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
Band Gap Energy (eV)
7
6.2eV
AlN
6
5
4
UV 3.4eV GaN
3
Visible light
2
1.95ev InN
IR
1
2.6
2.8
3.0
3.2
3.4
3.6
o
Lattice Constant a (A)
3.8
4.0
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 Information technology Displays
(data storage)
Solid state lighting
Traffic lights
Wireless
communications
Electric power industry
Health care
The Market for GaN
Devices
From Strategies
Unlimited (1997)
The Market for GaN Devices
Slaes of GaN Devices
(US $ Millions)
3500
20%
3000
19%
2500
17%
2000
1500
12%
1000
500
2%
3% 3%
4%
5%
7%
0
1997 1998 1999 2000 2001 2002 2003 2004 2005 2006
Year
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
•
•
•
•
•
•
•
•
•
•
SiC and GaNbased switches
Smart power controls
Smart cars
Smart manufacturing
SiC and GaNSmart transportation
based sensors
Smart house (energy management)
SiC and GaNInformation technology
non-volatile memories
Displays
GaN-based light emitters
Solid state lighting
Medicine
GaN-solar blind
Defense
detectors
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.
8 mm
Prototypes:
Toshiba displays for billboards
Sharp displays for Pachinko
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)
Non-volatile solid state memories
These memories will make hard drives obsolete and
revolutionize banking, medical record-keeping, and
information storage.
Bipolar NVRAM Cell
n-implanted emitter
Word Line
Bit Line
After W. Xie, J. A. Cooper, Jr.,
1x1019 cm-3
SiO2
p-base
5x1017 cm-3
n-floating collector
8x1018 cm-3
Storage
Capacitor
M. R. Melloch,
J. W. Palmour, and C. H. Carter, Jr.,
" IEEE Electron Device Lett.,
15, 212 (1994).
0.55 µm
0.55 µm Access
Transistor
2 µm
p-type 5x1018 cm-3
p+ 6H-SiC Substrate
Flat panel, high-resolution low power displays
for computer and medical applications.
8 mm
Prototypes:
Toshiba displays for
billboards
Sharp 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 Lights
One 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 longlasting, energy-efficient products will
also improve the planet, by reducing
waste and power consumption.
How does a semiconductor
laser work?
Absorption and Emission
E
photon
in
1
 n1 
 exp[ ( E1  E0 )]
 n0 
  1/ kBT
photon out
Eo
Stimulated vs. Spontaneous
Emission
Spontaneous
Stimulated
E
1
photon
in
photon out
Eo
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
Negative
bias
photon out
p-type
n-type
depleted region
(electric field)
History of Lasers
 First operating Laser in 1960 (Maser in 1958)
 Simulated emission concept from Einstein in 1905
 Townes (1964) and Schawlow (1981)
 First solid-state injection Laser in 1962
 First was Robert Hall but many competing groups
 Year before he had argued it was impossible
Violet Laser Diode
Nichia Laser Diode
p-contact
n-GaN blocking
layers
Active p-GaN/InGaN
MQW
p-Al0.15Ga0.85As
10,000 hours
operation!
p-GaN
n-Al0.15Ga0.85As
SiO2
n-contact
p-Al0.15Ga0.85As
n-GaN
sapphire substrate
Epitaxial Lateral Overgrowth material
Other Applications for Wide band gaps

High Power devices
 Large band gap allows semiconductor to be used at
high voltages
 Generally larger band gap means stronger bonds so
material can withstand higher currents and temperatures

High Temperature devices
 Much smaller effect of thermal excitation of carriers
 Tougher material
What are the hot
research topics?
How can the lifetime of the lasers be
improved?
improved growth
improved substrates
improved devices
What is the role of defects?
Very high field transport
Quantum Confinement
Quantum Wells
( x )
Quantum Mechanics
• Probability density given by
( x )
2
• Schroedinger’s Equation:
•
2

•
x
2
( x )  C ( E  U ) ( x )
where
16  2 m
C
h2
Quantum Mechanics
(cont.)
• Main point is that energy levels are
Quantized!
• Well defined energy level even at room
temperature.