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NEEP 541
Ionization in Semiconductors
Fall 2002
Jake Blanchard
Outline
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Ionization in Semiconductors
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Transients
Semiconductor fundamentals
Band theory
Doping
Junctions
Ionization Effects
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Radiation ionizes target through collisions
with electrons
As electrons slow, they remain free
electrons
In semiconductors, we think of positive
charges (holes) and negative charges
(electrons) as free particles
The key question concerns the average
densities of electrons and holes in the solid
and the subsequent transport of these
“particles”
Transient Effects
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The primary manifestation of ionization
is a transient increase in the electrical
conductivity and transient currents
across the semiconductor junctions
In optical materials, ionization changes
the absorption coefficient and
luminescence (we’ll discuss this later)
How a Semiconductor Works
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Si has four electrons in it’s outer shell
These form bonds with four neighboring
atoms
A perfect Si crystal is an insulator
because all these outer electrons are
tied up with neighboring atoms
By mixing in impurities, you can alter
this behavior
Doping
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Adding P or As makes N-type Si.
These have 5 outer electrons, so in Si
they have one free electron and thus
permit conduction
A small amount makes a big difference
N-type Si is a good conductor
Doping
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Adding B or Ga creates P-type Si
These have 3 electrons in the outer
shell, so they form “holes”
A Si electron is left free
P-type Si is also a good conductor
P-N junctions
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Combine a layer of Ptype with a layer of Ntype Si
The interface is a
“junction”
This forms a diode –
current can only flow
in one direction
P-N junction
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When diode is working, both holes and
electrons flow towards junction
They combine at interface
Net current results
P-N Junction
P-N Junction
IV Curve
Band Theory
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Fermi energy is the highest energy state that
would be occupied at 0 K
In solids, only certain energy levels can be
occupied by electrons
The allowed levels smear into bands, due to
periodicity of the lattice
In metals, the Fermi energy lies within an
allowed energy band
Hence, electrons close to Fermi level can
scatter into it (by electric fields) fairly easily
and it will conduct at 0 K
Band Theory
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In semiconductors, the electrons with the
highest energies exactly fill one energy band
at 0 K
The next higher band is empty
Resistivity is infinite
Filled band is the “valence band”
Higher band is the “conduction band”
The energy separation between the bands is
the “band gap”
The Fermi energy lies in this gap
Band Gap
EC
EF
EG
EV
Real Materials
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Previous comments are for perfect
crystals
Boundaries and defects disrupt
periodicity
This creates allowed energy levels in
gap
In single crystal Si, defects are isolated,
so the electrons in these levels are
bound
Semiconductors
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Intrinsic semiconductors have finite
probability (above 0 K) that some
electrons will reach conduction band
Extrinsic semiconductors have some
energy levels in the gap, due to defects
and impurities
These levels can capture holes and
electrons
Doping
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Most practical semiconductors rely on
impurities for their properties
Impurities can produce energy levels
with any charge at just about any
location within the gap
Donor defects give up electrons to the
conduction band
Acceptor defects capture an electron
from the valence band