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Engineering LibreTexts

Elemental Silicon

Silicon is the next element in the carbon family (Group IV/IVA) and shares some of the same atomic structure and tetrahedral bonding tendencies. Unlike carbon, silicon compounds do not have the ability to form double or triple bonds. In crystalline form, silicon does not have the different forms as carbon ( i.e., either diamond, graphite or fullerine graphite). Crystalline silicon forms only a cubic diamond-like structure.  Solid silicon can either be amorphous, single-crystal, or polycrystalline.

Unusual or Noteworthy Properties

1. Ease of  crystal-pulling

In the molten state ( nominally 1414C), liquid silicon exhibits a 2 C temperature range where it can be super-cooled. It has an unusually high surface tension of 720 dynes/cm at its nominal melting point, but that can increase up to 830 dynes/cm when it is the super-cooled molten state. Like ice, molten silicon expands about 8% when solidified. These characteristics allow purified molten silicon to be "pulled" upwards from a molten pool,  as a large diameter single-crystal cylinder.  After subsequent slicing into thin "wafers", and polishing, these wafers are the main structural elements of most solid-state electronic devices ( like computer chips) and photovoltaic solar cells.

2. Variable resistivity

By allowing parts-per-billion levels of phosphorus or boron (i.e., dopants) to be present in the high purity crystal, the electrical resistivity of silicon can be highly customized. These dopant levels can also be adjusted to deposit molecularly thin layers epitaxially. This allows for highly responsive semi-conductors to be made, or to produce the familiar solar cells.

3. Photovoltaic Effect

Silicon is the only element that has a photovoltaic response to natural sunlight ( other photovoltaics require combinations of elements - some of which are toxic or rare).  A significant amount of the sun's visible light frequencies will cause direct current to flow, if a purified silicon wafer is given polarity. Optimally a silicon solar cell can convert 25% of the incident sunlight to electrical energy, although commercial solar cells only operate in the 15-20% efficiency range.

Main Applications and Uses

1.  Solar Cells

The solar cell application requires the doping of a polished wafer, typically 100-150 mm diameter x 100-500 microns thick that has an optimal  resistivity of 1 ohm-cm. The upper surface of the wafer is ideally negative, made so by epitaxially depositing a thin layer of "N" type silicon, using a Group V hydride gas dopant such as phosphine or arsine.  The bottom surface is made positive by epitaxially depositing a thin layer of "P" type silicon, using diborane ( a Group IIIA hydride).  More about the construction of solar cells can be found at < http://pveducation.org/pvcdrom/design/solar-cell-parameters > . The attached graphic illustrates the details.

 2. Semi-Conductors

 The semi-conductor application of pure silicon is more complex, and typically features thinner wafers of 40-100 microns. Individual “NPN” or “PNP”  transistors are no longer used, but rather systems of programmable transistors, called integrated circuits ( IC). Like the solar cell application, the silicon wafer can be given polarity by epitaxial growth using either “N” or “P” dopants, but only in extremely precise regions on the wafer – sometimes only 20-50 atoms in thickness or width.  Or a region’s polarity can be altered by targeted ion implantation.

Using a system of photo masking, and etchant gases, extremely precise amounts of silicon are removed or added to the silicon wafer. The distinction between semi-conductor “device” and the interconnecting “wire” has been lost.  Modern IC’s are now so sophisticated, large, and fast that they referred to as processors, such as the latest Intel Pentium 64-bit processor.  It has cycle speeds of over 3 GHz ( 3 billion times per second), and consists of over 1 billion individual transistors. Linear signal speed approaches the speed of light (about 3x109 meters/second). The attached file “Integrated Circuit Semi-Conductors” provides greater detail.

3.  Metallurgical and Alloying

Silicon can be readily extracted from naturally occurring quartzite ore by reacting it with charcoal in an electric arc furnace at 1600C. The carbon from the charcoal combines with the oxygen in the quartzite, liberating the silicon metal in molten form.  Carbon Dioxide gas is formed.  Cast into ingots and crushed into granules, this metallurgical silicon is added to many molten metal alloys to form silicides that give the metal toughness ( e.g., ferric silicide helps steel to be tougher).

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