Download Semi-conductors

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information about electrical
characteristics of materials
Discussing how p-n junctions work
Describing the processing and
properties of semiconductors
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Semiconductor basics
Definition of the semiconductor
Common semiconductor material
Semiconductor crystal structure
How silicon crystal conducts electiricity?
P-type and N-type silicon
The PN Junction
Basic semiconductor devices
Semiconductor Manufacturing Process
Summary
References
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resistance
resistivity ,f(T)
conductor
insulator
semiconductor
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FIGURE 1 : Resistivity chart
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have electrical properties between a
conductor and insulator
do not conduct as well as conductors
do not insulate as well as insulators
their atoms are closely , have a
crystal structure
electronic devices, computers,
telephones made from semiconductors
conductivity can be controlled by
doping
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Most common
semiconductor material
four valence electron
covalent bond ,very
stable
FIGURE 2: structure and lattice of
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pure crystal of Silicon
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pure silicon
electrical conductor
by adding impurity
introduce an impurity such as
phosphorus, arsenic,
aluminum,gallium
can be exist unbounded electron
can exist a hole
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With elements that have three
valence electrons
Such as B, Al and Ga
Fourth bond cannot be formed
Positive charge
Not a complete connection
There is a empty place ‘hole’
Hole movement will exist
By diffusion of B2H6
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Electron leave the covalent bond
and move into hole
Therefore, another hole will be
existed
Holes will move
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These elements have five valence
electrons to share with other atoms
Such as Sb, P and As
Fifth electron cannot form a bond
Negative charge
Greatly increasing the conductivity
By diffusion of PH3
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A single type (p or n ) semiconductor
material is not very useful
For useful applications, semiconductor
crystal must contain both P-type and Ntype
PN junction can only be created by
inserting different impurities into different
parts of a single crystal
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N region will has positive charge after missing
some electrons
Those electrons will fill the holes in the P region,
which will therefore has a negative charge.
So there will be potential difference and electrical
current
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p-type
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n-type
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p-type
n-type
Semiconductors p-type and n-type are brought together
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p-type
n-type
Electrons and holes migrate across the junction
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p-type
n-type
The depletion layer is formed
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Electric field
p-type
n-type
A potential difference is set up across the depletion layer
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Higher dopant concentration, more carriers
(electrons or holes)
Higher conductivity, lower resistivity
Electrons move faster than holes
N-type silicon has lower resistivity than P-type
silicon at the same dopant concentration
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Resistor
Transistors
Capacitors
Diode
www.made-in-china.com/.../Resistor-Capacitor.jpg
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Resistors are made by doped silicon or
polysilicon on an IC chip.
Resistance is determined by length, line
width,height, ana dopant concentration
V=I*R ( Ohm law)
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Charge storage device
The capacitor's function is to
store electricity, or electrical
energy.
The capacitor also functions as
a filter, passing alternating
current (AC), and blocking direct
current (DC).
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blogs.courant.com/.../capacitor.jpg
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P-N Junction(transistor),electron tube
with anode ana cathode
Allows electric current go through
only in one direction
Used commonly in bridge form to
convert AC to DC voltage.
“Rectification”
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Various types of Bridge Rectifiers
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PNP or NPN
Switch
Amplifier
Anolog,digital circuit
Fast, high power device
www.ehobbycorner.com/images/trn_type.jpg
injection
diffusion
collection
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Metal-oxide semiconductor
Also called MOSFET( MOS Field Effect Transistor)
Simple, symmetric structure
Switch, good for digital, logic circuit
Most commonly used devices in the semiconductor
industry
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www.doitpoms.ac.uk/.../images/mosfet.jpg
NPN-PNP configuration
Market share reducing rapidly
Still used for anolog systems
power devices
TV,Cellar phone, etc.
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static.howstuffworks.com/gif/amplifier-transi...
Advantages
Switch signal high speed
Handle large circuit(high power
amplifier)
Wireless transmitter
Not effective weak signaal
amplification
A standard bipolar transistor
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1.Silicon Manufacturing
a) Wafer Manufacturing
b) Crystal structure
2.Photolithography
a) Photoresists
b) Photomask and Reticles
c) Patterning
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3.Oxide Growth & Removal
a) Oxide Growth & Deposition
b) Oxide Removal
c) Other effects
d) Local Oxidation
4. Diffusion & Ion Implantation
a) Diffusion
b) Other effects
c) Ion Implantation
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Czochralski Process is
a Technique in Making
Single-Crystal Silicon
A Solid Seed Crystal is
Rotated and Slowly
Extracted from a Pool
of Molten Si
Requires Careful
Control to Give
Crystals Desired Purity
and Dimensions
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The Silicon Crystal is Sliced by Using a Diamond-Tipped Saw
into Thin Wafers
Sorted by Thickness
Damaged Wafers Removed During Lapping
Etch Wafers in Chemical to Remove any Remaining Crystal
Damage
Polishing Smoothes Uneven Surface Left by Sawing Process
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Photolithography is a
technique that is used to
define the shape of
micro-machined
structures on a wafer.
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The first step in the photolithography process is to
develop a mask, which will be typically be a
chromium pattern on a glass plate.
Next, the wafer is then coated with a polymer which
is sensitive to ultraviolet light called a photoresist.
Afterward, the photoresist is then developed which
transfers the pattern on the mask to the photoresist
layer.
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There are two basic types of Photoresists
Positive and Negative.
Positive resists
Positive resists decomposes ultraviolet light.
The resist is exposed with UV light wherever
the underlying material is to be removed.
Negative resists
Exposure to the UV light causes the negative
resist to become polymerized, and more
difficult to dissolve. Therefore, the negative
resist remains on the surface wherever it is
exposed, and the developer solution removes
only the unexposed portions.
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Photomask
This is a square glass plate with a patterned
emulsion of metal film on one side. The mask is
aligned with the wafer, so that the pattern can be
transferred onto the wafer surface. Each mask
after the first one must be aligned to the previous
pattern.
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When a image on the photomask is projected several time
side by side onto the wafer, this is known as stepping and
the photomask is called a reticle.
A common reticle is the 5X
The patterns on the 5X reticle are reduced 5 times when
projected onto the wafer. This means the dies on the
photomask are 5 times larger than they are on the final
product. There are other kinds of reduction reticles (2X,
4X, and 10X), but the 5X is the most commonly used.
Reduction reticles are used on a variety of steppers, the
most common being ASM, Canon, Nikon.
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
The last stage of Photolithography is a process called ashing. This process
has the exposed wafers sprayed with a mixture of organic solvents that
dissolves portions of the photoresist .
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Conventional methods of ashing require an oxygen-plasma ash, often in
combination with halogen gases, to penetrate the crust and remove the
photoresist. Usually, the plasma ashing process also requires a follow-up
cleaning with wet-chemicals and acids to remove the residues and nonvolatile contaminants that remain after ashing. Despite this treatment, it is
not unusual to repeat the "ash plus wet-clean" cycle in order to completely
remove all photoresist and residues.
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Dry oxide - Pure dry oxygen is employed
Disadvantage
- Dry oxide grows very slowly.
Advantage
- Oxide layers are very uniform.
- It has especially low surface state charges and thus make
ideal dielectrics for MOS transistors.
Wet oxide - In the same way as dry oxides, but steam is injected
Disadvantage
- Hydrogen atoms liberated by the decomposition of the water
molecules produce imperfections that may degrade the oxide quality.
Advantage
- Wet oxide grows fast.
- Useful to grow a thick layer of field oxide
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
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Diffusion
--A uniformly doped
ingot is sliced into wafers.
--An oxide film is then
grown on the wafers.
--The film is patterned
and etched using
photolithography exposing
specific sections of the
silicon.
Diffusion is a cheaper and
more simplistic method, but
can only be performed from
the surface of the wafers.
Dopants also diffuse
unevenly, and interact with
each other altering the
diffusion rate.
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
Ion Implantation
--A particle accelerator is used
to accelerate a doping atom so that
it can penetrate a silicon crystal to
a depth of several micron
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Ion implantation is more expensive
and complex. It does not require
high temperatures and also allows
for greater control of dopant
concentration and profile.
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Semiconductors; materials, conductivity between conductor
and insulator
Its conductivity can be controlled by dopant concentration and applied
voltage
Silicon, germanium, and gallium arsenate
Silicon most popular: abundant and stable oxide
Boron doped semiconductors is p-type, majority carriers are holes
P, As or Sb doped semiconductor is P-type, the majority carriers are
electrons
Higher dopant concentration, lower resistivity
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R=g *l / A
C=KA/d
Capacitors are mainly used in DRAM
Bipolar transistors can amplify electric
signal, mainly used for analog systems
MOSFET electric controlled switch,
mainly used for digital systems
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http://www.playhookey.com/semiconductors/pn_junction.html
http://rel.intersil.com/docs/lexicon/manufactur
e.html
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