fisika zat padat

1. 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

2. What is a Transistor?
A Transistor is an
electronic device
composed of layers of a
semiconductor
material which regulates
current or
voltage flow and acts as a
switch or gate
for electronic circuit.

3. History of the Transistor
John Pierce –supervised the BellJohn Pierce –supervised the Bell
Labs team which built the firstLabs 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 newAkio Morita, who founded a new
company named Sony Electronicscompany named Sony Electronics
that mass-produced tinythat mass-produced tiny
transistorized radios (1961)transistorized radios (1961)

4. Processor development followed
Moore’s Law

5. Applications
Switching
Amplification
Oscillating Circuits
Sensors

6. 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

7. P-N Junction (Basic diode):
- Bringing P and N Semiconductors in contact
P Type N Type
- Creation of a Depletion Zone

8. Reverse Biased
=> No Current
Applying –Voltage to Anode
increases
Barrier Voltage & Inhibits
Current Flow
• Applying Voltage to Cathode
• Barrier Voltage to Anode allows
current flow
Forward Biased
=> Current

9. Types Of TransistorsTypes Of Transistors
NPN: transistor where
the majority current
carriers are electrons
The majority current carriers in the PNP
transistor are holes

10. Transistor OperationTransistor Operation

11. 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.

12. 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.

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

14. 4/11/2006 BAE 5413 14
Application of photodiodes
A brief overview

15. 4/11/2006 BAE 5413 15
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)

16. 4/11/2006 BAE 5413 16
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.

17. 4/11/2006 BAE 5413 17
Photodiode structure
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

18. 4/11/2006 BAE 5413 18
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

19. 4/11/2006 BAE 5413 19
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=

20. 4/11/2006 BAE 5413 20
Photodiode current/voltage characteristics

21. 4/11/2006 BAE 5413 21
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 −=
0≈oI
01 ≈V
sffff IRIRV −==
sffout IRVV −==
–Note: current to voltage conversion

22. 4/11/2006 BAE 5413 22
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

23. 4/11/2006 BAE 5413 23
For the photovoltaic mode
• I = thermal component + photon flux related current
• where
I = photodiode current
V = photodiode voltage
I0 = reverse saturation current of diode
e = electron charge
k = Boltzman's constant
T = temperature (K)
ν = frequency of light
h = Plank’s constant
P = optical power
η = probability that hv will elevate an electron across the band gap
ν
η
h
eP
eII kT
eV
−







−= 10

24. 4/11/2006 BAE 5413 24
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

25. THE LIGHT EMITTINGTHE LIGHT EMITTING
DIODEDIODE
Chapter 6Chapter 6

26. CB
VB
When the electronWhen the electron
falls down fromfalls down from
conduction band andconduction band and
fills in a hole infills in a hole in
valence band, there isvalence band, there is
an obvious loss ofan obvious loss of
energy.energy.
The question is;The question is;
where does that energy go?where does that energy go?

27. In order to achieve aIn order to achieve a
reasonable efficiencyreasonable efficiency
for photon emission,for photon emission,
the semiconductorthe semiconductor
must have a directmust have a direct
band gap.band gap.
CB
VB
The question is;The question is;
what is the mechanismwhat is the mechanism
behind photon emission in LEDs?behind photon emission in LEDs?

28. For example;For example;
SiliconSilicon is known as anis known as an indirect band-gapindirect band-gap
material.material.
as an electron goes from the bottom ofas an electron goes from the bottom of
the conduction band to the top of thethe conduction band to the top of the
valence band;valence band;
it must also undergo ait must also undergo a
significantsignificant change inchange in
momentum.momentum.
CB
VB
What this means is thatWhat this means is that
E
k

29.  As we all know, whenever something changesAs we all know, whenever something changes
state, one must conserve not only energy, butstate, one must conserve not only energy, but
also momentum.also momentum.
 In the case of an electron going fromIn the case of an electron going from
conduction band to the valence band in silicon,conduction band to the valence band in silicon,
both of these things can only be conserved:both of these things can only be conserved:
The transition also creates a
quantized set of lattice vibrations,
called phonons, or "heat“ .

30.  Phonons possess both energy and momentum.Phonons possess both energy and momentum.
 Their creation upon the recombination of anTheir creation upon the recombination of an
electron and hole allows for completeelectron 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 inAll of the energy which the electron gives up in
going from the conduction band to the valencegoing from the conduction band to the valence
band (1.1 eV) ends up in phonons, which isband (1.1 eV) ends up in phonons, which is
another way of saying that the electron heats upanother way of saying that the electron heats up
the crystal.the crystal.

31. In a class of materials calledIn a class of materials called direct band-gapdirect band-gap
semiconductorssemiconductors;;
the transition from conduction bandthe transition from conduction band
to valence band involves essentiallyto valence band involves essentially
no change in momentumno change in momentum..
Photons, it turns out, possess a fairPhotons, it turns out, possess a fair
amount of energy ( several eV/photonamount of energy ( several eV/photon
in some cases ) but they have veryin some cases ) but they have very
little momentum associated withlittle momentum associated with
them.them.

32.  Thus, for a direct band gap material, the excessThus, for a direct band gap material, the excess
energy of the electron-hole recombination canenergy of the electron-hole recombination can
either be taken away as heat, or more likely, aseither be taken away as heat, or more likely, as
a photon of light.a photon of light.
 This radiative transition thenThis radiative transition then
conserves energy and momentumconserves energy and momentum
by giving off light whenever anby giving off light whenever an
electron and hole recombine.electron and hole recombine. CB
VB
This gives rise toThis 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).

33. Mechanism is “injectionMechanism is “injection
Electroluminescence”.Electroluminescence”.
LuminescenceLuminescence
part tells us that we are producing photons.part tells us that we are producing photons.
Electro part tells us thatElectro part tells us that
the photons are being producedthe photons are being produced
by an electric current.by an electric current.
e-
Injection tells us thatInjection tells us that
photon production is byphoton production is by
the injection of current carriers.the injection of current carriers.
Mechanism behind photonMechanism behind photon
emission in LEDs?emission in LEDs?
e-

34. Producing photonProducing photon
Electrons recombine with holes.Electrons recombine with holes.
Energy of photon is the energy ofEnergy of photon is the energy of
band gap.band gap.
CB
VB
e-
h

35. 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 highside where they will be combine with the high
density of majority carriers.density of majority carriers.
n-side
p-side
-
+
I

36. Notice that:Notice that:
 Photon emission occurs whenever we havePhoton emission occurs whenever we have
injected minority carriers recombining with theinjected minority carriers recombining with the
majority carriers.majority carriers.
 If the eIf the e--
diffusion length is greater than the holediffusion length is greater than the hole
diffusion length, the photon emitting region willdiffusion length, the photon emitting region will
be bigger on the p-side of the junction than thatbe 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 considerConstructing a real LED may be best to consider
a na n++++
p structure.p structure.
 It is usual to find the photon emitting volumeIt 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.

37. MATERIALS FOR LEDSMATERIALS FOR LEDS
 The semiconductor bandgapThe semiconductor bandgap
energy defines the energy of theenergy defines the energy of the
emitted photons in a LED.emitted photons in a LED.
 To fabricate LEDs that can emitTo fabricate LEDs that can emit
photons from the infrared to thephotons from the infrared to the
ultraviolet parts of the e.m.ultraviolet parts of the e.m.
spectrum, then we must considerspectrum, then we must consider
several different materialseveral different material
systems.systems.
 No single system can span thisNo single system can span this
energy band at present, althoughenergy band at present, although
the 3-5 nitrides come close.the 3-5 nitrides come close.
CB
VB

38.  Unfortunately, many of potentiallly useful 2-6Unfortunately, many of potentiallly useful 2-6
group of direct band-gap semiconductorsgroup of direct band-gap semiconductors
(ZnSe,ZnTe,etc.)(ZnSe,ZnTe,etc.) come naturally dopedcome 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 areThe material reasons behind this are
complicated and not entirely well-known.complicated and not entirely well-known.
 The same problem is encountered in the 3-5The 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 aboutAlGaN, and InAlGaN. The amazing thing about
3-5 nitride alloy3-5 nitride alloy systems is that appear to besystems is that appear to be
direct gapdirect gap throughout.throughout.

39.  When we talk about light ,it is conventional toWhen 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 ofVisible 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.
( )
( )
hc
nm
E eV
λ =
1242
( )
( )
nm
E eV
λ =

40. 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.
GaNGaN
ZnSeZnSe
violet blue
GaP:NGaP:N
green yellow
GaAsGaAs.14.14
pp
8686
GaAsGaAs.35.35
pp
6565
redorange
GaAsGaAs.6.6
pp
44

41. Properties of InGaNProperties of InGaN
 InGaN alloy has one composition at a time only.InGaN alloy has one composition at a time only.
 This materialThis material will emit one wavelengthwill emit one wavelength onlyonly
corresponding to this particular composition.corresponding to this particular composition.
 An InGaN LED wouldAn InGaN LED would not emit white lightnot emit white light (the(the
whole of the visible spectrum at once) since itswhole of the visible spectrum at once) since its
specific compositionspecific composition..
 For aFor a white light sourcewhite light source we have to form awe have to form a
complicated multilayercomplicated multilayer devicedevice emitting lots ofemitting lots of
different wavelengths.different wavelengths.

42. Properties of InGaNProperties of InGaN
 A LED fabricatedA LED fabricated in ain a graded materialgraded material
where on either side of the junction regionwhere on either side of the junction region
thethe material changes slowly from InN tomaterial changes slowly from InN to
GaNGaN via InGaN alloys.via InGaN alloys.
 Minority carriers need to get through theMinority carriers need to get through the
whole of this alloy region if efficient photonwhole of this alloy region if efficient photon
production at all visible wavelengths wasproduction at all visible wavelengths was
to occur.to occur.

43. GaNGaN InNInN
Concentration:Concentration:
The highlyThe highly
gallium richgallium rich
alloyalloy
The highlyThe highly
indium richindium rich
alloyalloy
Band gap:Band gap: 3.3eV3.3eV 2 eV2 eV
Wavelength ofWavelength of
photons:photons:
376 nm376 nm 620 nm620 nm
Part of thePart of the
electromagneticelectromagnetic
spectrum:spectrum:
In the ultravioletIn the ultraviolet In the visibleIn the visible
(orange)(orange)

44. GaN
InN
3.3 eV(376 nm)
ultraviolet

45. GaN
InN
3 eV (414 nm)
violet

46. GaN
InN
2.7 eV(460 nm)

47. GaN
InN
2.4 eV(517 nm)

48. GaN
InN
2.1 eV(591 nm)

49. GaN
InN
2.00 eV
2 eV(620 nm)

50. 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).
GaAs
GaP
1.42 eV

51. GaAs
GaP
1.52 eV

52. GaAs
GaP
1.62 eV

53. GaAs
GaP
1.72 eV

54. GaAs
GaP
1.80 eV

55. GaAs
GaP
1.90 eV

56. GaAs
GaP
2.00 eV

57. GaAs
GaP
2.26 eV

58. _
hν
+
Energy
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.)

59. 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!

60. 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.

61. Materials for visible wavelength LEDsMaterials for visible wavelength LEDs
 We see them almost everyday, either on calculatorWe 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 availableYellow, 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.

62. Red LEDsRed LEDs
 can be made in the GaAsPcan 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 goesfor x>0.45 the gap goes
indirect andindirect and
 for x=0.45 the band gapfor x=0.45 the band gap
energy is 1.98 eV.energy is 1.98 eV.
 Hence it is useful for redHence it is useful for red
LEDs.LEDs.
N-GaAs substrate
N-GaAsP P = 40 %
p-GaAsP region
Ohmic Contacts
Dielectric
(oxide or nitride)
Fig. GaAsP RED LED on a GaAs sub.

63. Isoelectronic CentreIsoelectronic Centre
 IsoelectronicIsoelectronic means that tmeans that the centre being introducedhe centre being introduced has thehas the
same number of valance electronssame number of valance electrons as the element it isas the element it is
replacing.replacing.
 For example, nitrogen can replace some of the phosphorusFor example, nitrogen can replace some of the phosphorus
in GaP. It is isoelectronic with phosphorus, butin GaP. It is isoelectronic with phosphorus, but behavesbehaves
quite differentlyquite differently allowing reasonably efficient green emission.allowing reasonably efficient green emission.

64. How isoelectronic centres work?How isoelectronic centres work?
 For our isoelectronic centreFor our isoelectronic centre
thethe position is very well-position is very well-
defined,defined, hence there is ahence there is a
considerableconsiderable spread in itsspread in its
momentummomentum state.state.
 Isoelectronic centre has theIsoelectronic centre has the
same valance configurationsame valance configuration
as the phosphorus it isas the phosphorus it is
replacing.replacing.
 It doesn't act as a dopantIt doesn't act as a dopant..
E
dE
Isoelectronic
centre
CB edge
electrons
Electron-hole
recombination
Holes
VB edge
k = 0

65. Isoelectronic centres provide a ‘Isoelectronic centres provide a ‘stepping stonestepping stone’ for’ for
electrons in E-k space so that transitions can occurelectrons in E-k space so that transitions can occur
that are radiatively efficient.that are radiatively efficient.
The recombination eventThe recombination event
shown has no change inshown has no change in
momentum, so itmomentum, so it
behaves like a directbehaves like a direct
transition.transition.
E
dE
Isoelectronic
centre
CB edge
electrons
Electron-hole
recombination
Holes
VB edge
k = 0
Because the effective transition is occurring betweenBecause the effective transition is occurring between
the isoelectronic centre and VB edge, the photon thatthe isoelectronic centre and VB edge, the photon that
is emitted has a lower energy than the band-gapis emitted has a lower energy than the band-gap
energy.energy.

66. GaP : NGaP : N
 (dE = 50 meV) Photon energy(dE = 50 meV) Photon energy
is less than the semiconductoris less than the semiconductor
band-gap energy it means thatband-gap energy it means that
the photon is not absorbed bythe photon is not absorbed by
the semiconductor, and so thethe semiconductor, and so the
photon is easily emitted fromphoton is easily emitted from
the material.the material.
 This lack of absorption pushesThis lack of absorption pushes
up the efficiency of the diodeup the efficiency of the diode
as a photon source.as a photon source.
E
dE
Isoelectronic
centre
CB edge
electrons
Electron-hole
recombination
Holes
VB edge
k = 0
50 meV

67.  For emission in the red part of the spectrum using GaPFor emission in the red part of the spectrum using GaP
the isoelectronic centre introduced contathe isoelectronic centre introduced contaiins zinc (Zn) andns zinc (Zn) and
oxygen (O). These red LEDs are usually designatedoxygen (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 isTheir main drawback is that their emission at 690 nm is
in a region where the eye sensitivity is rather low, whichin a region where the eye sensitivity is rather low, which
means that commercially, the AlGaAs/GaAs diodes aremeans that commercially, the AlGaAs/GaAs diodes are
more successful devices.more successful devices.

68.  Orange (620 nm) and yellow (590 nm) LEDs areOrange (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-gapas we have just seen above, the required band-gap
energy for emission at these wavelengths means theenergy 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 theseand 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 theThe 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

69. Blue LEDsBlue LEDs
 Blue LEDs are commercially available and are fabricatedBlue LEDs are commercially available and are fabricated
using silicon carbide (SiC). Devices are also madeusing silicon carbide (SiC). Devices are also made
based on gallium nitride (GaN).based on gallium nitride (GaN).
 Unfortunately both of these materials systems haveUnfortunately 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 anThe 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.

70. Blue LEDsBlue LEDs
 The transitions that give rise to blue photon emission inThe transitions that give rise to blue photon emission in
SiC are between the bands and doping centres in theSiC are between the bands and doping centres in the
SiC. The dopants used in manufacturing SiC LEDs areSiC. The dopants used in manufacturing SiC LEDs are
nitrogen for n-type doping, and aluminium for p-typenitrogen for n-type doping, and aluminium for p-type
doping.doping.
 The extreme hardness of SiC also requires extremelyThe extreme hardness of SiC also requires extremely
high processing temperatures.high processing temperatures.

71. Gallium Nitride (GaN)Gallium Nitride (GaN)
 Gallium nitride has the advantage of being a direct-gapGallium nitride has the advantage of being a direct-gap
semiconductor, but has the major disadvantage that bulksemiconductor, 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 anLight emitting structures are made by producing an
intrinsic GaN layer using heavy zinc doping. Lightintrinsic 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.

72.  A possible device structure isA possible device structure is
shown in fig.shown in fig.
 Unfortunately, the recombinationUnfortunately, the recombination
process that leads to photonprocess that leads to photon
production involves the Znproduction involves the Zn
impurity centres, and photonimpurity centres, and photon
emission processes involvingemission processes involving
impurity centres are much lessimpurity centres are much less
efficient than band-to-bandefficient than band-to-band
processes.processes.
Sapphire
Substrate
(transparent)
n + GaN
i-GaN
Ohmic Contacts
Dielectric
(oxide or nitride)
Fig. Blue LED
Blue photons

73.  It is generally true to say that if we order the photonIt is generally true to say that if we order the photon
producing processes (in semiconductors) in terms ofproducing 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-efficiencySo, the current situation is that we do have low-efficiency
blue LEDs commercially available. We are now awaitingblue LEDs commercially available. We are now awaiting
a new materials system, or a breakthrough in GaN ora new materials system, or a breakthrough in GaN or
SiC technology, for blue LEDs of higher brightness andSiC technology, for blue LEDs of higher brightness and
higher efficiency to be produced.higher efficiency to be produced.

74. Color NameColor Name
WavelengthWavelength
(Nanometers)(Nanometers)
SemiconductorSemiconductor
CompositionComposition
InfraredInfrared 880880 GaAlAs/GaAsGaAlAs/GaAs
Ultra RedUltra Red 660660 GaAlAs/GaAlAsGaAlAs/GaAlAs
Super RedSuper Red 633633 AlGaInPAlGaInP
Super OrangeSuper Orange 612612 AlGaInPAlGaInP
OrangeOrange 605605 GaAsP/GaPGaAsP/GaP
YellowYellow 585585 GaAsP/GaPGaAsP/GaP
IncandescentIncandescent
WhiteWhite
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

75. MaterialMaterial
WavelengthWavelength
(µm)(µm)
MaterialMaterial
WavelengthWavelength
(µm)(µm)
ZnSZnS
ZnOZnO
GanGan
ZnSeZnSe
CdSCdS
ZnTeZnTe
GaSeGaSe
CdSeCdSe
CdTeCdTe
0.330.33
0.370.37
0.400.40
0.460.46
0.490.49
0.530.53
0.590.59
0.6750.675
0.7850.785
GaAsGaAs
InPInP
GaSbGaSb
InAsInAs
TeTe
PbSPbS
InSbInSb
PbTePbTe
PbSePbSe
0.84-0.950.84-0.95
0.910.91
1.551.55
3.13.1
3.723.72
4.34.3
5.25.2
6.56.5
8.58.5