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The key element of electric
engineering, computer engineering, or pretty much everything else that is
remotely "technical" in the last thirty years of the 2nd
millennium, is the integrated
transistor in a Silicon crystal -
everything else comes in second - at best. |
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Integrated
means that there is more than one transistor on the same piece of Si
crystal and thus in the same package. And "more
than one" means at the present stage of technology some
107 transistors per
cm2 of Silicon. |
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Silicon
crystal means that we use huge, extremely perfect single crystals of
Si to do the job. Why Si and not. for exampleGe,
GaAs or SiC? Because if you look at the sum total of the most
important properties you are asking for (crystal size and perfection,
bandgap, extremely good and process compatible
dielectric, ...) Si and its oxide, SiO2 are so vastly
superior to any possible contender that there is simply no other semiconductor
that could be used for complex integrated circuitry. |
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The lowly integrated circuit (IC), mostly selling for a few Dollars, is
the most marvelous achievement of Materials Science in the second half of the
20th century. Few people have an idea of the tremendous amount of
science and engineering that was (and still is) needed to produce a
state-of-the art chip, the little piece of Si
crystal with some other materials in precise arrangements, that already starts
to rival the complexity of the brains of lower animals and might at some day in
the not so distant future even rival ours. |
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If we want to make a circuit out of many transistors (some of which we
use as resistors) and maybe some capacitors, we need three basic ingredients - no matter if we do this in
an integrated fashion or by soldering the components together - and on occasion
some "spices", some special additions: |
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1. Ingredient: Transistors. "Big" and "small"
ones (with respect to the current they can switch), for low or high voltage,
fast or not so fast - the whole lot. We have two basic types to chose
from: |
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Bipolar transistors
(the hopefully familiar pnp- or npn-structures) are usually drawn as follows |
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Note right here that no real transistor looks
even remotely like this structure! It's only and purely a schematic drawing to
show essentials and and has nothing whatsoever to do with a real
transistor. |
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The name bipolar comes
from the fact that two kinds of carriers,
the negatively charged electrons and the positively charged holes, are
necessary for its function. |
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MOS
Transistors or
unipolar Transistors, more or less
only exist in integrated form and usually are drawn as follows: |
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2. Ingredient: Insulation. Always needed between transistors and
the other electrically active parts of the circuit. |
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In contrast to circuits soldered together where you simply use
air for insulation, it does not come "for free" in ICs but has
to be made in an increasingly complex way. |
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3. Ingredient: Interconnections between the various transistors
or other electronic elements - the wires in the discrete circuit. |
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The way you connect the transistors will determine the
function of the device. With a large bunch of transistors you can make
everything - a microprocessor, a memory, anything - only the interconnections
must change! |
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Then we may have special
elements |
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These might be capacitors, resistors or diodes on your chip.
Technically, those elements are more or less subgroups of transistors (i.e. if
you can make a transistor, you can also make these (simpler) elements), so we
will not consider them by themselves. |
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If you did the required reading, you should be
familiar with the basic physics of the two transistor types; otherwise do it now!!! |
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The list of necessary ingredients given above
automatically implies that we have to use several different materials. At the
very minimum we need a semiconductor (which
is practically always Silicon; only GaAs has a tiny share of the
IC market, too), an insulator and a
conductor. As we will see, we need many
more materials than just those three basic types, because one kind of material
cannot meet all the requirements emerging from advanced Si
technology. |
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Since this lecture course is about electronic materials, it may appear that all we need
now is a kind of list of suitable materials for making integrated circuits. But
that would be far too short sighted. In IC technology, materials and processes must be seen as a unit -
one cannot exist without the other. |
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We therefore have to look at both, materials with their
specific properties and their integration into a process flow. |
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Todays integrated circuits contain mostly
MOS transistors, but we will start with considering the integration of
bipolar transistors first. That is not only because historically bipolar
transistors were the first ones to be integrated, but because the basic
concepts are easier to understand. |
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© H. Föll (Electronic Materials - Script)
