Gallium ArsenideGaAs

By:Nhi Mai, Ricardo Lopez, Eric Leal,

Hector Leal Brol, James Lillian

ENGR2300 – Materials EngineeringNovember 2011

Outline Introduction History Properties Manufacturing Processes Cost Applications Conclusion

Introduction

Gallium Arsenide (GaAs) is a combination of one gallium atom (atomic no. 31) and one arsenic atom (atomic no. 33). The atoms are arranged in a cubic sphalerite lattice. It has a FCC symmetry. The unit cell contains four GaAs molecules. Gallium atoms bond to four arsenic and each arsenic atom bonds to 4 gallium atoms.

Gallium Arsenide

Elemental Gallium does not occur naturally in nature but in trace amounts with bauxite and zinc. It is a byproduct during those ores’ processing. In its pure form gallium is a soft, brittle and silvery metallic in color. Elemental gallium melts slightly above room temperature and will melt in your hand.

Introduction Gallium

Introduction Arsenide

Arsenic is a highly toxic element that is found with many minerals, especially sulfur, and can be found naturally as a pure substance. Its main industrial use is as a strengthener in alloys with copper and lead. Arsenic is also in declining use in pesticides and herbicides.

History Dmitri Mendeleev

predicted all elements’ properties on the periodic table

1875, Lecoq de Boibaudran discovered Gallium in the Pyrenees Mountains

Method to retrieve Gallium: electrolysis

Gallium

History Arsenic

Been known since the Bronze Age

1250, discovered by Albert the Great

Used as an impurity in bronze

Later found that Arsenic was poisonous

Has symptoms that were difficult to distinguish

History Gallium Arsenide

Discovered by Victor Goldschmidt

First to combine these elements

1954, produced a large amount for research on use of this compound

Since 1960s, used in many devices such as solar cells, light-emitting diodes, lasers, and optoelectronic devices

Properties & Crystal Structure

Gallium, a poor metal, has atomic number 31 and it is located in group 13, period 4, and block “p”. Its mas is 69.72 amu, and has no electronegativity according to Pauling.

Arsenic, a metalloid, has atomic number 33 and it is located in group 15, period 4, and block “p”. Its mas is 74.92 amu, and has an electronegativity of 2.18 according to Pauling.

Electrical &Thermal Properties Electrical Properties

Band gap (eV) = 1.42 Electrical conductivity

1X10-6 (Ω*m)-1 Electron mobility 7.7 m2 /

V*s Hole mobility .07 m2 / V*s Electron thermal velocity

4.4X105 m/s Hole thermal velocity

1.8X105 m/s

Thermal Properties

GaAs has a thermal conductivity of 0.55 W/cm-C

Melting point 1238 °C Specific heat 0.33 J g-1°C -1

Thermal diffusivity 0.31 cm2s-1

Thermal expansion, linear 5.73X10-6 °C -1

Crystal StructureThe GaAs crystal is

composed of two sublattices.

Each sublattice are face centered cubic (fcc) lattice.

They offset with respect to each other by half the diagonal of the fcc cube.

This crystal configuration is known as cubic sphalerite or zinc blende.

GaAs has basically outdone silicon in every imaginable way due to its incredible characteristics.

These characteristics make GaAs and ideal candidate for use in mobile phones, satellite communications, microwave point-to-point links, and some radar systems.

GaAs Properties (Room

Temperature)

GaAs vs. SiliconGaAs

Higher saturated electron velocity and higher electron mobility.

GaAs devices generate less noise than silicon and have higher breakdown voltages.

It has a direct bandgap which means that it can be used to emit light.

Silicon Considerably cheaper

than GaAs Existence of silicon

dioxide (one of the best known insulators of any kind)

Posses a much higher hole mobility which allows for the construction of higher-speed P-channel field effect transistors.

Manufacturing Process Ignot

Growing Ingot is a mold or cast of

a substance in bulk quantity ex: gold bars

Ingots can be grown 2 ways: Bridgman Method, or Czochralski Method

CM: Molten GaAs is slowly pulled upwards with a seed crystal while cooling

BM: melting pure GaAs and cooling it from one side. No pulling.

Centromax.net

Manufacturing Process Wafer

Processing GaAs wafers are cut

from ingots. Requires mechanical

and chemical steps such as etching, cropping, thermal treatment and polishing.

Single crystal wafers are then used for another process, Epitaxy.

Manufacturing Process Epitaxy

Epitaxy- gowing a thin single crystal layer over a single crystal substrate

Molecular Beam Epitaxy (MBE)

Molecular beams sending molecules to the substrate through a vacuum, substrate is heated exciting molecules eventually sticking to substrate

Metal Organic Vapor Chemical Deposition (MOVCD)

Similar to MBE except layer is formed by chemical reactions rather than physical placement

CostExpensiveGallium is rareAttempts to lower cost

by recycling substrates or using cheaper substrates such as germanium or silicon

Silicon max efficiency 25%

GaAs efficiency 30%

Applications

Limit cost Improve integrated circuits in all aspects of technologies.

Due to its unique qualities, GaAs can be used in computers, lasers, solar cells, aerospace technology, medical devices etc.

Applications Solar Cells

Solar panels consist of photovoltaic solar cells Leading focus in respect to applying it to solar

cellsUsed in photovoltaic solar cells for numerous

reasons: – GaAs has a high absorption rate – Very thin– Records for highest efficiency (25%)

Has a direct band gap, because of efficiency rate, leads to shrinking of solar cell sizes thus reduction in area

Photovoltaic applications include:– Satellites, cars, calculators highway signs

Applications Aerospace Projects such as the Rovers Spirit and

opportunity, which are exploring Mars’ surface, have used GaAs for its solar cell power source

Radiation hardness. “Single effect events,” (SEEs) associated with the transit of heavy ions through semiconductor junctions have to be minimized to reduce failure risk. Radiation effects from particles can cause degradation, and also failure of the electronic systems in space vehicles or satellites

GaAs has promising future aerospace applications in satellites, space crafts, signal devices, sensors

Applications Transistors

Having a higher electron mobility, this allows flow of electricity in transistors to flow much faster and can lead to many future applications such as: – faster computers– better wireless communication devices

Faster response times for transistors from GaAs

Less propagation delay, less noise to date using molecular beam epitaxial GaAs FETs

Applications

GaAs power amplifiers operate at higher power levels, have higher linearity and sharper edges, can be operated at higher power levels because they have higher breakdown voltages and allow maximum channels to be used

More efficient solar cells GaAs technologies allows to design at lower

frequencies, with less power consumption, over a smaller area, providing a size and power advantage to multi-function integrated solutions for circuits and many more technologies

Conclusion Cost expensive, but hopefully will decrease over

the years GaAs has excellent electronic properties.

(It is superior to Silicon.) Powerful in the electronic industry:

Aerospace Transistors Solar Cells