Transcript
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Semiconductor Physics
Semiconductor fundamentals
Doping
Pn junction
The Diode Equation Zener diode
LED
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What Is a Semiconductor?
Many materials, such as most metals, allow electrical current to
flow through them
These are known as conductors
Materials that do not allow electrical current to flow through
them are called insulatorsPure silicon, the base material of most transistors, is considered
a semiconductor because its conductivity can be modulated by
the introduction of impurities
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Semiconductors
A material whose properties are such that it is not quite aconductor, not quite an insulator
Some common semiconductors
elemental
Si - Silicon (most common) Ge - Germanium
compound
GaAs - Gallium arsenide
GaP - Gallium phosphide
AlAs - Aluminum arsenide
AlP - Aluminum phosphide
InP - Indium Phosphide
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Crystalline Solids
In a crystalline solid, the periodic arrangement of atoms is
repeated over the entire crystal
Silicon crystal has a diamond lattice
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Crystalline Nature of Silicon
Silicon as utilized in integrated circuits is crystalline in nature
As with all crystalline material, silicon consists of a repeating
basic unit structure called a unit cel l
For silicon, the unit cell consists of an atom surrounded by fourequidistant nearest neighbors which lie at the corners of the
tetrahedron
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Whats so special about Silicon?
Cheap and abundantAmazing mechanical, chemical and
electronic properties
The material is very well-known to
mankind
SiO2: sand, glass
Si is column IV of the
periodic table
Similar to the carbon
(C) and the
germanium (Ge)
Has 3s and 3p
valence electrons
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Nature of Intrinsic Silicon
Silicon that is free of doping impurities is called
intrinsic
Silicon has a valence of 4 and forms covalent
bonds with four other neighboring silicon atoms
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Semiconductor Crystalline Structure Semiconductors have a regular
crystalline structure
for monocrystal, extends
through entire structure
for polycrystal, structure is
interrupted at irregularboundaries
Monocrystal has uniform 3-
dimensional structure
Atoms occupy fixed positions
relative to one another, but
are in constant vibration about
equilibrium
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Semiconductor Crystalline Structure
Silicon atoms have 4electrons in outer shell
inner electrons are veryclosely bound to atom
These electrons are shared
with neighbor atoms onboth sides to fill the shell
resulting structure isvery stable
electrons are fairly
tightly bound no loose electrons
at room temperature, ifbattery applied, verylittle electric currentflows
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Conduction in Crystal Lattices
Semiconductors (Si and Ge) have 4 electrons in their outer shell
2 in the s subshell
2 in the p subshell
As the distance between atoms decreases the discrete subshells
spread out into bands As the distance decreases further, the bands overlap and then
separate
the subshell model doesnt hold anymore, and the electronscan be thought of as being part of the crystal, not part of the
atom4 possible electrons in the lower band (valence band)
4 possible electrons in the upper band (conduction band)
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Energy Bands in Semiconductors
The space
between the
bands is the
energy gap, or
forbidden band
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Insulators, Semiconductors, and Metals
This separation of the valence and conduction bands determinesthe electrical properties of the material
Insulators have a large energy gap electrons cant jump from valence to conduction bandsno current flows
Conductors (metals) have a very small (or nonexistent) energy gapelectrons easily jump to conduction bands due to thermal
excitationcurrent flows easily
Semiconductors have a moderate energy gap
only a few electrons can jump to the conduction band leaving holesonly a little current can flow
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Insulators, Semiconductors, and Metals
(continued)
Conduction
Band
Valence
Band
Conductor Semiconductor Insulator
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Hole - Electron Pairs
Sometimes thermal energy is enough to cause an electron tojump from the valence band to the conduction band
produces a hole - electron pair Electrons also fall back out of the conduction band into the
valence band, combining with a hole
pair elimination
hole electron
pair creation
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Improving Conduction by Doping
To make semiconductors better conductors, add impurities(dopants) to contribute extra electrons or extra holes
elements with 5 outer electrons contribute an extra electron to
the lattice (donordopant)
elements with 3 outer electrons accept an electron from the
silicon (acceptordopant)
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Improving Conduction by Doping
(cont.) Phosphorus and arsenic are
donor dopants if phosphorus is
introduced into the siliconlattice, there is an extraelectron free to movearound and contribute toelectric current
very loosely bound toatom and can easily jumpto conduction band
produces n type silicon sometimes use + symbol
to indicate heavierdoping, so n+ silicon
phosphorus becomespositive ion after giving upelectron
i C i i
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Improving Conduction by Doping
(cont.)
Boron has 3 electrons in its outershell, so it contributes a hole if itdisplaces a silicon atom
boron is an acceptordopant
yieldsp type silicon
boron becomes negative ionafter accepting an electron
E i i l
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Epitaxial
Growth of
Silicon Epitaxy grows silicon on top of
existing silicon
uses chemical vapordeposition
new silicon has samecrystal structure asoriginal
Silicon is placed in chamber athigh temperature
1200 o C (2150 o F) Appropriate gases are fed into
the chamber other gases add
impurities to the mix Can grow n type, then switch to
p type very quickly
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Diffusion of Dopants It is also possible to introduce
dopants into silicon by heatingthem so they diffuse into thesilicon
no new silicon is added high heat causes diffusion
Can be done with constantconcentration in atmosphere
close to straight lineconcentration gradient
Or with constant number of atomsper unit area
predepositionbell-shaped gradient
Diffusion causes spreading ofdoped areas
top
side
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Diffusion of Dopants (continued)
Concentration of dopant insurrounding atmosphere kept
constant per unit volume
Dopant deposited on
surface - constantamount per unit area
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Ion Implantation of Dopants
One way to reduce the spreading found with diffusion is to use ionimplantation also gives better uniformity of dopant yields faster devices lower temperature process
Ions are accelerated from 5 Kev to 10 Mev and directed at silicon higher energy gives greater depth penetration total dose is measured by flux
number of ions per cm2 typically 1012 per cm2 - 1016 per cm2
Flux is over entire surface of silicon use masks to cover areas where implantation is not wanted
Heat afterward to work into crystal lattice
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Hole and Electron Concentrations
To produce reasonable levels of conduction doesntrequire much doping
silicon has about 5 x 1022 atoms/cm3
typical dopant levels are about 1015 atoms/cm3
In undoped (intrinsic) silicon, the number of holes andnumber of free electrons is equal, and their productequals a constant
actually, ni increases with increasing temperature
This equation holds true for doped silicon as well, soincreasing the number of free electrons decreases the
number of holes
np = ni2
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INTRINSIC (PURE) SILICON
At 0 Kelvin Silicon
density is 5*10 particles/cm
Silicon has 4 valence
electrons, it covalently bonds
with four adjacent atoms in
the crystal latticeHigher temperatures create
free charge carriers.
A hole is created in theabsence of an electron.
At 23C there are 10
particles/cm of free carriers
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DOPING
The N in N-type stands for negative.
A column V ion is inserted.
The extra valence electron is free to
move about the lattice
There are two types of doping
N-type and P-type.
The P in P-type stands for positive.A column III ion is inserted.
Electrons from the surrounding
Silicon move to fill the hole.
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Energy-band Diagram
A very important concept in the study of semiconductors is theenergy-band diagram
It is used to represent the range of energy a valence electron can
have
For semiconductors the electrons can have any one value of acontinuous range of energy levels while they occupy the valence
shell of the atom
That band of energy levels is called the valence band
Within the same valence shell, but at a slightly higher energylevel, is yet another band of continuously variable, allowed energy
levels
This is the conduction band
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Band Gap
Between the valence and the conduction band is a range of energy
levels where there are no allowed states for an electron
This is the band gap
In silicon at room temperature [in electron volts]: Electron voltis an atomic measurement unit, 1 eV energy is
necessary to decrease of the potential of the electron with 1 V.
EG
E eVG
11.
1eV 1.602 10 joule19
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Impurities
Silicon crystal in pure form isgood insulator - all electrons are
bonded to silicon atom
Replacement of Si atoms can alter
electrical properties ofsemiconductor
Group number - indicates number
of electrons in valence level (Si -
Group IV)
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Impurities
Replace Si atom in crystal with Group V atom
substitution of 5 electrons for 4 electrons in outer shell
extra electron not needed for crystal bonding structure
can move to other areas of semiconductor
current flows more easily - resistivity decreases
many extra electrons-->
donor or n-type material Replace Si atom with Group III atom
substitution of 3 electrons for 4 electrons
extra electron now needed for crystal bonding structure
hole created (missing electron)
hole can move to other areas of semiconductor if electrons continually
fill holes
again, current flows more easily - resistivity decreases
electrons needed --> acceptor or p-type material
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COUNTER DOPING
Insert more than onetype of Ion
The extra electron and
the extra hole cancel out
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A LITTLE MATH
n= number of free electrons
p=number of holes
ni=number of electrons in intrinsic silicon=10/cm
pi-number of holes in intrinsic silicon= 10/cm
Mobile negative charge = -1.6*10-19 Coulombs
Mobile positive charge = 1.6*10-19 Coulombs
At thermal equilibrium (no applied voltage) n*p=(ni)2
(room temperature approximation)
The substrate is called n-type when it has more than 10 free
electrons (similar for p-type)
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P-N Junction
Also known as a diode
One of the basics of semiconductor technology -
Created by placing n-type and p-type material in closecontact
Diffusion - mobile charges (holes) in p-type combine with
mobile charges (electrons) in n-type
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P-N Junction
Region of charges left behind (dopants fixed in crystallattice)
Group III in p-type (one less proton than Si- negative
charge)
Group IV in n-type (one more proton than Si - positivecharge)
Region is totally depleted of mobile charges - depletion
region
Electric field forms due to fixed charges in the depletion
region
Depletion region has high resistance due to lack of mobile
charges
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THE P-N JUNCTION
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The Junction
The potential or voltage acrossthe silicon changes in the depletion
region and goes from + in the n
region toin the p region
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Biasing the P-N Diode
Forward BiasApplies - voltage
to the n region
and + voltage to
the p region
CURRENT!
Reverse BiasApplies + voltage to
n region andvoltage to p region
NO CURRENT
THINK OF THE
DIODE AS A
SWITCH
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P-N JunctionReverse Bias
positive voltage placed on n-type material
electrons in n-type move closer to positive terminal, holes
in p-type move closer to negative terminal
width of depletion region increases
allowed current is essentially zero (small drift current)
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P-N JunctionForward Bias
positive voltage placed on p-type material
holes in p-type move away from positive terminal, electrons in n-
type move further from negative terminal
depletion region becomes smaller - resistance of device decreases
voltage increased until critical voltage is reached, depletion region
disappears, current can flow freely
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P-N Junction - V-I characteristics
Voltage-Current relationship for a p-n junction (diode)
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Current-Voltage Characteristics
THE IDEAL DIODE
Positive voltage yields
finite current
Negative voltage yields
zero current
REAL DIODE
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The Ideal Diode Equation
I IqV
kT
whereI diode current with reverse bias
q coulomb the electronic ch e
keV
K
Boltzmann s cons t
0
0
19
5
1
1602 10
8 62 10
exp ,
. , arg
. , ' tan
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Semiconductor diode - opened region
The p-side is the cathode, the n-side is the anode
The dropped voltage, VD is measured from the cathode
to the anode
Opened: VD VF:
VD = VF
ID = circuit limited, in our model the VD cannot exceed VF
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Semiconductor diode - cut-off region
Cut-off: 0 < VD < VF:
ID 0 mA
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Semiconductor diode - closed region
Closed: VF < VD 0:
VD is determined by the circuit, ID = 0 mA
Typical values of VF: 0.5 0.7 V
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Zener Effect
Zener break down: VD
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LED
Light emitting diode, made from GaAs
VF=1.6 V
IF >= 6 mA
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