Download Semiconductors Silicon Atom Structure

Semiconductors
Semiconductor
Elements
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© 2009 Associate Professor PhD. T.Vasileva
Silicon Atom Structure
W
WGe<
WSi < WGaAs
Nucleus and electrons
Conduction band
W
Bandgap energy
Valence band
As a whole the
atom is neutral.
Energy Bands
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Intrinsic Semiconductors
Free
electron
ni = pi
Energy
ni.pi= ni2
Hole
niGe > ni Si > niGaAs
W
Covalent bonds
Equal numbers of free electrons and holes
are created in the covalent bonds breaking up.
Heat from the surrounding air is the primary
source for the required energy.
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Exitation of an electron from the
valence to the conductive 3band.
© 2009 Associate Professor PhD. T.Vasileva
Electron and Hole Currents
Drift movement
vE = µE
µn > µp
Holes move slower than electrons by drifting
from one covalent bond to another.
The movement of one hole requires the
movement of many more valence electrons.
Jp
Jn
J = Jn +Jp
Free electrons and holes are
referred to as mobile carriers.
Carriers flow
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Current Direction
The direction of the hole's flow is opposite to that of the free electrons.
The electric current is conventionally represented by the flow of
positive charges.
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Impure Semiconductors – n-Type
Covalent bonds
Neutral phosphorus (P) atom
Donor atom – V valence electrons
Majority
carriers
n >> p
Free electron
Minority
carriers
Positively charged ion
A donor atom will easily donate an
extra electron to the silicon crystal.
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Carriers in n-Type Semiconductor
Majority carriers are formed in donor ionisation
process. One free electron will be created, but
without a hole.
Minority carriers are created by
breaking covalent bonds.
nno = ND
Whenever a neutral donor atom looses one
electron, it will become a positively charged ion.
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Carriers Concentration
T = const
Relation between majority/minority
carriers’ concentrations
nno.pno= ni2
Carrier
type
Semiconductor
type
nno = ND
pno=
ni2
Intrinsic
concentration
nno = const (T)
pno = f (T)
nno
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p-Type Semiconductors
Covalent bonds
Majority
carriers
Neutral boron (B) atom
Acceptor atom – III valence
electrons
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Hole
p >> n
Minority
carriers
Negatively charged ion
When a neutrally charged acceptor
gains an extra electron it will become
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a fixed negatively charged ion.
© 2009 Associate Professor PhD. T.Vasileva
Carriers in p-Type Semiconductor
Majority carriers are formed in accepror
ionisation process. One hole will be created,
but without an electron.
Minority carriers are created by
breaking covalent bonds.
ppo = NA
Whenever a neutral boron atom gains an extra
electron, it will become a negatively charged ion.
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Currents in Impurity Semiconductors
In n-type semiconductors, electrons are the
majority carriers.
In p-type semiconductors, holes are the
majority carriers.
The value of the electron current In will far
exceed the value of the hole current Ip.
In p-type semiconductors the hole current
Ip will far exceed the electron current In.
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