Wide Bandgap Semiconductors

What is a wide bandgap semiconductor?

Larger energy gap allows higher power and temperature operation and the generation of more energetic (i.e. blue) photons

The III-nitrides (AlN, GaN and InN), SiC have recently become feasible. Other materials (like diamond) are being investigated.

What are they good for?

WBG nitride for photonics

2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.01

2

3

4

5

6

7

o

InN

GaN

AlN

B

an

d G

ap

En

erg

y (e

V)

Lattice Constant a (A)

Visible light

UVUV

IRIR 1.95ev1.95ev

3.4eV3.4eV

6.2eV6.2eV

Market for III-Nitride Devices

Blue / UV solid state diodes and lasers UV optical detectors High power microwave devices High power switches High temperature devices High density data storage devices

Additional Specialty Applications: Surface acoustic wave (SAW) devices (for wireless

communication)

High thermal conductivity substrates

Impact

Automotive industry Avionics and defense

Traffic lights

Solid state lighting

Electric power industry

Health care

Information technology(data storage)

Wirelesscommunications

Displays

The Market for GaN Devices

The Market for GaN Devices

19%

20%

17%

12%

7%5%

4%3%3%2%0

500

1000

1500

2000

2500

3000

3500

1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

Year

Sla

es

of

GaN

De

vice

s (

US

$ M

illio

ns

)

From Strategies Unlimited (1997)

Nichia estimates that the LD market alone will be worth $10B.

Violet Laser Diode

Nichia announced commercial release of VIOLET LASER DIODE (Model No.NLHV500A) on October 1, 1999

Costs $2000 apiece.

Possible new applications

• Smart power controls

• Smart cars

• Smart manufacturing

• Smart transportation

• Smart house (energy management)

• Information technology

• Displays

• Solid state lighting

• Medicine

• Defense

SiC and GaN-based switches

SiC and GaN-based sensors

SiC and GaN-non-volatile memories

GaN-based light emitters

GaN-solar blinddetectors

Solid State Lighting

• GaN-based solid state display lighting is nearly an order of magnitude more efficient than incandescent lamps and twice as efficient for general lighting.

• Practically does not require replacement.

• Will affect the energy industry, construction, automotive, and avionics applications.

Compact power switches

Compact efficient power switches for power distribution, automotive, avionics, and industrial applications. These switches should allow energy savings up to 10%. This may allow us to avoid the deployment of new power plants, cut our dependence on imported oil, and improve reliability of power distribution in order to minimize blackouts during natural disasters.

Example of research by WBS faculty:M. S. Shur, SiC Transistors, in "SiC Materials and Devices", ed. Y. S. Park, (1998), Academic Press;see also http://nina.ecse.rpi.edu/shur/GaN.htm

UV detectors and sources

UV sources and sensitive and fast UV detectors for applications in medicine, biology, chemical industry, and defense.

Example of research by WBS scientists:M. S. Shur and M. A. Khan, GaN and AlGaN Ultraviolet Photodetectors, Academic Press, T. Moustakos and J. Pankove, Editors (1998)see also http://nina.ecse.rpi.edu/shur/GaN.htm

Flat panel, high-resolution low power displays for computer and medical applications.

Example of research by WBS scientists:M. S. Shur, M. D. Jacunski, H. Slade, and M. Hack, Analytical Models for Amorphous-Silicon and Polysilicon Thin-Film Transistors for High Definition Display Technology, in Journ. Society for Information Display, vol. 3, No. 4, pp. 223-236, Dec. (1995)

8 mmPrototypes: Toshiba displays for billboardsSharp displays for Pachinko

Non-volatile solid state memories

These memories will make hard drives obsolete and revolutionize banking, medical record-keeping, and information storage.

p+ 6H-SiC Substrate

p-type 5x1018 cm-3

n-implanted emitter

p-base

n-floating collector

1x1019 cm-3

8x1018 cm-3

5x1017 cm-3 AccessTransistor

StorageCapacitor

2 µm

0.55 µm0.55 µm

Bipolar NVRAM Cell

SiO2

Word Line Bit LineAfter W. Xie, J. A. Cooper, Jr., M. R. Melloch, J. W. Palmour, and C. H. Carter, Jr.," IEEE Electron Device Lett., 15, 212 (1994).

Flat panel, high-resolution low power displays for computer and medical applications.

8 mm

Prototypes: Toshiba displays for

billboardsSharp displays for

Pachinko

Wireless Applications

AlN attractive for surface acoustic wave devices due to large piezoelectric effect. Sound velocity 10x materials currently used.

Laser Diode for Mass Data Storage•Optical Data Storage Market will use over 300M

LDs in 1999 (Compound Semicond., March 1999)

•HD-DVD will use GaN or SHG laser; will dominate future market with 15GB capacity or greater

•Market expects laser cost to be approx. $10 but current cost ~$2000.

Light-

emitting Diode (LED)

First visible LED

Blue LED

Traffic LightsOne of the first

applications of the new nitride semiconductor technology. The Green light uses 10% of the power and last more than 10x longer.

a Philips Lighting and Agilent Technologies joint venture that's changing the future of light. In the next century, LED-based lighting will quickly replace conventional lighting in a wealth of commercial, industrial and consumer applications. LumiLeds‘ LED-based solutions will bring irresistible value to lighting solutions of all kinds, earning us a leadership position in a fast-growing and lucrative marketplace. Our long-lasting, energy-efficient products will also improve the planet, by reducing waste and power consumption.

How does a semiconductor laser work?

Absorption and Emission

Eo

E1

n

nE E1

01 0exp[ ( )]

photon out

photonin

1 / k TB

Stimulated vs. Spontaneous Emission

Eo

E1

photonin

Spontaneous

photon out

Stimulated

Stimulated vs. Spontaneous Emission (Cont.)

Derived in 1917 by Einstein. (Required for thermal equilibrium was it was recognized that photons were quantized.)

However, a “real” understanding of this was not achieved until the 1950’s.

Population Inversion by Photopumping

Biased junction

n-type

p-type

depleted region(electric field)

Negativebias

photon out

History of Lasers

First operating Laser in 1960 (Maser in 1958)Simulated emission concept from Einstein in 1905Townes (1964) and Schawlow (1981)

First solid-state injection Laser in 1962First was Robert Hall but many competing groupsYear before he had argued it was impossible

Violet Laser Diode

Nichia Laser Diode

SiO2

n-contact

p-contact

p-GaN

n-GaN

sapphire substrate

p-Al0.15Ga0.85As

p-Al0.15Ga0.85As

n-Al0.15Ga0.85As

n-GaN blockinglayers

Active p-GaN/InGaNMQW

Epitaxial Lateral Overgrowth material

10,000 hoursoperation!

Other Applications for Wide band gaps

High Power devicesLarge band gap allows semiconductor to be used at

high voltagesGenerally larger band gap means stronger bonds so

material can withstand higher currents and temperatures

High Temperature devicesMuch smaller effect of thermal excitation of carriersTougher material

What are the hot research topics?

How can the lifetime of the lasers be improved?improved growthimproved substratesimproved devices

What is the role of defects?Very high field transportQuantum Confinement

Quantum Wells

( )x

Quantum Mechanics

• Probability density given by

• Schroedinger’s Equation:

•

• where

( )x 2

2

2xx C E U x ( ) ( ) ( )

Cm

h

16 2

2

Quantum Mechanics (cont.)

• Main point is that energy levels are Quantized!

• Well defined energy level even at room temperature.