Transcript Semiconductors

Semiconductor Materials and pn
Junctions
T. Floyd, “Electronic Devices”, Maxwell Macmillan
International Editions, Chapter 2
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Atoms
• An atom is the smallest particle of an element that retains the characteristics of that
element.
• It is made out of a nucleus and a number negatively charged electrons. Electrons
are orbiting in one or more shells around the nucleus.
• The nucleus consists of positively charged particles, called protons, and
uncharged particles called neutrons.
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Examples of Atoms
• Some typical elements are the Hydrogen that has only 1 electron and one proton,
the Helium that has 2 electrons, two protons and two neutrons, and the Silicon that
has 14 electrons and 14 protons.
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Hydrogen
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Helium
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Silicon
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The Bohr Model
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Valance Electrons, Free Electrons and Conductivity
• Orbiting electrons in an atom are attracted by the positively charged protons.
• Electrons in orbits farther from the nucleus are less tightly bound to the atom than
those closer to the nucleus.
• Electrons in the outermost shell of an atom are relatively loosely bound to the atom.
These electrons are called the valance electrons and contribute to chemical
reactions and bonding within the structure of a material.
• When an atom absorbs energy from a heat source or from light, its electrons gain
energy and move to an orbit farther from the nucleus.If a valance electron gains
sufficient amount of energy, it can be completely removed from the atom, and
become a free electron.
• The number of free electrons in a material determines its resistance to electric
current. This number increases as the temperature increases.
• Materials with relatively few free electrons are called insulators, and posses a high
resistance to electric current. Materials with relatively large number of free
electrons are called conductors, and posses a low resistance to electric current.
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Semiconductor Crystals
• Two widely used types of semiconductor
materials are Silicon and Germanium.
• Their atoms have 4 valance electrons.
• When their atoms combine to form a solid
material, they arrange themselves in a
fixed pattern called a crystal.
• The atoms within a crystal are held
together by covalent bonds, which are
created by the valance electrons of each
atom.
• Covalent bonds the atoms together,
because the valance electrons of adjacent
atoms are attracted equally by the protons
in the nucleus of the atoms.
• A small number of valance electrons can
escape and become free electron, leaving
a positively charged Hole in the atom.
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Si
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Si
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Si
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n-Type Semiconductors
• Intrinsic (pure) semiconductor materials have
a low conductivity, due to the small number of
free electrons.
• Their conductivity can be increased and
controlled by the addition of impurities. This
process is called doping.
• Atoms such as the Antimony (Sb) have 5
valance electrons. When these atoms are
added in pure silicon, then each antimony
atom forms a covalent bond with the four
adjacent silicone atoms, leaving one free
electron that is not attracted by any atom.
• The number of free electrons can be
controlled by the amount of impurity added to
the silicon.
• The semiconductor produced by adding
pentavalent (impurities with 5 valance
electrons) impurities to pure semiconductors
is called the n-type semiconductor.
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Si
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Free
Electron
Si
Sb
Si
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p-Type Semiconductors
• Atoms such as the Boron (B) have 3 valance
electrons. When these atoms are added in
pure silicon, then each boron atom forms a
covalent bond with the four adjacent silicone
atoms. An electron is though missing since
boron has only 3 valance electrons, leaving a
positively charged hole.
• The number of holes in the semiconductor
material can be controlled by the amount of
impurity added to the silicon.
• The semiconductor produced by adding
trivalent (impurities with 3 valance electrons)
impurities to pure semiconductors is called
the p-type semiconductor.
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Si
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Hole
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Si
B
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pn- junction
• A pn junction is formed by connecting a ptype semiconductor with an n-type
semiconductor.
• Initially, at the point of contact, the free
electrons of the n-type region recombine
with the holes of the p-type region.
• Due to the force of attraction between the
electrons and holes, some electrons move
to the p-region while some holes move to
the n-region.
• At some point the force on the
electrons that moved in the p-region is
the same as the force due to the holes
that moved in the n-region, thus these
electrons are trapped in the p-region.
• The same applies to the holes that
moved in the n-region.
• This creates a barrier between the two
regions, called the Depletion Layer.
Depletion
Layer
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p-type
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VD
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Biasing the pn-junction:- Forward Bias (V < VD)
Depletion
Layer
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n-type
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VD
V < VD
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• If a voltage source is connected to a pn-junction with the positive terminal connected
on the p-region and the negative on the n-region then the junction is forward biased.
• In this case the positive terminal of the source push the holes of the p-region towards
the n-region, while the negative terminal push the electrons towards the p-region.
• This reduces the width of the depletion layer. If the source voltage is less than the
depletion voltage, then the width of the depletion layer is only reduced, not
eliminated. Thus, only a very small current flows through the circuit.
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Biasing the pn-junction:- Forward Bias (V >= VD)
p-type
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• If the source voltage is greater than the depletion voltage, then the depletion layer is
eliminated, and a large number of electrons gain enough energy to become valance
electrons. Thus a large current flows through the pn-junction.
• In this case the pn-junction behaves like a resistance (Rp + Rn) with a voltage
source (VD).
• The bulk resistance (Rp + Rn) is very low (few ohms), thus to avoid damaging the
pn-junction, a liming resistor is usually connected in series.
• The depletion voltage for silicon pn-junctions is 0.7V and for germanium 0.3V.
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Biasing the pn-junction:- Reverse Bias
Depletion
Layer
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p-type
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n-type
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VD
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• If the source voltage is connected in such a way so that the negative terminal is connected on
the p-region and the positive on the n-region, then the pn-junction is reversed biased.
• In this case the source voltage widens the depletion layer. Only a very small currents flows the
circuit due to the small number of minority curriers in the pn-junction.
• If the source voltage increases further, then the depletion layer widens more, creating a gap
between the p-region and the n-region, that behaves as a capacitance.
• Further increase in the source voltage will lead to a state known as the avalanche
breakdown, where a large reverse current flows through the circuit.
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The PN-Junction Diode
• A pn junction is formed by connecting a p-type semiconductor with an n-type
semiconductor.
• Initially, at the point of contact, the free electrons of the n-type region recombine
with the holes of the p-type region.
• Due to the force of attraction between the electrons and holes, some electrons move
to the p-region while some holes move to the n-region.
• At some point the force on the electrons that moved in the p-region is the same as
the force due to the holes that moved in the n-region, thus these electrons are
trapped in the p-region.
• The same applies to the holes that moved in the n-region.
• This creates a barrier between the two regions, called the Depletion Layer.
Depletion
Layer
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p-type
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Terminal Identification
Symbol
Cathode
Anode
Cathode Anode
VD
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Biasing the Junction Diode:- Forward Bias (V < VD)
Depletion
Layer
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p-type
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n-type
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VD
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• If a voltage source is connected to a pnjunction with the positive terminal
connected on the p-region and the
negative on the n-region then the
junction is forward biased.
• In this case the positive terminal of the
source pushes the holes of the p-region
towards the n-region, while the negative
terminal pushes the electrons towards
the p-region.
• This reduces the width of the depletion
layer. If the source voltage is less than
the depletion voltage, then the width of
the depletion layer is only reduced, not
eliminated. Thus, only a very small
current flows through the circuit.
V < VD
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I (mA)
V (V)
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Biasing the Junction Diode:- Forward Bias (V >= VD)
p-type
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n-type
p-type
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Rp
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Rn
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VD
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V >= V D
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• If the source voltage is greater than the depletion voltage, then the depletion layer is
eliminated, and a large number of electrons gain enough energy to become valance
electrons. Thus a large current flows through the pn-junction.
• In this case the pn-junction behaves like a resistance (Rp + Rn) with a voltage
source (VD).
• The bulk resistance (Rp + Rn) is very low (few ohms), thus to avoid damaging the
pn-junction, a liming resistor is usually connected in series.
• The depletion voltage for silicon pn-junctions is 0.7V and for germanium 0.3V.
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Junction Diode Forward Characteristics
• Ideal Junction Diode: (Assume that the internal resistance of the diode is zero)
– Silicon diode: if V<0.7V then I = 0. if V>0.7V then I = ∞. For germanium V=0.3V.
I (mA)
VD
0.7V
V (V)
0.7V
• Non-ideal Junction Diode:
– Need to consider the internal resistance of the pn-junction
I (mA)
rD
VD
0.7V
V (V)
0.7V
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Biasing the pn-junction:- Reverse Bias
I (mA)
Depletion
Layer
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p-type
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n-type
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V (V)
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VD
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• If the source voltage is connected in such a way so that the negative terminal is
connected on the p-region and the positive on the n-region, then the pn-junction is
reversed biased.
• In this case the source voltage widens the depletion layer. Only a very small currents
flow in the circuit due to the small number of minority curriers in the pn-junction.
• If the source voltage increases further, then the depletion layer widens more,
creating a gap between the p-region and the n-region, that behaves as a
capacitance.
• Further increase in the source voltage will lead to a state known as the avalanche
breakdown, where a large reverse current flows through the circuit.
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