Gallium nitride is a binary III/V direct bandgap semiconductor commonly used in
bright light-emitting diodes since the 1990s. The compound is a very hard
material that has a Wurtzite crystal structure. Its wide band gap of 3.4 eV
affords it special properties for applications in optoelectronic,
high-power and high-frequency devices. For example, GaN is the substrate which
makes violet laser diodes possible, without use of nonlinear optical
frequency-doubling. Its sensitivity to ionizing radiation is
low, making it a suitable material for solar cell arrays for satellites.
Military and space applications could also benefit as devices have shown
stability in radiation environments. Because GaN transistors can operate at
much higher temperatures and work at much higher voltages than gallium
arsenide transistors, they make ideal power amplifiers at microwave
frequencies. Physical properties
GaN is a very hard, mechanically stable wide bandgap semiconductor material with
high heat capacity and thermal conductivity. In its pure form it
resists cracking and can be deposited in thin film on sapphire or silicon
carbide, despite the mismatch in their lattice constants. GaN can be doped with
silicon or with oxygen to n-type and with magnesium to p-type; however, the
Si and Mg atoms change the way the GaN crystals grow, introducing tensile
stresses and making them brittle. Gallium nitride compounds also tend to
have a high spatial defect frequency, on the order of a hundred million to ten
billion defects per square centimeter. Developments
GaN with a high crystalline quality can be obtained by depositing a buffer layer
at low temperatures. Such high-quality GaN led to the discovery of p-type GaN,
p-n junction blue/UV-LEDs and room-temperature stimulated emission.
This has led to the commercialization of high-performance blue LEDs and
long-lifetime violet-laser diodes, and to the development of nitride-based
devices such as UV detectors and high-speed field-effect transistors.
High-brightness GaN light-emitting diodes completed the range of primary
colors, and made applications such as daylight visible full-color LED
displays, white LEDs and blue laser devices possible. The first GaN-based
high-brightness LEDs were using a thin film of GaN deposited via MOCVD on
sapphire. Other substrates used are zinc oxide, with lattice constant mismatch of
only 2% and silicon carbide. Group III nitride semiconductors are in general
recognized as one of the most promising semiconductor family for fabricating
optical devices in the visible short-wavelength and UV region.
The very high breakdown voltages, high electron mobility and saturation
velocity of GaN has also made it an ideal candidate for high-power and
high-temperature microwave applications, as evidenced by its high Johnson's
figure of merit. Potential markets for high-power/high-frequency devices based
on GaN include microwave radio-frequency power amplifiers and high-voltage
switching devices for power grids. A potential mass-market application for
GaN-based RF transistors is as the microwave source for microwave ovens,
replacing the magnetrons currently used. The large band gap means that the
performance of GaN transistors is maintained up to higher temperatures
than silicon transistors. The first gallium nitride metal semiconductor
field-effect transistors were experimentally demonstrated in 1993 and
they are being actively developed. In 2010 the first enhancement-mode
gallium nitride transistors became generally available. These devices were
designed to replace power MOSFETs in applications where switching speed or
power conversion efficiency is critical. These transistors, also called eGaN
FETs, are built by growing a thin layer of GaN on top of a standard silicon
wafer. This allows the eGaN FETs to maintain costs similar to silicon power
MOSFETs but with the superior electrical performance of GaN.
Applications GaN-based violet laser diodes are used
to read Blu-ray Discs. When doped with a suitable transition metal such as
manganese, GaN is a promising spintronics material. The mixture of GaN
with In or Al with a band gap dependent on ratio of In or Al to GaN allows the
manufacture of light-emitting diodes with colors that can go from red to
ultra-violet. GaN HEMTs have been offered commercially
since 2006, and have found immediate use in various wireless infrastructure
applications due to their high efficiency and high voltage operation.
Second generation technology with shorter gate lengths will be addressing
higher frequency telecom and aerospace applications. GaN based MOSFET and
MESFET transistors also offer many advantages in high power electronics,
especially in automotive and electric car applications. Nanotubes of GaN are
proposed for applications in nanoscale electronics, optoelectronics and
biochemical-sensing applications. They are also useful in military
electronics such as active electronically scanned array radars.
SAAB's JAS 39 Gripen NG is using an electronic warfare system based on
gallium nitride technology. GaN-based electronics has the potential
to drastically cut energy consumption, not only in consumer applications but
even for power transmission utilities. MIT spinout Cambridge Electronics Inc's
claim their transistors have at least one-tenth the resistance of such
silicon-based transistors. This allows for much higher energy efficiency, and
orders-of-magnitude faster switching frequency. This has huge implications
not only for energy usage of power electronics systems, but their physical
size and stability. Unlike silicon transistors which switch off due to
power surges, GaN transistors are typically normally-on, however the
company along with US government and university assistance developed GaN
transistors via materials engineering to be "normally off" just as silicon based
transistors. Synthesis
= Bulk substrates= GaN crystals can be grown from a molten
Na/Ga melt held under 100 atmospheres of pressure of N2 at 750 °C. As Ga will not
react with N2 below 1000 °C, the powder must be made from something more
reactive, usually in one of the following ways:
2 Ga + 2 NH3 → 2 GaN + 3 H2 Ga2O3 + 2 NH3 → 2 GaN + 3 H2O
= Molecular beam epitaxy= Commercially, GaN crystals can be grown
using molecular beam epitaxy. This process can be further modified to
reduce dislocation densities. First, an ion beam is applied to the growth
surface in order to create nanoscale roughness. Then, the surface is
polished. This process takes place in a vacuum.
Safety GaN dust is an irritant to skin, eyes
and lungs. The environment, health and safety aspects of gallium nitride
sources and industrial hygiene monitoring studies of MOVPE sources have
been reported recently in a review. Bulk GaN is non-toxic and biocompatible,
and therefore may be used in the electrodes and electronics of implants
in living organisms. See also
Schottky diode Semiconductor devices
Molecular-beam epitaxy Epitaxy
References External links
Ioffe data archive