basics i unit
TRANSCRIPT
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Atomic Structure
Atoms go around the nucleolus in their orbitsdiscrete distances
Each orbit has some energy level
The closer the orbit to the nucleus the less energy ithas
Group of orbits called shell
Electrons on the same shell have similar energy level
Valence shellis the outmost shell Valence shell has valence electronsready to be
freed
Number of electrons (Ne) on each shell (n)
First shell has 2 electrons Second shell has 8 electrons (not shown here)
Ne = 2n2
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Atomic Structure
Elements are made of atoms 110 Elements; each has an atomic
structure
Today, quarksand leptons, and theirantiparticles, are candidates for being the
fundamentalbuilding blocks from whichall else is made!
Bohr Model
Atoms have planetary structure
Atoms are made of nucleus(Protons (+)
& Neutrons) and electrons(-)
110 th element is called Darmstadtium (Ds)
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Valence Shell
Atoms are made of valence shell
and core
Core includes nucleus and other
inner shells
For a Carbon atom the atomic
number is 6
Core charge = 6 P + 2 e = (+6)+(-
2)=(+4)
Remember the first shell has 2
electrons
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Elements
Basic categories Conductors
Examples: Copper, silver One valence electron , the ecan
easily be freed
Insulators Valence electrons are tightly
bounded to the atom
Semiconductors Silicon, germanium (single
element) Gallium arsenide, indium
phosphide (compounds) They can act as conductors or
insulators Conduction bandis where
the electron leaves the
valence shell and becomes
free
Valence bandis where the
outmost shell is
Always free
electrons
Free electrons
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Semiconductors Remember the further away from the
nucleus the less energy is required tofree the electrons
Germanium is less stable Less energy is required to make the
electron to jump to the conduction band
When atoms combine to form a solid,they arrange themselves in asymmetrical patterns
Semiconductor atoms (silicon) formcrystals
Intrinsiccrystals have no impurities
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Conduction Electrons and Holes
Electrons exist only withinprescribed energy bands
These bands are separated byenergy gaps
When an electron jumps to the
conduction band it causes a hole When electron falls back to its
initial valence recombinationoccurs
Consequently there are twodifferent types of currents
Hole current (electrons are theminority carriers)
Electron current (holes are theminority carriers)
Remember: We are interested in electrical current!
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Doping
By adding impurities to the intrinsicsemiconductor we can change theconductivity of the materialthis is calleddoping
N-type doping
P-type doping
N-type:pentavalent (atom with 5 valence
electrons) impurity atoms are added [Sb(Antimony) + Si]
Negative charges (electrons) aregenerated
N-type has lots of free electrons
P-type: trivalent (atom with 3 valence
electrons) impurity atoms are added [B(Boron) + Si]
Positive charges (holes) are generated
P-type has lots of holes
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What is a Semiconductor?
Low resistivity => conductor
High resistivity => insulator
Intermediate resistivity => semiconductor
conductivity lies between that of conductors and insulators
generally crystalline in structure for IC devices
In recent years, however, non-crystalline semiconductors have
become commercially very important
polycrystalline amorphous crystalline
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Semiconductor Materials
Gallium
(Ga)
Phosphorus
(P)
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Silicon
Atomic density: 5 x 1022atoms/cm3
Si has four valence electrons. Therefore, it can form
covalent bonds with four of its nearest neighbors.
When temperature goes up, electrons can becomefree to move about the Si lattice.
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Electronic Properties of Si
Silicon is a semiconductor material. Pure Si has a relatively high electrical resistivity at room temperature.
There are 2 types of mobile charge-carriers in Si: Conduction electronsare negatively charged; Holesare positively charged.
The concentration (#/cm3) of conduction electrons & holes in asemiconductor can be modulated in several ways:
1. by adding special impurity atoms ( dopants )
2. by applying an electric field3. by changing the temperature
4. by irradiation
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Electron-Hole Pair Generation
When a conduction electron is thermally generated,
a hole is also generated.
A hole is associated with a positive charge, and is
free to move about the Si lattice as well.
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Carrier Concentrations in Intrinsic Si
The band-gap energy Egis the amount of energy
needed to remove an electron from a covalent bond.
The concentration of conduction electrons in intrinsic
silicon, ni, depends exponentially on Egand theabsolute temperature (T):
600Kat/101
300Kat/101
/2
exp102.5
315
310
32/315
cmelectronsn
cmelectronsn
cmelectronskT
ETn
i
i
g
i
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Doping (N type)
Si can be doped with other elements to change its
electrical properties.
For example, if Si is doped with phosphorus (P), each
P atom can contribute a conduction electron, so thatthe Si lattice has more electrons than holes, i.e.itbecomes N type: Notation:
n= conduction electron
concentration
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Doping (P type)
If Si is doped with Boron (B), each B atom cancontribute a hole, so that the Si lattice has moreholes than electrons, i.e.it becomes P type:
Notation:
p= hole concentration
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Summary of Charge Carriers
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Electron and Hole Concentrations
Under thermal equilibrium conditions, the productof the conduction-electron density and the hole
density is ALWAYS equal to the square of ni:2
innp
P-type material
A
i
A
Nnn
Np
2
D
i
D
N
np
Nn
2
N-type material
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Terminology
donor: impurity atom that increases nacceptor: impurity atom that increasesp
N-type material: contains more electrons than holes
P-type material: contains more holes than electrons
majority carrier: the most abundant carrier
minority carrier: the least abundant carrier
intrinsic semiconductor: n=p= ni
extrinsic semiconductor: doped semiconductor
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Summary
The band gap energy is the energy required to free anelectron from a covalent bond.
Egfor Si at 300K = 1.12eV
In a pure Si crystal, conduction electrons and holes are
formed in pairs. Holes can be considered as positively charged mobile particles
which exist inside a semiconductor.
Both holes and electrons can conduct current.
Substitutional dopants in Si: Group-V elements (donors) contribute conduction electrons
Group-III elements (acceptors) contribute holes
Very low ionization energies (