(gallium nitride) is one of the third generation of wide band gap
semiconductors in Materials Chemistry. Compared with formal semiconductor
materials, GaN (gallium nitride) has a higher frequency, higher power, and
higher density for making integrated electronics, besides, the strong radiation
resistance ability for GaN (gallium nitride) could also make great
contributions in microwave power devices filed too.
Properties for GaN (gallium nitride)
nitride) is very hard and stable chemical compound and it’s melting point is
about 2000K. Generally, the atomic structure for GaN (gallium nitride) is
closed-packed hexagonal structure and that results in relatively low symmetry
of lattice and strong piezoelectricity and ferroelectricity. The
piezoelectricity describes when the lattice suffers from certain direction
pressure or tension, the vertical surface of the coming force will obtain equal
quantity inverse charges at two sides. The Ferroelectricity describes the
spontaneous polarization when the structure of lattice don’t have a center of
symmetry that makes the gravity center of positive and negative charges don’t coincide
together leading to the electric moment and is not equal to 0. The two factors
lead to very strong piezoelectric polarization and spontaneous polarization,
totally it generates 5(MV/cm) which’s called breakdown electric energy.
GaN (Gallium nitride) is regard as the third
generation wide band gap semiconductor. The band gap is 3.4 eV and thermal
conductivity is 1.3 W/cm*K. The band gap is presented on electronic band
structure which’s between valence band and conduction band. Normally valence
bond refers to the band of energy occupied by the valence electrons and usually
it’s the highest occupied band. Conduction Band is empty or may be defined as
the lowest unfilled energy band. The term ‘band gap’ refers to the energy
difference between the top of the valence bond and the bottom of the conduction
band. Electrons could gain enough energy to jump to the conduction band by
absorbing either a phonon or photon.
This two factors lead to the GaN (Gallium
nitride) has a high working temperature and breakdown voltage and a strong
ability of radiation resistance. The bottom of conduction band of GaN is at ? position
which makes a huge energy difference with other with other valley to resist the
scattering between different valleys. As a result, GaN has a very high
saturated drift velocity of electrons.
Comparing semiconductors with insulators, semiconductors
have a relatively smaller band gap and though both of them behave as insulator
at absolute zero, for semiconductors , it allow thermal excitation of electrons
into its conduction band at the temperature below its melting point. Generally,
wide-band gap semiconductors materials have band gaps in the range of 2-4 eV,
whereas typical semiconductors have band gaps in the range of 1-1.5 eV. Higher
energy of band gap makes it suitable for working in a high temperature. Wide
band gap semiconductors are associated with a high voltage. This is due to a
large electric filed to generate carries through impact mechanism.
GaN also has its shortcomings. Because of it structure of energy bond, the
electron mobility is relatively low while the charge carriers have a high
Preparation for GaN (gallium nitride)
preparation of GaN (gallium nitride) includes four main steps: metalorganic
chemical vapor deposition, hydride vapor phase epitaxy, separation and second
the MOCVD step, ultra-pure gases are transferred into a reactor and finally result
in a deposition of a very thin layer of atoms onto a semiconductor wafer. For
instance, Pin can be grown in a heated substrate by trimethylindium and
precursor molecular decomposition happens in the absence of oxygen. As to the
equipment, the reaction chamber is the main body that is composed by reactor
walls, liner, susceptor, gas injection units and temperature control units.
two temperature should be paid attention when we heat the substrate. One is
around 823K and another is
around 1273K. In the low temperature condition, there will be a buffer layer growing
firstly. However, in the high temperature, GaN (gallium nitrate) will grow
directly. So the temperature should be
The hydride vapor
phase epitaxy (HVPE) makes the GaN (gallium nitrate) grow continually. The hydrogen
chloride is reacted at elevated temperature while the group (III) metal producing
gaseous metal chlorides and then it will react with ammonia to produce group (III)
As to the separation
part, the technique of laser lift-off is better than natural separation which
uses high power pulsed laser directly to the surface. The energy of light is
between Esubstrate and EGaN.
One of the typical application for GaN (gallium nitride) is power
devices. Compared GaN (gallium nitride) with other materials, it has relatively
small volume and high efficiency to transport. Nowadays, as the popularization
of 4G cell site and wireless power, the potential market could be expected too.
Besides from the strong ability for GaN
(gallium nitride) to transport information, the high color rendeing index and
luminous efficiency of GaN (gallium nitride) also could be applied to the LED.
For instance, many companies have put
their eye on the research and exploitation on GaN (gallium nitride) materials,
like Samsung, Mitsubishi etc.
According to the graph mentioned on the
slides, it can easily show us the promising future of GaN (gallium nitride),
the statistics also show that the total value in US in 2015 has arrived at 298
Dollars, and many of the cost is
concentrated on wireless infrastructure.