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Monday, October 28, 2013Tennessee Technological University 1
PHYSICAL ELECTRONICS(ECE3540)APPLICATIONS OF
PHYSICAL ELECTRONICS – PART IAPPLICATIONS OF
PHYSICAL ELECTRONICS – PART I
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IntroductionIn the following slides, we will discuss the summaryof the Reading Assignment:the concepts of a large reverse-bias voltage thatcause a Junction Breakdown, the Zener Effect and theAvalanche Effect.1. Junction Breakdown, Avalanche Breakdown,
Tunneling Breakdown2. Zener Diodes3. Tunnel Diodes4. Applications of Physical Electronics I:
PN Junction Diodes.More discussion on these concepts in Chapter 12.
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PHYSICAL ELECTRONICS(ECE3540)
Explanation of the Reading Assignment
Zener Diodes and Tunnel Diodes
Junction Breakdown
Dominant if both sides of a junction are very heavily doped.
Can be classified into two:1. Zener Breakdown2. Avalanche Breakdown
V/cm106 critp EEV
I
Breakdown
Empty StatesFilled States -
Ev
Ec
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1. Zener Breakdown
A Zener diode is designed to operate in the breakdown mode.
V
I
VB, breakdown
P NA
R
Forward Current
Small leakageCurrent
voltage
3.7 V
R
IC
A
B
C
D
Zener diode
Peak Electric Field
2/1
|)|(2)0(
rbi
sp VqN
EE
bicrits
B qNV
2
2E
N+ PNa
Neutral Region
0 xp(a)
increasingreverse bias
Deletion layer
x
E
xp
(b)
increasing reverse biasEp
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2. Avalanche Breakdown • Impact ionization: an energetic electron generating electron and hole, which can also cause impact ionization.
qNV crits
B 2
2E
• Impact ionization + positive feedbackavalanche breakdown
daB N
1N1
N1V
EcEFn
Ec
Ev
EFp
originalelectron
electron-holepair generation
Ev
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Quantum Mechanical Tunneling
)( )(82exp 2
2
EVh
mTP H Tunneling probability:
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A tunnel diode or Esaki diode is a type of semiconductor thatis capable of very fast operation, well intothe microwave frequency region, made possible by the use ofthe quantum mechanical effect called tunneling.
Fig. 1 Quantum Mechanical Tunneling
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Tunnel Diode Under normal forward bias operation, as voltage
begins to increase, electrons at first tunnel through thevery narrow p–n junction barrier because filledelectron states in the conduction band on the n-sidebecome aligned with empty valence band hole stateson the p-side of the p-n junction.
As voltage increases further these states become moremisaligned and the current drops – this iscalled negative resistance because current decreases withincreasing voltage. As voltage increases yet further, thediode begins to operate as a normal diode.
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Tunnel Diode In the reverse direction, tunnel diodes are
called back diodes (or backward diodes) and canact as fast rectifiers with zero offset voltage andextreme linearity for power signals (they have anaccurate square law characteristic in the reversedirection).
Under reverse bias, filled states on the p-sidebecome increasingly aligned with empty states onthe n-side and electrons now tunnel through the PNjunction barrier in reverse direction.
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Tunnel Diode
Fig. 2:a) Simplified Energy band diagram of a tunnel diode with a reverse bias
voltageb) I-V Characteristic of a Tunnel Diode with a reverse-bias voltage
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PHYSICAL ELECTRONICS(ECE3540)
APPLICATIONS OF PN JUNCTION DIODES
APPLICATIONS OF PN JUNCTION DIODES
The PN Junction as a Temperature Sensor
What causes the IV curves to shift to lower V at higher T ?
)1(0 kTVqeII
an
n
dp
pi NL
DNL
DAqnI 2
0
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Fig. 3: PN Junction diode as a Temperature Sensor
•Solar Cells are also known asphotovoltaic cells (PV).
•Convert sunlight to electricitywith 10-30% conversionefficiency.
•1 m2 solar cell generate about150 W peak or 25 Wcontinuous power.
•Low cost and high efficiencyare needed for widedeployment.
Solar Cells
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Fig. 4: World Energy Consumption
Solar Cell Basics
sckTVq IeII )1(0
V0.7 V
–Isc Maximum
power-output
Solar CellIV
I
Dark IV
0
Eq.(4.9.4)N P
-
Short Circuit
lightIsc
+(a)
Ec
Ev
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Light Absorption
)(24.1
(eV)Energy Photon
m
hc
x-e (x)intensity Light
α(1/cm): absorption coefficient
A thinner layer of direct-gap semiconductor can absorb most of solarradiation than indirect-gap semiconductor. Compare Si and Ge.
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Fig. 5: Photon Energy vs. AbsorptionCoefficient
Output Power
FFVI ocsc PowerOutput
•Theoretically, the highest efficiency (~24%) can be obtained with 1.9eV >Eg>1.2eV. Larger Eg lead to too low Isc (low light absorption); smaller Eg leads to too low Voc.•Tandem solar cells gets 35% efficiency using large and small Egmaterials tailored to the short and long wavelength solar light.
A particular operating point on the solar cell I-V curve maximizes the output power (I x V).
•Si solar cell with 15-20% efficiency dominates the market now
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Light emitting diodes (LEDs)• LEDs are made of compound semiconductors such as InP and
GaN.
• Light is emitted when electron and hole undergo radiativerecombination.
Ec
Ev
Radiative recombination
Non-radiative recombination through traps
Light Emitting Diodes and Solid-State Lighting
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LED Materials and Structure
)(24.1
energy photon24.1 m) ( h wavelengtLED
eVEg
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Fig. 7: LED Materials and Structure
LED Materials and Structure)(eVEg
redyellowblue
Wavelength (μm) Color
Lattice constant
(Å)
InAs 0.36 3.44 6.05
InN 0.65 1.91 infrared 3.45
InP 1.36 0.92 5.87
GaAs 1.42 0.87 5.66
GaP 2.26 0.55 5.46
AlP 3.39 0.51 5.45
GaN 2.45 0.37 3.19
AlN 6.20 0.20 UV 3.11
Table: Light-emitting diode materials
compound semiconductors
binary semiconductors:- Ex: GaAs, efficient emitter
ternary semiconductor :- Ex: GaAs1-xPx , tunable Eg (to vary
the color)
quaternary semiconductors:- Ex: AlInGaP , tunable Eg and lattice constant (for growing high quality epitaxial films on inexpensive substrates)
violet
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Common LEDsSpectral
rangeMaterial System
Substrate Example Applications
Infrared InGaAsP InP Optical communication
Infrared-Red
GaAsP GaAsIndicator lamps. Remote control
Red-Yellow
AlInGaPGaA or
GaP
Optical communication. High-brightness traffic signal lights
Green-Blue
InGaNGaN or sapphire
High brightness signal lights. Video billboards
Blue-UV AlInGaNGaN or sapphire
Solid-state lighting
Red-Blue
Organic semicon-ductors
glass Displays
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Solid-State Lighting
Incandescent lamp
Compact fluorescent lamp
Tube fluorescent lamp
White LED
Theoretical limit at peak of eye sensitivity ( λ=555nm)
Theoretical limit (white light)
17 60 50-100 90 683 ~340
luminosity (lumen, lm): a measure of visible light energy normalized to the sensitivity of the human eye at different wavelengths
Luminous efficacy of lamps in lumen/watt
Terms: luminosity measured in lumens, luminous efficacy
Organic Light Emitting Diodes (OLED) : has lower efficacy than nitride or aluminide based compound semiconductor LEDs.
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Diode Lasers
(d) Net Light Absorption
(e) Net Light Amplification
Stimulated emission: emitted photon has identical frequency and directionality as the stimulating photon; light wave is amplified.
(b) Spontaneous Emission
(c) Stimulated Emission
(a) Absorption
Light Amplification
Light amplification requires population inversion: electron occupation probability is larger for higher E states than lower E states.
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Laser ApplicationsRed diode lasers: CD, DVD reader/writer
Blue diode lasers: Blu-ray DVD (higher storage density)
1.55 m infrared diode lasers: Fiber-optic communication
Photodiodes: Reverse biased PN diode. Detects photo-generatedcurrent (similar to Isc of solar cell) for optical communication, DVDreader, etc.
Avalanche photodiodes: Photodiodes operating near avalanchebreakdown amplifies photocurrent by impact ionization.
Photodiodes
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Picture Credits Semiconductor Physics and Devices, Donald Neaman, 4th
Edition, McGraw Hill Publications. Modern Semiconductor Devices for Integrated Circuits, Prof.
Chenming Calvin Hu, UC Berkeley.http://www.eecs.berkeley.edu/~hu/Book-Chapters-and-Lecture-Slides-download.html
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