Download Semiconductors * Learning Outcomes

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Semiconductors – Learning Outcomes
 Interpret I-V graphs of semiconductors.
 Discuss conduction in semiconductors – charge carriers,
intrinsic/extrinsic, p-type, n-type.
 Discuss current flow in a p-n junction.
 Demonstrate current flow in a p-n junction in forward
and reverse bias.
 Give uses of semiconductors.
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Semiconductors
 Semiconductors have resistivity between that of a good
conductor and a good insulator.
 e.g. silicon, germanium, cadmium sulphide
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Semiconductors
 At zero kelvin,
semiconductors (e.g.
silicon) are insulators.
 They have no free charges
that can carry a current.
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Semiconductors
 At higher temperatures,
electrons are freed from
the structure and may
conduct current.
 They leave behind a
positive hole, or hole.
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Semiconductors
 Strictly speaking, positive
holes aren’t “real”
conductors, electrons are.
 It is much easier to consider a
single hole moving than many
electrons one-by-one filling
and leaving holes.
 Holes are positively charged
because they move towards
the negative terminal of the
power supply.
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Intrinsic Conduction
 Pure semiconductors have an equal number of free
electrons and positive holes.
 These are intrinsic semiconductors and exhibit intrinsic
conduction.
 These are terrible conductors.
 Intrinsic conduction can be increased by increasing the
temperature (releasing more free electrons and holes) or
for certain semiconductors, shining light on them
(likewise, more detail later).
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Light-Dependent Resistor
 A light-dependent resistor (LDR) is
a semiconductor that decreases
its resistance when light shines on it.
 Light hitting the resistor releases
electrons from the molecules,
allowing them to conduct electricity.
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Thermistor
 A thermistor is a semiconductor
designed to decrease its resistance
as its temperature increases.
 The heat energy frees electrons
from the material, allowing them
to be used for conduction.
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Extrinsic Conduction
 Semiconductors with impurities may have more
electrons or holes than pure semiconductors, increasing
their conduction.
 Extrinsic conduction is increased conduction in a
semiconductor due to impurities.
 Doping is the adding of small controlled amounts of
certain impurities to a semiconductor to increase its
conduction.
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Doping
 An n-type semiconductor
has more free electrons
available for conduction
due to its impurity.
 e.g. phosphorus in silicon.
 4 valence electrons are
used to bond to the
structure, leaving 1 free
electron for conduction.
 Electrons are the “majority”
carriers.
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Doping
 An p-type semiconductor
has more positive holes
available for conduction
due to its impurity.
 e.g. boron in silicon.
 3 valence electrons are
used to bond to the
structure, leaving 1 hole in
the structure.
 Holes are the “majority”
carriers.
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P-N Junctions (Diodes)
 A p-n junction is a piece of semiconductor which is
partly doped p-type and partly doped n-type.
 Where the two meet, free electrons from the n-type fill
holes from the p-type, forming a neutral region called
the depletion layer which acts as an insulator.
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Diodes in Circuits – Forward Bias
 In forward bias, the p-type
is connected to a positive
terminal and the n-type is
connected to a negative
terminal.
 Holes move away from
the positive terminal into
the depletion layer.
 Free electrons move away
from the negative terminal
into the depletion layer.
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Diodes in Circuits – Forward Bias
 For small voltages, this
barely affects the
resistance.
 Once a threshold value
called the junction
voltage is reached, the
depletion layer is
eliminated.
 Resistance decreases
rapidly beyond this
voltage.
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Diodes in Circuits – Reverse Bias
 In reverse bias, the p-type is
connected to a negative
terminal and the n-type is
connected to a positive
terminal.
 Holes move towards the
negative terminal away
from the depletion layer.
 Free electrons move
towards the positive
terminal away from the
depletion layer.
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Diodes in Circuits – Reverse Bias
 The depletion layer widens
and the diode continues to
be an insulator.
 In reverse bias, the diode
conducts minimal current
(due to minority carriers).
 If a “breakdown voltage” is
reached, the depletion
region is flooded with
carriers causing high
current.
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Uses of Semiconductors
 Since diodes only conduct
current one way, a.c. will
only be allowed to flow in
one direction. This is called
rectification of a.c.
 Semiconductors are used to
make transistors, which
along with other
components make
integrated circuits (ICs), the
building blocks of
computers.