Gallium Nitride (GaN)A Potential Candidate of

Future Electronics

Lecture10

1

Overview Overview

Introduction

Structure & Physical Properties

GaN vs. other semiconductors GaN vs. other semiconductors

Current research,

Limiting factors

Applications

2

Introduction Introduction

GaNGaN is the next important is the next important semiconductor after Sisemiconductor after Si.

Operate at High Temperatures.High Temperatures.

Key material for the next generation Key material for the next generation of high frequency high frequency and high power high power applicationsapplications.

Belongs Wide Band Gap (WBG) Belongs Wide Band Gap (WBG) semiconductor familysemiconductor family..

http://www.phy.mtu.edu/yap/images/galliumnitride.jpg

3

WBG Semiconductor with WBG Semiconductor with Direct Direct Bandgap EnergyBandgap Energy

Useful as Optoelectronics Useful as Optoelectronics DevicesDevices

oo Blue & Blue/green light emittersBlue & Blue/green light emitters

Transistors withstand Transistors withstand extreme heatextreme heat andand

Why Why GaNGaN is important ?is important ?

Transistors withstand Transistors withstand extreme heatextreme heat andand

High High frequencies frequencies and and power levelspower levels

GaN based Amplifiers more efficient at the base GaN based Amplifiers more efficient at the base

stationsstations

oo SiSi--only 10% power used and 90% wasted as heatonly 10% power used and 90% wasted as heat

4

GaNGaN Crystal StructureCrystal Structure

GaNGaN grown ingrown in

Wurtzite crystal structure

Zinc-blende crystal structure Zinc-blende crystal structure

The band gap, Eg, effected by crystal structure

5

WurtziteWurtzite Crystal StructureCrystal Structure

Wurtzite crystal structure is a member of the hexagonal crystal system

Several other compounds can take the wurtzite structure, including Agl, ZnO, CdS, CdSe, and other semiconductors.

Energy gap: 3.4 eV

An ideal angle: 1090

Nearest neighbor: 19.5 nm

Energetically favorable

Hexagonal Diamond http://en.wikipedia.org/wiki/Image:Wurtzite-unit-cell-3D-balls.png

CdS, CdSe, and other semiconductors.

6

ZincZinc--blendeblende Crystal StructureCrystal Structure

Energy gap 3.2 eV

An ideal angle: 109.470 An ideal angle: 109.470

Nearest neighbor: 19.5 nm

http://en.wikipedia.org/wiki/Image:Sphalerite-unit-cell-depth-fade-3D-balls.png7

Tetrahedral bonds

sp3 hybridization

Bonding angle: 109.47

Bond Length: 19.5 nm

GaNGaN Bonding PropertiesBonding Properties

Bond Length: 19.5 nm

Ga-N bonds significantly stronger than Ga-Gainteractions (based on distance)

8

IonicityIonicity

GaN exhibits mixed ionic-covalent bondingIonicity of a bond is the fraction fi of ionic character compared to the fraction of fh of covalent character

By Paulings definition

Modern definition is the ionicity phase angle

1http://www.bcpl.net/~kdrews/bonding/bonding2.html 9

GaNGaN Bonding PropertiesBonding PropertiesBased on calculations using both methods, typical values are

Compound Pauling ionicity Modern ionicity2

AlN 0.430 0.449

AlP 0.086 0.307

AlAs 0.061 0.274

GaN 0.387 0.500

GaP 0.061 0.327GaP 0.061 0.327

GaAs 0.039 0.310

InN 0.345 0.578

InP 0.039 0.421

InAs 0.022 0.357

NaCl 0.668 > 0.9

C (Diamond) 0 0

2J.C. Phillips, Bonds and Bands in Semiconductors 1973

Bond Character dependent on electronegativity10

GaNGaN--Crystal GrowthCrystal GrowthGaNGaN--Crystal GrowthCrystal Growth

Substrates for Epigrowth

650nm

AlN

BN

MgO

3C-SiC

6H-SiC

AlN Sapphire5.5

6.5

ZnOHexagonalCubic

650nm650nm

650nm

650nm

AlN

BN

GaNGaN

InN

Lattice Constant /

Ban

d g

ap /

eV 4.5

3.5

2.5

1.52 2.5 3 3.5 4 4.5 5 5.5

11

Thermal & lattice mismatch Thermal & lattice mismatch -- Strain and DefectsStrain and Defects

Why Why GaNGaN based HEMTs important? based HEMTs important?

High Voltage OperationHigh Voltage Operation High power densities 4 to 8 watts/mm at 28 and

50 volt operation respectively High Frequency Performance High Frequency Performance High Efficiency High Efficiency Low Quiescent CurrentLow Quiescent Current Low Quiescent CurrentLow Quiescent Current High Native LinearityHigh Native Linearity Low capacitance per peak wattLow capacitance per peak watt

12% of LDMOS and 21% of GaAs MESFET supports broad bandwidths

Almost constant CAlmost constant CDSDS as a function of Vas a function of VDSDS Enable new amplifier architecturesEnable new amplifier architectures

12

GaNGaN HEMT HistoryHEMT History

1960 GaN small crystals was made.

1980 Takashi Takashi MinuraMinura, Fujitsu laboratories , Fujitsu laboratories designed the features of the first HEMTfirst HEMT.

1985 HEMT was announced the lowest noise HEMT was announced the lowest noise 1985 HEMT was announced the lowest noise HEMT was announced the lowest noise device.device.

1994 KahnKahn demonstrated the first first AlGaNAlGaN//GaNGaNHEMT.HEMT.

13

Available Substrates Substrate Cost

GaN HEMT Substrate GaN HEMT Substrate

14

GaN HEMT on Silicon Substrate has major advantages of highpower, cost effectiveness and reliability.

1989-Started III-V nitride research.

1990- a new two-flow MOCVD equipment was developed forgrowth of high quality single crystal GaN layers.

1992-grow InGaN single crystal layers for the production ofheterostructures

1995-Developed high-brightness SQW structure blue/greenLEDs with a luminous intensity of 2 cd and 10 cd, anddeveloped a violet laser diode using III-V nitride materials for

GaNGaN LED History LED History (Nakamura)

developed a violet laser diode using III-V nitride materials forthe first time.

1996-The first current infection III-V nitride based LDs werefabricated.

1996-Announces the first CW blue GaN based injection laserat room temperature.

15

Comparison of Comparison of GaNGaN with other with other Comparison of Comparison of GaNGaN with other with other semiconductors semiconductors

16

GaN ComparisonGaN Comparison

Silicon cannot provide the power-bandwidth product for military applications 17

GaNGaN Comparison Comparison

1x100

1x105

SiC

-3)

The small intrinsic carrier concentration in GaN at room temperature enables the high power and temperature applications.

GaN epitaxialfor 1025.2(300) 3-10i cmn

18

2.5 3.0 3.5 4.010

-20

10-15

1x10-10

1x10-5

GaN

ni(T)=1.98x10

16T

(3/2)exp(-20488/T)

SiC

Intr

insi

c co

ncen

trat

ion

(cm

-3

Temperature (1000/K)

Intrinsic carrier concentration in SiC and GaN as a function of temperature. Ref. R. Kolessar et al., 2001.

0.01

0.1

1

GaN Schottky rectifiers AlGaN-UF

AlGaN-UF

AlGaN-UF

GaN-UF

GaN-UFGaN-UF

GaN-Caltech

Si

Spe

cifi

c on

-sta

te r

esis

tanc

e (

-cm

2 )

GaNGaN comparison comparison

19

102

103

104

1E-4

1E-3

0.01

GaN-UF

GaN-UF

GaN-UTGaN

6H-SiC

Spe

cifi

c on

-sta

te r

esis

tanc

e (

Reverse breakdown voltage (V)

GaNfor 104.2 5.212 BVRON

Breakdown VoltageBreakdown Voltage

3

104

Non-punchthrough theoretical limit

GaN punchthrough diodeTheoretical breakdown voltage50 m

30 m

20 m

10 m

5 m

Bre

akdo

wn

volt

age

(V)

0

2

2PTB

PTcPT

WqNWEBV

GaNGaN punchthroughpunchthrough diodediode

n- n n+

20

The calculated reverse breakdown voltage of punch-through diode as a function of doping concentration and standoff

region thickness

where EC is critical electric field, WP drift region thicknessNA doping concentration,

and permittivity

1E15 1E16 1E17

102

103

3 m

1 m

Bre

akdo

wn

volt

age

(V)

Doping concentration (cm-3)

02

3 m GaN epi can give more than 900V of reverse breakdown

voltage with the doping concentration of 1016 cm-3

Current Research and Current Research and Limiting factors Limiting factors Limiting factors Limiting factors

21

Current Research Current Research

Fundamental Physics

Improving crystal quality (still very poor)

Ultraviolet lasers

Lattice matching with quaternary alloys (AlGaInN)

Nitride heterostructures and accompanying applications

Nitride heterostructures and accompanying applications

Current Issues in III- V Nitrides

1. Inability to grow good quality crystals

2. Inability to grow p-type crystals

22

Crystal QualityCrystal Quality

Problem: No lattice matched substrates, high growth Problem: No lattice matched substrates, high growth

temperature results in convection currentstemperature results in convection currents

Sapphire is closest but is 15% off.Sapphire is closest but is 15% off.

SiCSiC is too expensiveis too expensive

MOCVD growth too fast for good control (few MOCVD growth too fast for good control (few mmm/min)m/min)

Solution: Buffer layers, new growth system

First grow GaN or AlN buffer layer

Two-flow MOCVD system

Still many many dislocations in material (1010 cm-2) but

dislocations dont matter?

23

Research QuestionsResearch Questions

Even undoped, carrier densities in AlGaN/GaNheterostructures is 10 to 100 times larger than those in similar (AlGaAs/GaAs) systems.

What is the source of these carriers?

Carrier mobilities in AlGaN/GaN heterostructuresare 10 to 100 times lower than in the AlGaAs/ GaAs system.

What are the principle mechanisms limiting the mobility?

24

Limiting FactorsLimiting Factors

Scattering mechanisms

Coulomb fields

Phonons

Alloy Disorder

25

Coulomb ScatteringCoulomb Scattering

Electrons are affected by the long-range Coulomb

fields of randomly distributed ionized donor

atoms.

Thicker barriers move ionized surface donors further Thicker barriers move ionized surface donors further

away from carriers.

Large 2-DEG densities screen the effect of these

Coulomb fields.

26

Phonon scatteringPhonon scatteringPhonons are lattice vibrations in a crystal.Phonons are lattice vibrations in a crystal.

Acoustic phononsAcoustic phonons

Both types of atoms move in-phase

Low energy vibrations

Optical phononsOptical phonons

Atoms of different types move out-of-phase Atoms of different types move out-of-phase

High energy vibration

27

Phonon ScatteringPhonon Scattering

Phonons scatter carriers by creating small Phonons scatter carriers by creating small fluctuating dipoles between atoms fluctuating dipoles between atoms (piezoelectric mode).(piezoelectric mode).

Phonons scatter carriers by disturbing the Phonons scatter carriers by disturbing the periodicity of the crystal lattice periodicity of the crystal lattice (deformation potential mode).(deformation potential mode).

28

Alloy DisorderAlloy Disorder

Electron Electron wavefunctionwavefunctionpenetrates into penetrates into AlGaNAlGaNbarrier.barrier.

Al and Al and GaGa atoms are atoms are distributed randomly in distributed randomly in distributed randomly in distributed randomly in AlGaNAlGaN

Randomly varying Randomly varying potential scatters potential scatters electronselectrons.

29

Overview of RFOverview of RF--Device TechnologiesDevice Technologies(Theoretical Thermal Limitations) (Theoretical Thermal Limitations)

30

Advantage of GaN HEMT Device Advantage of GaN HEMT Device

Si vs GaN technology

31

GaNGaN Applications Applications

32

GaNGaN NanotubesNanotubes

Single Crystal Single Crystal NanotubesNanotubesfabricatedfabricated

Gallium Nitride Gallium Nitride nanotubesnanotubeshave diameter between 30 have diameter between 30 200 nm200 nm

33

GaNGaN Laser DiodeLaser Diode

Normally emit ultraviolet

radiation

Indium doping allows variation in

band gap sizeband gap size

Band gap energies range from

0.7eV 3.4eV

Applications in: Blu-Ray technology Laser Printing

http://www.lbl.gov/Science-Articles/Archive/assets/images/2002/Dec-17-2002/indium_LED.jpg 34

BluBlu--ray Discray Disc

35

GaNGaN Solar CellsSolar Cells

Indium doped (Indium doped (InGaNInGaN)) Conversion of many wavelengths for energy Theoretical 70% maximum conversion rate.

Multiple layers attain higher efficiency.Multiple layers attain higher efficiency. Need many layers to attain 70%Need many layers to attain 70%

Lattice matching not an issue

Advantages:Advantages:Advantages:Advantages: High heat capacity Resistant to effects of strong radiation High efficiency

DifficultiesDifficulties: Too many crystal layers create system damaging stress Too expensive

36

Comparison Comparison -- bulbs bulbs vs.vs. LEDsLEDsComparison Comparison -- bulbs bulbs vs.vs. LEDsLEDs

37

High Power RectifiersHigh Power Rectifiers

38

SiC high power rectifier product

Current ratings of 1A to 20A at 600V, and 5A to 10A at 1200V

http://www.cree.com

The applications of IGBT modules

- UPS Power Supply,Servo Drive, Medical Power Supply, Motor Drives, Inverters

http://www.pwrx.com

ObjectiveObjectiveDevelop GaN-based rectifiers at power levels above 1MW

Military Applications Military Applications Electronic warfareElectronic warfare

Broadband and high-power microwave emission Interrupt and jam RF signals

AESA RadarAESA Radar Arrays of hundreds or thousands T/R modules

Phase shifted to form and steer the beamPhase shifted to form and steer the beam Phase shifted to form and steer the beamPhase shifted to form and steer the beam So many modules place a premium on size, weight, power

efficiency and high power performance

Tactical Radio CommunicationsTactical Radio Communications To operate over a frequency range of 30-3000 MHz at power levels up to 100W,

Accommodate portable, Accommodate portable, manpackmanpack and vehicle mount form and vehicle mount form

39

GaN Device Manufacturers GaN Device Manufacturers

Cree is the biggest US GaN chipmakers

Raytheon is running project on X to Ku-band

(7-11.2 GHz) applications

TriQuint has independent program for S-band

(2-4GHz) and wideband (6-18GHz) MMICs

Northrop Grumman has MMIC project for Ku-

band (7-11GHz).

40

RF RF Device Technology ComparisonDevice Technology Comparison

41

Pricing of LDMOS has reached ~$0.20s/W. Does Pricing of LDMOS has reached ~$0.20s/W. Does GaNGaN HEMT HEMT compete this price? compete this price?

Thank you to participate in Thank you to participate in this course this course this course this course

Hope all of you enjoy this course and enhance Hope all of you enjoy this course and enhance knowledge as wellknowledge as well

Good Luck for exam Good Luck for exam

42