Ask A Scientist Chemistry Archive

(Created prior to 1993)

Question: Why is silicon used in making computer chips?  Is it because of 
special chemical properties?  Could other crystals be used equally well?
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Hi rita, great question! I have a few guesses in answer to this.  
One thing that makes silicon desirable for manufacturing purposes is that it 
is abundant (it is the second most abundant substance in the earth's crust) 
and therefore cheap.  Another, more directly relevant reason, is that silicon 
is an "impurity semiconductor."  What this means is that if you deliberately 
make a sample of silicon with certain impurities lodged in the crystal, you 
can control the conductivity of the sample by varying the concentration of 
impurities.  What is more, you can do it over a range of conductivity values 
that correspond to small voltages.  And best of all, you can either increase 
or decrease the conductivity, depending on what kind of impurity you 
introduce.  Germanium (Ge) also has these properties, but is more expensive.  
These properties make form the basis for a wide variety of semiconductor 
devices.  Also, I believe that silicon wafers can be etched by chemical 
process (involving fluorine, I think) to change the conductivity along well-
defined channels in the wafer.  This combination of properties makes silicon a 
uniquely good choice for making computer chips . . . it has special chemical 
AND physical properties.  I just received this note from "Eric Peterson" who 
has agreed to let me post it in this space. - robert
Great answer to the question on silicon use in chips.  As an expansion, the 
use of Gallium Arsenide is becoming more popular as a base material for doping 
than silicon, or in conjunction with it.  Gas chips are much faster because of 
two reasons.  1)  Wafers can be sub-micron, and therefore quicker because of 
the less amount of circuitry involved, and 2)  GAs substrate has a faster 
charge transfer rate than does Silicon.  There are several good trade 
publications that delve into this subject at great length.  One is called 
"surface Mount Technology" and another is a weekly tabloid called "Electronics 
Products News".  Both are very good sources for those who are interested in 
keeping up with the latest new tech stuff in chip making. --- 
Eric Peterson
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