does that energy go?where does that energy go?

In order to achieve a In order to achieve a reasonable efficiency reasonable efficiency for photon emission, for photon emission, the semiconductor the semiconductor must have a direct must have a direct band gap.band gap.

CB

VB

The question is; The question is; what is the mechanism what is the mechanism

behind photon emission in LEDs?behind photon emission in LEDs?

For example;For example;SiliconSilicon is known as an is known as an indirect band-gap indirect band-gap

material.material.

as an electron goes from the bottom of as an electron goes from the bottom of the conduction band to the top of the the conduction band to the top of the valence band;valence band;

it must also undergo a it must also undergo a significant significant change in change in momentum.momentum.

CB

VB

What this means is thatWhat this means is that

E

k

As we all know, whenever something changesAs we all know, whenever something changes state, one must conserve not only energy, but also state, one must conserve not only energy, but also

momentum.momentum. In the case of an electron going from conduction In the case of an electron going from conduction

band to the valence band in silicon, both of these band to the valence band in silicon, both of these things can only be conserved:things can only be conserved:

The transition also creates a quantized set of lattice vibrations, called phonons, or "heat“ .

Phonons possess both energy and momentum.Phonons possess both energy and momentum. Their creation upon the recombination of an Their creation upon the recombination of an

electron and hole allows for complete electron and hole allows for complete conservation of both energy and momentum. conservation of both energy and momentum.

All of the energy which the electron gives up in All of the energy which the electron gives up in going from the conduction band to the valence going from the conduction band to the valence band (1.1 eV) ends up in phonons, which is band (1.1 eV) ends up in phonons, which is another way of saying that the electron heats up another way of saying that the electron heats up the crystal.the crystal.

In a class of materials called In a class of materials called direct band-gap direct band-gap semiconductorssemiconductors; ;

the transition from conduction band the transition from conduction band to valence band involves essentially to valence band involves essentially no change in momentumno change in momentum..

Photons, it turns out, possess a fair Photons, it turns out, possess a fair amount of energy ( several eV/photon amount of energy ( several eV/photon in some cases ) but they have very in some cases ) but they have very little momentum associated with little momentum associated with them.them.

Thus, for a direct band gap material, the excess Thus, for a direct band gap material, the excess energy of the electron-hole recombination can energy of the electron-hole recombination can either be taken away as heat, or more likely, as either be taken away as heat, or more likely, as a photon of light.a photon of light.

This radiative transition then This radiative transition then conserves energy and momentum conserves energy and momentum by giving off light whenever an by giving off light whenever an electron and hole recombine.electron and hole recombine. CB

VB

This gives rise to This gives rise to (for us) a new type (for us) a new type of device;of device;

the light emitting diode (LED).the light emitting diode (LED).

Mechanism is “injection Mechanism is “injection Electroluminescence”. Electroluminescence”. Luminescence Luminescence part tells us that we are producing photons.part tells us that we are producing photons.

Electro part tells us that Electro part tells us that the photons are being produced the photons are being produced by an electric current.by an electric current.

e-

Injection tells us that Injection tells us that photon production is by photon production is by the injection of current carriers.the injection of current carriers.

Mechanism behind photon Mechanism behind photon emission in LEDs?emission in LEDs?

e-

Producing photonProducing photon

Electrons recombine with holes.Electrons recombine with holes.

Energy of photon is the energy of Energy of photon is the energy of band gap.band gap.

CB

VB

e-

h

Method of injectionMethod of injection We need putting a lot of eWe need putting a lot of e--’s where there are lots ’s where there are lots

of holes.of holes. So electron-hole recombination can occur.So electron-hole recombination can occur. Forward biasing a p-n junction will inject lots of eForward biasing a p-n junction will inject lots of e--’s ’s

from n-side, across the depletion region into the p-from n-side, across the depletion region into the p-side where they will be combine with the high side where they will be combine with the high density of majority carriers.density of majority carriers.

n-side

p-side

-+

I

Notice that:Notice that: Photon emission occurs whenever we have Photon emission occurs whenever we have

injected minority carriers recombining with the injected minority carriers recombining with the majority carriers.majority carriers.

If the eIf the e- - diffusion length is greater than the hole diffusion length is greater than the hole diffusion length, the photon emitting region will diffusion length, the photon emitting region will be bigger on the p-side of the junction than that be bigger on the p-side of the junction than that of the n-side.of the n-side.

Constructing a real LED may be best to consider Constructing a real LED may be best to consider a na n++++p structure.p structure.

It is usual to find the photon emitting volume It is usual to find the photon emitting volume occurs mostly on one side of the junction region.occurs mostly on one side of the junction region.

This applies to LASER devices as well as LEDs.This applies to LASER devices as well as LEDs.

MATERIALS FOR LEDSMATERIALS FOR LEDS The semiconductor bandgap The semiconductor bandgap

energy defines the energy of the energy defines the energy of the emitted photons in a LED.emitted photons in a LED.

To fabricate LEDs that can emit To fabricate LEDs that can emit photons from the infrared to the photons from the infrared to the ultraviolet parts of the e.m. ultraviolet parts of the e.m. spectrum, then we must consider spectrum, then we must consider several different material several different material systems.systems.

No single system can span this No single system can span this energy band at present, although energy band at present, although the 3-5 nitrides come close.the 3-5 nitrides come close.

CB

VB

Unfortunately, many of potentiallly useful 2-6 Unfortunately, many of potentiallly useful 2-6 group of direct band-gap semiconductors group of direct band-gap semiconductors (ZnSe,ZnTe,etc.) (ZnSe,ZnTe,etc.) come naturally doped come naturally doped either p-either p-type, or n-type, but they don’t like to be type-type, or n-type, but they don’t like to be type-converted by overdoping.converted by overdoping.

The material reasons behind this are The material reasons behind this are complicated and not entirely well-known.complicated and not entirely well-known.

The same problem is encountered in the 3-5 The same problem is encountered in the 3-5 nitrides and their alloys InN, GaN, AlN, InGaN, nitrides and their alloys InN, GaN, AlN, InGaN, AlGaN, and InAlGaN. The amazing thing about AlGaN, and InAlGaN. The amazing thing about 3-5 nitride alloy 3-5 nitride alloy systems is that appear to be systems is that appear to be direct gap direct gap throughout.throughout.

When we talk about light ,it is conventional to When we talk about light ,it is conventional to specify its wavelength, specify its wavelength, λλ, instead of its , instead of its frequency. frequency.

Visible light has a wavelength on the order of Visible light has a wavelength on the order of nanometers. nanometers.

Thus, a semiconductor with a 2 eV band-gap should give a light at about 620 nm (in the red). A 3 eV band-gap material would emit at 414 nm, in the violet.

The human eye, of course, is not equally responsive to all colors.

( )( )hcnm

E eV

1242( )( )

nmE eV

Relative response of the human eye to various colors

350 400 450 500 550 600 650 700 750

100

10-1

10-2

10-3

10-4

Relative eye responseRelative eye response

Wavelength in nanometers

The materials which are used for important light emitting diodes (LEDs) for each of the different spectral regions.

GaN

GaN

ZnSe

ZnSe

violet blueG

aP:N

GaP

:Ngreen yellow

GaA

sG

aAs .1

4.1

4pp 8686

GaA

sG

aAs .3

5.3

5pp 6565

redorange

GaA

sG

aAs .6.6

pp 44

Properties of InGaNProperties of InGaN InGaN alloy has one composition at a time only.InGaN alloy has one composition at a time only. This material This material will emit one wavelengthwill emit one wavelength only only

corresponding to this particular composition.corresponding to this particular composition. An InGaN LED would An InGaN LED would not emit white light not emit white light (the (the

whole of the visible spectrum at once) since its whole of the visible spectrum at once) since its specific compositionspecific composition. .

For a For a white light sourcewhite light source we have to form a we have to form a complicated multilayercomplicated multilayer device device emitting lots of emitting lots of different wavelengths. different wavelengths.

Properties of InGaNProperties of InGaNA LED fabricated A LED fabricated in a in a graded materialgraded material

where on either side of the junction region where on either side of the junction region the the material changes slowly from InN to material changes slowly from InN to GaNGaN via InGaN alloys. via InGaN alloys.

Minority carriers need to get through the Minority carriers need to get through the whole of this alloy region if efficient photon whole of this alloy region if efficient photon production at all visible wavelengths was production at all visible wavelengths was to occur. to occur.

GaNGaN InNInN Concentration:Concentration:

The highly The highly gallium rich gallium rich

alloyalloy

The highly The highly indium rich indium rich

alloyalloy

Band gap:Band gap: 3.3eV3.3eV 2 eV2 eVWavelength of Wavelength of photons:photons:

376 nm376 nm 620 nm620 nm

Part of the Part of the electromagnetic electromagnetic spectrum:spectrum:

In the ultravioletIn the ultraviolet In the visible In the visible (orange)(orange)

GaNInN

3.3 eV(376 nm)ultraviolet

GaNInN

3 eV (414 nm)violet

GaNInN

2.7 eV(460 nm)

GaNInN

2.4 eV(517 nm)

GaNInN

2.1 eV(591 nm)

GaN

InN

2.00 eV

2 eV(620 nm)

A number of the important LEDs are based on the GaAsP system.GaAsGaAs is a direct band-gap S/C with a band gap of 1.42 eV1.42 eV (in the

infrared). GaPGaP is an indirect band-gap material with a band gap of 2.26 eV2.26 eV

(550nm, or green).

GaAsGaP

1.42 eV

GaAsGaP

1.52 eV

GaAsGaP

1.62 eV

GaAsGaP

1.72 eV

GaAsGaP

1.80 eV

GaAsGaP

1.90 eV

GaAsGaP

2.00 eV

GaAsGaP

2.26 eV

_

h

+

Ener

gy

Momentum

• Addition of a nitrogen recombination center to indirect GaAsP .

Both As and P are group V elements. (Hence the nomenclature of the materials as III-V compound semiconductors.)

We can replace some of the As with P in GaAs and make a mixed compound semiconductor GaAs1-xPx.

When the mole fraction of phosphorous is less than about 0.45 the band gap is direct, and so we can "engineer" the desired color of LED that we want by simply growing a crystal with the proper phosphorus concentration!

X CB

Minimum

Γ VB

Maximum

N Level

Γ CB

Minimum

(a) Direct-gap GaAs

N Level

(b) Crossover GaAs0.50P0.50

N Level

(c) Indirect-gap GaP

Schematic band structure of GaAs, GaAsP, and GaP. Also shown is the nitrogen level. At a P mole fraction of about 45-50 %, the direct-indirect crossover occurs.

Materials for visible wavelength LEDsMaterials for visible wavelength LEDs

We see them almost everyday, either on calculator We see them almost everyday, either on calculator displays or indicator panels.displays or indicator panels.

Red LED use as “ power on” indicatorRed LED use as “ power on” indicator Yellow, green and amber LEDs are also widely available Yellow, green and amber LEDs are also widely available

but very few of you will have seen a blue LED.but very few of you will have seen a blue LED.

Red LEDsRed LEDs

can be made in the GaAsP can be made in the GaAsP (gallium arsenide phosphide). (gallium arsenide phosphide).

GaAsGaAs1-x1-xPPxx

for 0<x<0.45 has direct-gapfor 0<x<0.45 has direct-gap for x>0.45 the gap goes for x>0.45 the gap goes

indirect andindirect and for x=0.45 the band gap for x=0.45 the band gap

energy is 1.98 eV.energy is 1.98 eV. Hence it is useful for red Hence it is useful for red

LEDs.LEDs.

N-GaAs substrate

N-GaAsP P = 40 %p-GaAsP region

Ohmic ContactsDielectric(oxide or nitride)

Fig. GaAsP RED LED on a GaAs sub.

Isoelectronic CentreIsoelectronic Centre

IsoelectronicIsoelectronic means that t means that the centre being introduced he centre being introduced has the has the same number of valance electrons same number of valance electrons as the element it is as the element it is replacing.replacing.

For example, nitrogen can replace some of the phosphorus For example, nitrogen can replace some of the phosphorus in GaP. It is isoelectronic with phosphorus, but in GaP. It is isoelectronic with phosphorus, but behaves behaves quite differentlyquite differently allowing reasonably efficient green emission. allowing reasonably efficient green emission.

How isoelectronic centres work?How isoelectronic centres work? For our isoelectronic centre For our isoelectronic centre

the the position is very well-position is very well-defined,defined, hence there is a hence there is a considerable considerable spread in its spread in its momentum momentum state.state.

Isoelectronic centre has the Isoelectronic centre has the same valance configuration same valance configuration as the phosphorus it is as the phosphorus it is replacing.replacing.

It doesn't act as a dopantIt doesn't act as a dopant..

E

dE

Isoelectroniccentre

CB edgeelectrons

Electron-hole recombination

Holes

VB edge

k = 0

Isoelectronic centres provide a ‘Isoelectronic centres provide a ‘stepping stonestepping stone’ for ’ for electrons in E-k space so that transitions can occur electrons in E-k space so that transitions can occur that are radiatively efficient.that are radiatively efficient.

The recombination event The recombination event shown has no change in shown has no change in momentum, so it behaves momentum, so it behaves like a direct transition. like a direct transition.

E

dE

Isoelectroniccentre

CB edgeelectrons

Electron-hole recombination

Holes

VB edge

k = 0

Because the effective transition is occurring between Because the effective transition is occurring between the isoelectronic centre and VB edge, the photon that the isoelectronic centre and VB edge, the photon that is emitted has a lower energy than the band-gap is emitted has a lower energy than the band-gap energy.energy.

GaP : NGaP : N

(dE = 50 meV) Photon energy (dE = 50 meV) Photon energy is less than the semiconductor is less than the semiconductor band-gap energy it means that band-gap energy it means that the photon is not absorbed by the photon is not absorbed by the semiconductor, and so the the semiconductor, and so the photon is easily emitted from photon is easily emitted from the material.the material.

This lack of absorption pushes This lack of absorption pushes up the efficiency of the diode up the efficiency of the diode as a photon source.as a photon source.

E

dE

Isoelectroniccentre

CB edgeelectrons

Electron-hole recombination

Holes

VB edge

k = 0

50 meV

For emission in the red part of the spectrum using GaP For emission in the red part of the spectrum using GaP the isoelectronic centre introduced contathe isoelectronic centre introduced contaiins zinc (Zn) and ns zinc (Zn) and oxygen (O). These red LEDs are usually designated oxygen (O). These red LEDs are usually designated GaP:ZnO and they are quite efficient. GaP:ZnO and they are quite efficient.

Their main drawback is that their emission at 690 nm is in Their main drawback is that their emission at 690 nm is in a region where the eye sensitivity is rather low, which a region where the eye sensitivity is rather low, which means that commercially, the AlGaAs/GaAs diodes are means that commercially, the AlGaAs/GaAs diodes are more successful devices.more successful devices.

Orange (620 nm) and yellow (590 nm) LEDs are Orange (620 nm) and yellow (590 nm) LEDs are commercially made using the GaAsP system. However, commercially made using the GaAsP system. However, as we have just seen above, the required band-gap as we have just seen above, the required band-gap energy for emission at these wavelengths means the energy for emission at these wavelengths means the GaAsP system will have an indirect gap. GaAsP system will have an indirect gap.

The isoelectronic centre used in this instance is nitrogen, The isoelectronic centre used in this instance is nitrogen, and the different wavelengths are achieved in these and the different wavelengths are achieved in these diodes by altering the phosphorus concentration.diodes by altering the phosphorus concentration.

The green LEDs (560 nm) are manufactured using the The green LEDs (560 nm) are manufactured using the GaP system with nitrogen as the isoelectronic centre.GaP system with nitrogen as the isoelectronic centre.

Orange-Yellow & Green LEDsOrange-Yellow & Green LEDs

Blue LEDsBlue LEDs Blue LEDs are commercially available and are fabricated Blue LEDs are commercially available and are fabricated

using silicon carbide (SiC). Devices are also made using silicon carbide (SiC). Devices are also made based on gallium nitride (GaN).based on gallium nitride (GaN).

Unfortunately both of these materials systems have Unfortunately both of these materials systems have major drawbacks which render these devices inefficient.major drawbacks which render these devices inefficient.

The reason silicon carbide has a low efficiency as an The reason silicon carbide has a low efficiency as an LED material is that it has an indirect gap, and no ‘magic’ LED material is that it has an indirect gap, and no ‘magic’ isoelectronic centre has been found to date.isoelectronic centre has been found to date.

Blue LEDsBlue LEDs

The transitions that give rise to blue photon emission in The transitions that give rise to blue photon emission in SiC are between the bands and doping centres in the SiC are between the bands and doping centres in the SiC. The dopants used in manufacturing SiC LEDs are SiC. The dopants used in manufacturing SiC LEDs are nitrogen for n-type doping, and aluminium for p-type nitrogen for n-type doping, and aluminium for p-type doping. doping.

The extreme hardness of SiC also requires extremely The extreme hardness of SiC also requires extremely high processing temperatures.high processing temperatures.

Gallium Nitride (GaN)Gallium Nitride (GaN)

Gallium nitride has the advantage of being a direct-gap Gallium nitride has the advantage of being a direct-gap semiconductor, but has the major disadvantage that bulk semiconductor, but has the major disadvantage that bulk material cannot be made p-type.material cannot be made p-type.

GaN as grown, is naturally nGaN as grown, is naturally n++ . ++ .

Light emitting structures are made by producing an Light emitting structures are made by producing an intrinsic GaN layer using heavy zinc doping. Light intrinsic GaN layer using heavy zinc doping. Light emission occurs when electrons are injected from an nemission occurs when electrons are injected from an n+ +

GaN layer into the intrinsic Zn-doped region.GaN layer into the intrinsic Zn-doped region.

A possible device structure is A possible device structure is shown in fig.shown in fig.

Unfortunately, the recombination Unfortunately, the recombination process that leads to photon process that leads to photon production involves the Zn production involves the Zn impurity centres, and photon impurity centres, and photon emission processes involving emission processes involving impurity centres are much less impurity centres are much less efficient than band-to-band efficient than band-to-band processes.processes.

SapphireSubstrate

(transparent)

n + GaNi-GaN

Ohmic ContactsDielectric(oxide or nitride)

Fig. Blue LED

Blue photons

It is generally true to say that if we order the photon It is generally true to say that if we order the photon producing processes (in semiconductors) in terms of producing processes (in semiconductors) in terms of efficiency, we would get a list like the one below.efficiency, we would get a list like the one below. band-to-band recombination in direct gap material,band-to-band recombination in direct gap material, recombination via isoelectronic centres,recombination via isoelectronic centres, rreecombination via impurity (not isoelectronic) centres,combination via impurity (not isoelectronic) centres, band-to-band recombination in indirect-gap materials.band-to-band recombination in indirect-gap materials.

So, the current situation is that we do have low-efficiency So, the current situation is that we do have low-efficiency blue LEDs commercially available. We are now awaiting blue LEDs commercially available. We are now awaiting a new materials system, or a breakthrough in GaN or a new materials system, or a breakthrough in GaN or SiC technology, for blue LEDs of higher brightness and SiC technology, for blue LEDs of higher brightness and higher efficiency to be produced.higher efficiency to be produced.

Color NameColor Name WavelengthWavelength(Nanometers)(Nanometers)

SemiconductorSemiconductorCompositionComposition

InfraredInfrared 880880 GaAlAs/GaAsGaAlAs/GaAsUltra RedUltra Red 660660 GaAlAs/GaAlAsGaAlAs/GaAlAs

Super RedSuper Red 633633 AlGaInPAlGaInPSuper OrangeSuper Orange 612612 AlGaInPAlGaInP

OrangeOrange 605605 GaAsP/GaPGaAsP/GaP

YellowYellow 585585 GaAsP/GaPGaAsP/GaP

IncandescentIncandescentWhiteWhite 4500K (CT)4500K (CT) InGaN/SiCInGaN/SiC

Pale WhitePale White 6500K (CT)6500K (CT) InGaN/SiCInGaN/SiC

Cool WhiteCool White 8000K (CT)8000K (CT) InGaN/SiCInGaN/SiC

Pure GreenPure Green 555555 GaP/GaPGaP/GaP

Super BlueSuper Blue 470470 GaN/SiCGaN/SiC

Blue VioletBlue Violet 430430 GaN/SiCGaN/SiC

UltravioletUltraviolet 395395 InGaN/SiCInGaN/SiC

MaterialMaterial Wavelength Wavelength (µm)(µm) MaterialMaterial Wavelength Wavelength

(µm)(µm)

ZnS ZnS ZnOZnOGanGanZnSeZnSeCdSCdSZnTeZnTeGaSeGaSeCdSeCdSeCdTeCdTe

0.33 0.33 0.370.370.400.400.460.460.490.490.530.530.590.59

0.6750.6750.7850.785

GaAsGaAsInPInP

GaSbGaSbInAsInAsTeTe

PbSPbSInSbInSbPbTePbTePbSePbSe

0.84-0.950.84-0.950.910.911.551.553.13.13.723.724.34.35.25.26.56.58.58.5