Gallium Arsenide (GaAs) the Savior
of the Semiconductor
Neil Troy
Gallium Arsenide (GaAs)
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Why GaAs is special
Process to make GaAs wafers
Ion implantation
Uses for GaAs
Gallium Arsenide (GaAs)
GaAs’s advantages over silicon
in semiconductor use
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High electron mobility (~8x Silicon)
Significant reduction in signal noise
High power transmission
High breakdown voltages
Direct bandgap
Electron Mobility
In semiconductors, electron mobility is directly related to the current for a
given applied voltage,
Je=-mnncE
which states the current is the (negative) product of the electron mobility,
the number of carriers, and the electric field. For a the same applied
electric field GaAs would be able to exhibit an 8 times larger current in an
n-type material, or one can apply a smaller voltage to achieve the same
amount of current.
Smaller applied voltages means smaller power supplies (physically) as
well as less heat, in a semiconductor.
Larger currents can allow for more current can be easily carried (amplification).
Can be operated at much higher frequencies than silicon equivalents.
Noise & Power Transmission
Since GaAs has very low noise characteristics one can make many
changes to modern semiconductors.
The potential for smaller devices as noise and cross-talk of elements are
diminished.
Higher gain amplifiers can be made.
By having low noise characteristics one can amplify to a greater
extent without worrying about noise amplification. The worry of
larger circuitry is negated since GaAs also has superior power
transmission properties.
Direct Bandgap Semiconductor
E
Conduction Band
Valence Band
Which lends itself perfectly to LEDs
k
Silicon has an indirect bandgap and a phonon is emitted instead
GaAs Wafer Production
• Ingot Production
• Cutting and Polishing
• Ion Implantation
Ingot Production
Liquid Encapsulated Czochralski
Liquid Encapsulate (B2O3)
Seed GaAs
Melted Gallium & Arsenide
Lapping, Slicing, & Lapping Again
An ingot is then placed in a lathe where it can be ground down and then lapped
into its cylindrical shape.
A special circular saw called an “ID” saw is then used to cut thin wafers from this
cylindrical mass.
Diamond Cutting Wheel
625 mm
100 mm
Ion Implantation
Many semiconductors use a diffusion process to place impurities in the
semiconductor but GaAs evaporates at the temperatures required for diffusion. To
introduce the dopants a process called ion implantation is required.
Ion implantation has many advantages over diffusion:
-Direct control of the dopant and width of dopant
-Possibility for uniformity
-Able to create abnormal shapes of the dopant (can use masks)
-Room temperature operation
Ion Implantation
Magnetic Field
Dopant Source
Ions can be precisely placed deep
in a semiconductor (10 nm – 1mm)
Wafer
GaAs in Solar Cells
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Offer better efficiency than silicon equivalents
Can be made a few microns thick as opposed to 100s of microns for silicon
Extremely resilient to radiation damage (satellite applications)
References
GaAs Semiconductor Manufacturing,
http://www.mse.vt.edu/faculty/hendricks/mse4206/GaAsTEK/default.htm (Nov. 26, 2007)
US Department of Labor, OSHA, http://www.osha.gov/SLTC/semiconductors/gaas_index.html (Nov.
26, 2007)
SANDIA, Photovoltaic Systems Research and Development,
http://photovoltaics.sandia.gov/docs/PVFSCGallium_Arsenide_Solar_Cells.htm (Nov. 26, 2007)
Wait a sec...
If GaAs is so superior why is it not everywhere?
Gallium is not naturally found in a deposit. Ga diffuses itself into many
other substances and is only noticed spectrally or by melting of the
substance. This helps explain its relatively late discovery in 1875.
An extensive process must be carried out to actually get a “pure” form of
Ga. Firstly, one has to melt other materials to find Ga and then once a Ga
ingot can be made it must be purified further for semiconductor use,
commonly this is performed by zone melting.
End result, 99.9999% pure Ga costs ~$15 per gram.
...and...
Gallium’s melting point is ~30 C, and as such must be handled in special
fashions so that it does not melt and diffuse in its container or
surroundings. In its solid phase Ga is actually quite brittle, which again
complicates handling.
Add to this that GaAs is fairly brittle and wafers are normally limited to
about 4” in diameter whereas silicon wafers are typically made at about
12” diameter. This significant decrease in useable area adds again to the
expense of a GaAs wafer.
Approximate price for a 2” GaAs wafer is ~$100
Whereas a 12” wafer of Si is ~$200
...and...
Arsenic itself is a very toxic and needs to be handled delicately to prevent
adverse effects (think of Napoleon).
Ion implantation has key advantages over diffusion but from a cost and
time perspective it is at a severe disadvantage.
Although a key property of GaAs is that it has a high electron mobility
compared to silicon it has an inferior hole mobility which limits its uses to
mainly n-type.