Lecture #10 Electrons, Atoms, and
Materials
Reading:
Malvino chapter 2 (semiconductors)
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Electrons
• Electrons are not point particles
• Can think of them like a vibrating jelly
• Electrons take spatial shapes sometime
known as orbitals—would have been
better to call them states or modes
• Only one electron of each of two types can
be in the same mode (the types are called
spin up and spin down)
• This is called Fermi exclusion
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Here is what some of the orbitals
look like:
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Occupation
• Atomic nuclei have varying numbers of
protons in them, which have a positive
charge which has the same magnitude as
that on the electron.
• A nuclei will attract electrons to it until it
becomes neutral, filling the electronic
states around it, making an atom.
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• Since only two electrons can be in each
spatial state, they fill up the orbitals in
order.
• Lowest energy state first: 1S (spherical)
• One proton→one electron  Hydrogen
• Two protons→two electrons  Helium
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Chemistry
• Interactions of electrons in orbitals is what
we call chemistry
• The chemical reactions which a atom
takes part in are determined by the outer
orbitals.
• Because the inner orbitals are all full, and
that keeps out other electrons like a shield.
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Filling of orbitals
1s
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f
6s 6p 6d
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The orbitals fill up in the order shown
(simplified a bit)
Since the p, d, f orbitals have similar
shapes and they get filled up in
sequence, we get a periodicity in the
chemical properties, giving us the
periodic table
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Periodic table
The number in each element’s box is the atomic number, which is
the number of protons, and consequently the number of electrons
needed for neutrality
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Bonding
• “Bonding” is due to the fact that when two atoms
are close together, the outer orbitals are
wrapped around both of the nuclei, and the
electons are in these “shared” orbitals.
• These orbitals can have lower energy than those
of the two atoms would have if they were farther
away from each other.
• Since the energy decreases as the atoms get
close together, this provides a bonding force
• The shared electrons act like glue!
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Column 4 of the periodic table
IV
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Orbitals
• The periodic table can be understood by
the following
• The single S orbital can hold 2 electrons
• The three P orbitals can hold 6 electrons
• If the orbitals are all full, the atom does not
share and play well with friends. (Neon)
• While the other orbitals are filling, we get a
whole bunch of different metals.
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Silicon
• The elements of column four of the
periodic table can bond with each other in
a regular structure (crystallize)
• each atom bonded to the four nearest
neighbors
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Silicon structure and bonding
• There are four nearest neighbors, four
orbitals to share, and 4 electrons to
contribute
• Since each neighbor contributes one
electron for each orbital, the orbitals are all
full and the lattice is strong and stable
• Carbon in this form is called Diamond
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TEM picture of silicon
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Semiconductor
• Interestingly enough, even though this electronic
glue is everywhere in the crystal, they can not
contribute to current because they are locked
into this pattern.
• These electrons are said to be in the “valence
band”
• If there were a few extra electrons, they could
wander about.
• How do we get extra electrons into our crystal?
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Column 5 of the periodic table
V
If we look at column 5 of the periodic table, we can see that these
elements are very similar to silicon, except that they have one extra
proton—an extra quantum of positive charge.
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• So a column 5 element substituted for a silicon
atom results in what looks like a silicon crystal
but which has a fixed postive charge.
• This is called “doping” the silicon with Arsenic or
phosphorus.
• If the crystal is to be electrically neutral, then
there will be a mobile electron, called a
conduction band electron, hanging around.
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N type silicon
• So if we take a silicon crystal, and add a very small
amount of atoms of arsenic or phosphorous to it, then it
will have extra electrons which can wander around.
• Since these extra electrons can move, they can conduct
a current
• This is called N type because the carriers that can move
are negative.
• They are at a higher energy than they could have if they
could fall down into a bond
• Electrons which can wander around above a full set of
orbitals are said to be in the “conduction band”
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P type
But wait a minute: the electrons are always
what moves through a crystal, and they
are always negative!
• Lets look at what happens if we put a few
atoms from column 3 of the periodic table
into the crystal
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Column 3 of the periodic table
III
If we look at column 3 of the periodic table, we can see that these
elements are very similar to silicon, except that they have one
fewer protons—one less quantum of positive charge.
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P type semiconductors
• If we have a silicon crystal with a few Boron,
Aluminum, or Gallium atoms in it, then a few
orbitals will be missing an electron.
• Other electrons can hop into that orbital
• Since the electrons can now move, the crystal
can conduct an electrical current
• Since it is an unoccupied orbital that is moving, it
is called a “Hole”
• It moves like a positively charged particle would.
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Electrons and Holes
• If you were to have both electrons and holes in
the crystal, the electrons could fill up the holes
until one or the other was depleted.
• Silicon crystals with quite a few extra electrons
running around are called N-type, and they only
have a few holes in the valence band.
• Silicon crystals with many holes running around
are called P-type, and they only have a few
electrons in the conduction band.
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Applications
• We can take silicon wafers and put patterns of
doping on their surface, and control its
conductivity.
• We can further control the conductivity by
applying electric fields
• We can make use of the difference between P
and N type carriers moving through the crystal.
• This ability to control the conduction of silicon is
the basis for the function of transistors, the
foundation of the entire electronics industry
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