Semiconductors
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An atom consists of a positively charged nucleus of protons and neutrons and surrounded by
a cloud of orbiting negatively charged electrons.
In its normal state each atom has an equal number of protons and electrons and thus is
neutral: uncharged.
Electrons are grouped into shells. The last occupied shell is called the valence shell, and
electrons in this shell are valence electrons (free electron).
Valance shell (orbit): The outer orbit. It controls the electrical properties of the atom.
Materials are categorized electrically as conductors, insulators, or semiconductors, depending
on how many electrons they have in their valence shell.
o 1 valance electron-> conductor
o 8 valance electrons -> Insulator
o 4 valance electrons -> semiconductor
Copper Atom
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Silicon Atom
Best conductors are: gold, silver, copper. They have one valance electron.
Attraction between the atom and the valance electron is very weak, an outside force can
always dislodge this electron from the copper atom.
The slightest voltage will cause the free electron to flow from one atom to another. That is
why we call copper is a good conductor.
Best insulators have eight valance electrons.
Examples of insulators: glass, plastic, rubber.
Semiconductor: an element with an electrical properties between those of a conductor and
those of an insulator.
Examples of semiconductors: Germanium (obsolete), Silicon.
Silicon and Germanium Atoms
• Two types of semiconductive materials are silicon and germanium.
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Both the silicon and the germanium atoms have four valence electrons.
Silicon has 14 protons in its nucleus and germanium has 32.
The valence electrons in germanium are in the fourth shell while the ones in silicon are in the
third shell, closer to the nucleus.
This means that the germanium valence electrons are at higher energy levels than those in
silicon and require a smaller amount of additional energy to escape from the atom.
This property makes germanium more unstable than silicon at high temperatures.
Intrinsic Semiconductor
• When certain atoms combine to form a solid material, they arrange themselves in a fixed
pattern called a crystal.
• The atoms within the silicon crystal structure are held together by covalent bonds, which are
created by the sharing of the valence electrons of each atom.
• Figure below shows how each silicon (Si) atom positions itself with four adjacent atoms to
form a silicon crystal.
• A silicon atom with its four valence electrons shares an electron with each of its four
neighbors. This effectively creates eight valence electrons for each atom and produces a state
of chemical stability.
• Also, this sharing of valence electrons produces the covalent bonds that hold the atoms
together; each shared electron is attracted equally by two adjacent atoms.
• A silicon crystal is an intrinsic semiconductor if every atom in the crystal is a silicon atom.
Valance Saturation
• Each atom in a silicon crystal has eight electrons in its valence orbit. These eight electrons
produce a chemical stability that results in a solid piece of silicon material.
• No one is quite sure why the outer orbit of all elements has a predisposition toward having
eight electrons.
• When eight electrons do not exist naturally in an element, there seems to be a tendency for
the element to combine and share electrons with other atoms to have eight electrons in the
outer orbit.
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Energy Band Diagram
• An energy band diagram shown below is for a silicon crystal with only unexcited atoms (no
external energy such as heat).
The Hole
• An intrinsic (pure) silicon crystal at room temperature has sufficient heat (thermal) energy
for some valence electrons to jump the gap from the valence band into the conduction band,
becoming free electrons.
• When an electron jumps to the conduction band, a vacancy is left in the valence band within
the crystal. This vacancy is called a hole.
• For every electron raised to the conduction band by external energy, there is one hole left in
the valence band, creating what is called an electron-hole pair.
• Recombination occurs when a conduction-band electron loses energy and falls back into a
hole in the valence band.
• The existence of the hole is a critical difference between conductors and semiconductors.
Two types of flow
• When a voltage is applied across a piece of silicon, the thermally generated free electrons in
the conduction band, are easily attracted toward the positive end.
• This movement of free electrons is one type of current in a semiconductive material and is
called electron current.
• Electrons remaining in the valence band are still attached to their atoms and are not free to
move randomly in the crystal structure.
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However, a valence electron can move into a nearby hole, with little change in its energy
level, thus leaving another hole where it came from. The hole has effectively, although not
physically, moved from one place to another in the crystal structure. This current is called
hole current.
Doping a semiconductor
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Semiconductive materials do not conduct current well and are of little value in their intrinsic
state.
This is because of the limited number of free electrons in the conduction band and holes in
the valence band.
Intrinsic silicon must be modified by increasing the free electrons and holes to increase its
conductivity and make it useful in electronic devices.
This is done by adding impurities to the intrinsic crystal to alter its electrical conductivity.
Extrinsic semiconductor: a doped semiconductor.
A lightly doped semiconductor has a high resistance. A heavily doped semiconductor has a
low resistance.
n-type semiconductor
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Silicon doped with pentavalent impurity to increase the number of conduction-band electrons
is called n-type semiconductor, where n stands for negative.
pentavalent impurity are atoms with five valence electrons, such as arsenic (As), phosphorus
(P), and antimony (Sb) and are known as donor atoms because they provide an extra electron
to the semiconductor’s crystal structure.
Each pentavalent atom forms covalent bonds with four adjacent silicon atoms, leaving one
extra electron.
This extra electron becomes a conduction electron because it is not attached to any atom.
The number of conduction electrons can be controlled by the number of impurity atoms
added to the silicon.
Free Electrons outnumber the holes in an n-type semiconductor.
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p-type semiconductor
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Silicon doped with a trivalent impurity to increase the number of holes is called p-type
semiconductor, where p stands for positive.
Trivalent impurity are atoms with three valence electrons, such as aluminum (Al), boron (B),
and gallium (Ga) and are known as acceptor atoms because they leave a hole in the
semiconductor’s crystal structure.
Each trivalent atom forms covalent bonds with four adjacent silicon atoms.
Since four electrons are required, a hole is formed with each trivalent atom.
The number of holes can be controlled by the amount of trivalent impurity added to the
silicon.
Holes outnumber free electrons.
Diodes
• A piece of n-type semiconductor is about as useful as a carbon resistor.
• But when a manufacturer dopes a crystal so that one half of it is p-type and the other half is
n-type something new comes into existence.
• A PN junction is a junction formed by joining p-type and n-type semiconductors together in
very close contact.
• The term junction refers to the boundary interface where the two regions of the
semiconductor meet.
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Depletion Layer
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In the p-type region there are holes and in the n-type region there are conduction electrons.
With no external voltage, the conduction electrons in the n region are randomly drifting in all
directions.
When a p-n junction is formed, some of the electrons from the n-region diffuse across the
junction and combine with holes.
For each electron that crosses the junction and recombines with a hole, a pentavalent atom is
left with a net positive charge in the n region near the junction, making it a positive ion.
Also, when the electron recombines with a hole in the p region, a trivalent atom acquires net
negative charge, making it a negative ion.
A large number of positive and negative ions builds up near the p-n junction.
As this buildup occurs, the electrons in the n region must overcome both the attraction of the
positive ions and the repulsion of the negative ions in order to migrate into the p region.
Thus, as the ion layers build up, the area on both sides of the junction becomes essentially
depleted of any conduction electrons or holes and is known as the depletion region.
The depletion region now acts as a barrier.
Barrier Potential
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The electric field formed in the depletion region acts as a barrier.
External energy must be applied to get the electrons to move across the barrier of the electric
field.
The potential difference required to move the electrons through the electric field is called the
barrier potential.
Barrier potential of a PN junction depends on the type of semiconductor material, amount
of doping and temperature.
This is approximately 0.7V for silicon and 0.3V for germanium.
Biasing the PN Junction
• In electronics, bias refers to the use of a dc voltage to establish certain operating conditions
for an electronic device.
Forward Bias
• An external DC voltage is applied to a PN junction to bias it.
• Forward bias allows current through a PN junction.
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Bias voltage imparts sufficient energy to the free electrons, so they overcome the barrier
potential and move into the P region.
As more electrons and holes flow in the depletion region the number of positive and negative
ions is reduced, causing the depletion region to narrow.
The energy loss in overcoming the barrier potential results in a voltage drop across the PN
junction equal to the barrier potential. (0.7V for Si, 0.3V for Ge.)
Forward Bias
Reverse Bias
Reverse Bias
• Reverse bias is the condition that prevents current through the diode.
• The negative battery terminal attracts holes, and the positive battery terminal attracts the free
electrons.
• Holes and free electrons flow away from the junction.
• The depletion layer gets wider.
• The potential across the depletion region is equal to the bias voltage.
• Reverse current (saturation current) can flow through the PN junction, but it is very small.
• This current is typically in the µA or nA range.
• The reverse current is dependent primarily on the junction temperature and not on the amount
of reverse-biased voltage. A temperature increase causes an increase in reverse current.
Reverse Breakdown
• If the reverse bias voltage is increased to the reverse breakdown voltage, the reverse current
will drastically increase.
• This is due to collisions of fast minority carriers with atoms, knocking out valence electrons.
• This process avalanches and results in a high reverse current damaging the PN structure.
• To describe what happens: Assume that one minority conduction-band electron acquires
enough energy from the external source to accelerate it toward the positive end of the diode.
During its travel, it collides with an atom and imparts enough energy to knock a valence
electron into the conduction band. There are now two conduction-band electrons. Each will
collide with an atom, knocking two more valence electrons into the conduction band. There
are now four conduction band electrons which, in turn, knock four more into the conduction
band. This rapid multiplication of conduction-band electrons, known as an avalanche effect,
results in a rapid buildup of reverse current.
• Most diodes normally are not operated in reverse breakdown and can be damaged if they are.
• However, a particular type of diode known as a zener diode is specially designed for reversebreakdown operation.
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