ELCT 871: Advanced Semiconductor Devices

ELCT 871: Advanced Semiconductor Devices Course Outline lemental vs. compound semiconductors • GaN general properties • GaN growth techniques • GaN ...
Author: Amos Gardner
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ELCT 871: Advanced Semiconductor Devices

Course Outline lemental vs. compound semiconductors • GaN general properties • GaN growth techniques • GaN doping and materials characterization • Metal contacts to GaN • Nitride based heterostructures and HFETs • GaN optoelectronic devices: LEDs and Lasers • SiC material properties • SiC device applications

Semiconductor electronic devices tree Semiconductor devices Elemental semiconductors

Compound semiconductors*

High power Low power High power Low power High speed High speed Low speed High speed

High power High speed

LDMOS Si and strained Power BJT, pHEMTs, HBTs DMOS Si MOSFETs, IGBTs, Wide bandgap Thyristors etc. SiGe HBTs *Optoelectronic devices : Lasers and LEDs SiC MESFETs Nitride HEMTs

Lattice constant vs. bandgap for common semiconductors

Common semiconductors comparison Properties

Si (----)

GaAs (AlGaAs/ InGaAs)

Bandgap (eV)

1.11

µe (cm2/Vs) Vsat (× 107 cm/s)

InP (InAlAs/ InGaAs)

4H- SiC (----)

GaN (AlGaN/ GaN)

1.42

1.35

3.26

3.42

1500

8500 (10000)

5400 (10000)

700

900 (2000)

1

1 (2.1)

1 (2.3)

2

1.5 (2.7)

2DEG density (cm-2)

NA

< 4×1012

< 4×1012

NA

1-2 ×1013

EB (106 V/cm)

0.3

0.4

0.5

2

3.3

Dielectric constant

11.8

12.8

12.5

10

9

Wide bandgap applications

GaN is projected to be a $3 billion industry by 2007 !

Figures of merit for high frequency/high power devices Semiconductor

Electron mobility (cm2/Vsec)

Relative permittivity ε

Bandgap Eg (eV)

BFOM Ratio

JFM Ratio

Tmax (°C)

Si

1300

11.4

1.1

1.0

1.0

300

GaAs

5000

13.1

1.4

9.6

3.5

300

SiC

260

9.7

2.9

3.1

60

600

GaN

1500

9.5

3.4

24.6

80

700

BFOM: Baliga’s figure-of-merit

κµ n ECR 3

JFM: Johnson’s figure-of-merit ECR vsat 2π

Figure of Merit

CFOM = 400 200 0 Si GaAs 6H- 4H- GaN SiC SiC

2 χε o µv s E CR 2 (χε o µvs ECR )silicon

χ : thermal conductivity ECR : breakdown field µ : low field mobility vs : saturation velocity εo: dielectric constant

Advantages for Nitride Electronic Devices Properties • • • •

Advantages

High mobility High saturation velocity High sheet carrier concentration High breakdown field

• Wide bandgap ( ni = N C NV exp − E • Growth on SiC substrate

G

High microwave power, Power electronic devices / 2 kT

)

High temperature operation

• Chemical inertness • Good ohmic contacts • No micropipes

Holds promise for reliable device fabrication

• SiO2/AlGaN and SiO2/GaN good quality interfaces

Insulated Gate transistors possible

Output Power density (W/mm)

Power densities for AlGaN/GaN HEMTs vs. Time 35 30

Field-plated gate used to enhance breakdown voltage

25 20 15 10 5 0 96

98

00

02

04

06

Best SiC device ~5.5 W/mm Best GaAs device ~1.2 W/mm

Years

Highest reported value of power density of 32 W/mm, EDL, April, 2004

Size reduction with same output power

Applications in power electronics Schottky metal

RON

VF

+ Low doped GaN n substrate

Ohmic metal

= (4VB 2 / ε ⋅ µ ⋅ E M 3 ) + ρs ⋅ WS + RC (per unit area)

= (nkT / e )(ln[J

∗∗

F

2

]

/ A T + nφB + RON ⋅ A ⋅ J F

)

VB = Reverse breakdown voltage EM = Maximum electric field strength at breakdown ρs = substrate resistivity Ws = thickness of substrate A = cross-sectional area Advantage: Higher VB for same Ron, and lower Ron for the same VB

Optoelectronic applications: LEDs Ti/Au

t= 1-5 µm

Ni/Au

Pd/Ag/Au p+-GaN p-GaN n-InGaN MQW Ti/Al/Ti/Au

n-GaN n+-GaN

Al2O3

100 µm

• Recent applications include high brightness white LEDs

0 10

µm

Nitride based lasers

• Purple - Blue CW Lasers (> 104 hour lifetime easily achieved) • Blue lasers are used for increasing storage capacities of DVDs

Bio-agent and missile plume detection A. Bio-agent detection due to UV induced fluorescence

Deep UV Light

Bio organism

Spectrometer

Detector

B. Solar blind photo-detectors for missile plume detection

Solar Spectrum

Ozone layer

290 nm

Non line-of-sight communication • Uses UV LEDs with ~280 nm wavelength • Due to lower wavelength the UV light is scattered very strongly by the atmosphere and particulates • Useful in areas where the parties concerned cannot see each other, but very fast and highly reliable communication is essential

Advantages of nitride based light source

• Compact, light, inexpensive, efficient, and robust light sources can be made of III-nitrides

Gallium nitride structure • 2 interpenetrating HCP structures of Ga and N atoms each displaced from the other by 3/8 c • Structure is 2H type • Atoms of only Ga or N lie on any single plane normal to the c-axis ( or , called c-planes), but not for a-planes • Very strongly polar bond as N is the most electronegative of the Group V materials • The crystal structure is non-centrosymmetric, i.e. lacks inversion symmetry along the c-axis

SiC polytypes

Band structures of GaN Zinc Blende

Wurtzite

Zinc Blende

Wurtzite

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