Physical Electronics Slides - Applications of Physical...

Monday, October 28, 2013Tennessee Technological University 1

PHYSICAL ELECTRONICS(ECE3540)APPLICATIONS OF

PHYSICAL ELECTRONICS – PART IAPPLICATIONS OF

PHYSICAL ELECTRONICS – PART I


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.


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



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


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


Quantum Mechanical Tunneling

)( )(82exp 2

2

EVh

mTP H Tunneling probability:

Tennessee Technological University Monday, October 28, 2013 8

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


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.


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.


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


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


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


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


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.


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


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


LED Materials and Structure

)(24.1

energy photon24.1 m) ( h wavelengtLED

eVEg


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


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


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.


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.


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


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


Physical Electronics Slides - Applications of Physical...

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