Transcript 11.12-11.13 Crystalline Solids, Band Theory, and Doping

11.12-11.13 Crystalline Solids, Band Theory, and Doping
Crystalline Solids
• Molecular solids: solids whose composite units are molecules
(compounds made up of non-metals. i.e. H2O and CO2.)
– Held together by intermolecular forces.
• Dispersion forces, dipole-dipole interactions and hydrogen bonds.
– Tend to have low melting points.
• Ionic Solids: Those whose composite units are ions.
(compounds made up of metal and nonmetal, or cationanion. i.e. MgCl2, & NaCl)
– Strong coulombic forces (or ionic bonds) hold ions together in a
crystal lattice.
– Tend to have much higher melting points than molecular solids.
Atomic solids  Next slide
Crystalline Solids (Continued)
• Atomic Solids: Solids whose composite units are individual atoms.
• There are three categories of atomic solids:
– Nonbonding atomic solids (i.e. Xe)
• Held together by relatively weak dispersion forces.
• Have extremely low melting points.
– Metallic atomic solids (i.e. Fe)
• Held together by metallic bonds (metal cations with a sea of electrons
surrounding them)
• Most metals, not including mercury, have very high melting points.
• At some point we will discuss different types of alloying. This will be on the AP
exam.
– Network covalent atomic solids (i.e. SiO2, diamond, and graphite)
• Held together by covalent bonds (very strong).
• Have extremely high melting points, because all of the atoms are covalently
bonded.
Network Covalent Solids (Continued)
Straight from the college board:
A Little More on Network Solids
• Diamond does not conduct electricity because the
electrons in diamond are confined to covalent
bonds and are not free to flow.
• Graphite is used as a lubricant because it is slippery
due to the weak dispersion forces holding sheets of
benzene rings together. Since the pi bonds in
graphite, just like the pi bonds in benzene are
delocalized, graphite is a good electrical conductor.
Silicates and Doping (Controlling the
Conductivity of Semiconductors)
• Silicates (an extended array of silicon and oxygen atoms) are the most common
network covalent atomic solids, making up 90% of the earths crust.
• Silicon is a covalent network solid and a semiconductor.
• It forms a 3D structure similar to diamond
• Silicon’s conductivity increases with temperature. This is because electrons can
move from what is called the “valence band” to the “conduction band”.
Doping
•
Molecular orbital theory can be used to explain why the electrons of conductive material
can be easily excited to a higher energy level.
•
In material that is conductive, such as metals, there is no gap between the “valence band”
and the “conduction band”.
•
In semiconductors, such as silicon, there is a small energy gap between what is known as the
valence “band” (band of occupied molecular orbitals) and the conduction “band” (band of
unoccupied molecular orbitals).
– Since semiconductors are weak conductors, they can be doped. A dopant is another
substance (an impurity) that is added to silicon’s crystal structure and that adds
additional electrons to aid in conductance.
– Silicon can be doped with phosphorus, and since the electrons of silicon only fill the
valence band. Phosphorus has 5 valence electrons, so each phosphorus atom brings 1
extra electron that can go directly into the conductance band. These electrons are
mobile and can conduct electrical current. This is called an n-type semiconductor
because the charge carriers are negatively charged electrons in the conductance band.
– Silicon can also be doped with a group 13 (or 3A) element such as gallium. When
gallium is used as a dopant, there are electron holes in the valence band. This type of
semiconductor is called a p-type semiconductor, because each hole acts as a positive
charge.
– Most modern electronic devices are silicon chips containing millions of p-n junctions,
tiny spots that are p-type of one side and n-type on the other. These can function as
diodes, amplifiers, etc… because they can be used to control electron flow.
11.12-11.13 pg. 539 #’s 106, 108, 110 and 118
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