Transcript Lecture #11 Metals, insulators and Semiconductors, Diodes Reading: Malvino chapter 2 (semiconductors)

Lecture #11 Metals, insulators and
Semiconductors, Diodes
Reading:
Malvino chapter 2 (semiconductors)
9/24/2004
EE 42 fall 2004 lecture 11
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Energy states
• In any material, there are different roles
that an electron can play.
• Electrons can be tightly bond around the
core of atoms
• Electrons can participate in bonding
between atoms
• Electrons can be at higher energies, and
free to move
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Insulators
• In an insulator, all of the electrons are either
tightly wrapped around the neuclei, or they are
locked into bonds between atoms.
• Since no electrons or ions can move, the
material is an insulator.
• Examples:
– Silicon dioxide
– Plastic
– glass
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Metals
• In a metal, a very large number of
electrons are free to move.
• This gives metals there ductility and
strength, because the sea of electrons
binds the material in a distributed way
• It makes them good conductors of heat
• It also makes them very good conductors
of electricity.
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Semiconductors
• In a semiconductor, there is a gap in the
energies that an electron can take, between
those of the bonding (valence electrons) and
those that can move (conduction electrons)
• In an intrinsic semiconductor, all of the valence
(bonding) states are full of electrons, but no
electrons are left to go into the conduction band.
• So an intrinsic semiconductor is an insulator, but
if a few extra electrons are added, they can
move freely.
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Semiconductor (undoped)
All empty
}
Energy gap
All full of electrons
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Semiconductor (N type)
A few extra electrons
}
Energy gap
All full of electrons
Doped with Arsenic or Phosphorus atoms, each with one more
proton than a silicon atom, each becomes a fixed positive ion,
and neutrality means a few extra electrons will be available
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Semiconductor (p type)
All empty
}
Energy gap
A few empty states
Doped with Boron, Aluminum or Gallium atoms, each with one
fewer proton than a silicon atom, each becomes a fixed
positive ion, and neutrality means a few extra electrons will be
available
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Column 3 of the periodic table
If we look at column 3 of the periodic table, we can see that these
elements are very similar to silicon, except that they have one
fewer protons—one less quantum of positive charge.
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Energy bands
• In general, we won’t worry about all of the
core electrons, and we will just care about
electrons which are in the conduction
band, or missing electrons in the valence
band.
• Electrons in the conduction band are
called …wait for this… electrons
• Missing electrons from the valence band
are called holes
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Conduction and valence bands
Electrons
Energy gap, about
1 electron volt
Holes
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Recombination
Electrons
Energy gap, about
1 electron volt
Holes
If an electron comes near a hole, it can fall into the open state.
This is called recombination.
The extra energy can heat up the crystal, or it can be released
as light (as in a light emitting diode)
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Recombination
Electrons
Energy gap, about
1 electron volt
Holes
If some energy is available, on of the many electrons in the
valence band can jump up into the conduction band, leaving a
hole behind.
This process is called generation.
Of course, this always makes a pair, an electron and a hole.
In a solar cell, light causes this jump, and that is how sunlight is
converted into electrical energy (we just need to get the
electron and hole out before they recombine!)
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Electrons and Holes
• If you were to have both electrons and holes in
the crystal, the electrons could fill up the holes
until one or the other was depleted.
• Silicon crystals with quite a few extra electrons
running around are called N-type, and they only
have a few holes in the valence band.
• Silicon crystals with many holes running around
are called P-type, and they only have a few
electrons in the conduction band.
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Applications
• We can take silicon wafers and put patterns of
doping on their surface, and control its
conductivity.
• We can further control the conductivity by
applying electric fields
• We can make use of the difference between P
and N type carriers moving through the crystal.
• This ability to control the conduction of silicon is
the basis for the function of transistors, the
foundation of the entire electronics industry
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Diodes
• If we have a P doped material next to an N
doped material, the result is a diode.
• Remember:
• the P type material has extra fixed negative charge
– And mobile positive charge (holes)
• The N type material has extra fixed postive charge
– And mobile negative charge (electrons)
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Diode
• If we apply a voltage to push the electrons and
holes toward each other, they can recombine at
the junction and a current will flow (this is called
forward bias)
• If we apply a voltage which pulls the electrons
and holes away from each other, then the will
pull back from the junction, and very little current
will flow. (called reverse bias)
• So a diode acts like a one way valve for current
and the symbol is an arrow in the direction that
the current is allowed to flow
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• The diode can be used for many
purposes, for example to charge a power
supply capacitor from an AC source
~
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IV curve for a diode
• The IV curve for a ideal diode is to have
zero current in the reverse direction, and
no resistance when forward biased
Current 
Voltage →
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Unbiased diode
• If we were to make an N type material and join it
to a P type material, we would have extra
electrons right next to holes.
• Near the joint, the extra electrons and extra
holes recombine, leaving a region which has no
carriers called the depletion zone)
• The loss of the mobile charge leaves the fixed
charge from the dopants in place, and that
creates an electric field which keeps more
electrons and holes from wandering into the
deletion layer.
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Depletion zone
-
+ -+ +
+
-- + +
+ +
+- +
-
+ +
-
- +
+
+
- -+
+
+
- +
The mobile electrons and holes are held back from the joint by the
electric field from the the fixed charges
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Reverse leakage
• In a real diode, if a reverse bias voltage is put on
it, a small reverse current flows. This current is
called the leakage current
• It is caused by:
– Generation in the depletion zone (at higher
temperatures, there is more leakage current, and light
on a diode can cause a lot of leakage current)
– surface and edge effects.
• If enough reverse voltage is applied, the diode
will break down and conduct a lot of current (not
necessarily destructively)
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Real diode under forward bias
• In a real diode, as you reduce the
depletion width, pushing the mobile
carriers together, the current does not turn
on instantly, but rises exponentially with
the voltage
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Real diode IV curve
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