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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.
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