Transcript - SlideBoom

Semiconductors
Copyright © Declan O’Keeffe
Ard Scoil na nDéise,
Dungarvan
Enjoy!
http://physics.slss.ie/forum
Slide 1
Help
To view contents/show animation on each slide use

back/forward ‘arrow keys’ on keyboard

mouse click (mouse click must be outside any interactive flash
animation area present on a slide)
To view the ‘flash’ content in this presentation you need to
have Shockwave & Flash player installed on your system. If
you can see a flashing bulb (top right hand corner) then you
may proceed….if not click
and
and
to download & install.
Insulators
Insulators have tightly bound electrons in their outer shell
These electrons require a very large amount of energy to free
them for conduction
Let’s apply a potential difference across the insulator above…
The force on each electron is not enough to free it from its orbit
and the insulator does not conduct
Insulators are said to have a high resistivity / resistance
Slide 3
Conductors
Conductors have loosely bound electrons in their outer shell
These electrons require a small amount of energy to free them
for conduction
Let’s apply a potential difference across the conductor above…
The force on each electron is enough to free it from its orbit and
it can jump from atom to atom – the conductor conducts
Conductors are said to have a low resistivity / resistance
Slide 4
Semiconductors
Semiconductors have a resistivity/resistance between that
of conductors and insulators
Their electrons are not free to move but a little energy will
free them for conduction
The two most common semiconductors are silicon and
germanium
Slide 5
Semiconductor Industry in 2003



The semiconductor business: $166B.
– 1018 transistors produced during the
year.
US semiconductor industry: $80B.
$13B reinvested in research, $10B in
equipment. 226 000 jobs in US alone.
http://www.infras.com/Tutorial/sld001.htm
Typical Semiconductors
Silicon
GaAs
Diamond Cubic Structure
ZnS (Zinc Blende) Structure
4 atoms at (0,0,0)+ FCC translations 4 Ga atoms at (0,0,0)+ FCC translations
4 atoms at (¼,¼,¼)+FCC translations 4 As atoms at (¼,¼,¼)+FCC translations
Bonding: covalent
Bonding: covalent, partially ionic
The Silicon, Si, Atom
Silicon has a valency
of 4 i.e. 4 electrons in
its outer shell
This picture shows
the shared electrons
Each silicon atom
shares its 4 outer
electrons with 4
neighbouring atoms
These shared electrons
– bonds – are shown as
horizontal and vertical
lines between the
atoms
Slide 8
Silicon – the crystal lattice
If we extend this
arrangement
throughout a piece of
silicon…
We have the crystal
lattice of silicon
This is how silicon
looks when it is cold
It has no free electrons – it cannot conduct electricity – therefore it
behaves like an insulator
Slide 9
Electron Movement in Silicon
However, if we apply
a little heat to the
silicon….
An electron may gain
enough energy to
break free of its
bond…
It is then available
for conduction and is
free to travel
throughout the
material
Slide 10
Hole Movement in Silicon
Let’s take a closer
look at what the
electron has left
behind
There is a gap in the
bond – what we call
a hole
Let’s give it a little
more character…
Slide 11
Hole Movement in Silicon
This hole can also
move…
An electron – in a
nearby bond – may
jump into this hole…
Effectively causing
the hole to move…
Like this…
Slide 12
Heating Silicon
We have seen that,
in silicon, heat
releases electrons
from their bonds…
This creates
electron-hole pairs
which are then
available for
conduction
Slide 13
Intrinsic Conduction
Take a piece of
silicon…
And apply a potential
difference across it…
This sets up an
electric field
throughout the
silicon – seen here as
dashed lines
When heat is applied an electron is
released and…
Slide 14
Intrinsic Conduction
The electron feels a
force and moves in
the electric field
It is attracted to the
positive electrode
and re-emitted by the
negative electrode
Slide 15
Intrinsic Conduction
Now, let’s apply
some more heat…
Another electron
breaks free…
And moves in the
electric field.
We now have a
greater current than
before…
And the silicon has
less resistance…
Slide 16
Intrinsic Conduction
If more heat is
applies the process
continues…
More heat…
More current…
Less resistance…
The silicon is acting
as a thermistor
Its resistance decreases
with temperature
Slide 17
The Thermistor
The thermistor is a heat sensitive
resistor
 When cold it behaves as an
insulator i.e. it has a very high
resistance
 When heated, electron hole pairs
are released and are then available
for conduction as has been
described – thus its resistance is
reduced

Thermistor
Symbol
Slide 18
The Thermistor

Thermistors are used to measure
temperature

They are used to turn devices on,
or off, as temperature changes

They are also used in fire-warning
or frost-warning circuits
Thermistor
Symbol
Slide 19
The Thermistor in Action
Slide 20
The Light Dependent Resistor (LDR)
The LDR is very similar to the
thermistor – but uses light energy
instead of heat energy
 When dark its resistance is high
 As light falls on it, the energy
releases electron-hole pairs
 They are then free for conduction
LDR Symbol
 Thus, its resistance is reduced

Slide 21
The Light Dependent Resistor (LDR)

LDR’s are used as light meters

LDR’s are also used to control
automatic lighting

LDR’s are used where light is
needed to control a circuit – e.g.
Light operated burgler alarm
LDR Symbol
Slide 22
The LDR in Action.
Slide 23
The Phosphorus Atom
Phosphorus is
number 15 in the
periodic table
It has 15 protons and
15 electrons – 5 of
these electrons are in
its outer shell
Slide 24
Doping – Making n-type Silicon
Relying on heat or
light for conduction
does not make for
reliable electronics
Suppose we remove
a silicon atom from
the crystal lattice…
and replace it with a
phosphorus atom
We now have an electron that is not bonded – it is thus free for
conduction
Slide 25
Doping – Making n-type Silicon
Let’s remove another
silicon atom…
and replace it with a
phosphorus atom
As more electrons
are available for
conduction we have
increased the
conductivity of the
material
Phosphorus is called
the dopant
If we now apply a potential difference
across the silicon…
Slide 26
Extrinsic Conduction – n-type Silicon
A current will
flow
Note:
The negative
electrons move
towards the
positive
terminal
Slide 27
N-type Silicon




From now
on n-type
will be
shown like
this.
This type of silicon is called n-type
This is because the majority charge carriers are
negative electrons
A small number of minority charge carriers – holes –
will exist due to electrons-hole pairs being created in
the silicon atoms due to heat
The silicon is still electrically neutral as the number of
protons is equal to the number of electrons
Slide 28
The Boron Atom
Boron is number 5
in the periodic table
It has 5 protons and
5 electrons – 3 of
these electrons are
in its outer shell
Slide 29
Doping – Making p-type Silicon
As before, we
remove a silicon
atom from the crystal
lattice…
This time we replace
it with a boron atom
Notice we have a
hole in a bond – this
hole is thus free for
conduction
Slide 30
Doping – Making p-type Silicon
Let’s remove another
silicon atom…
and replace it with
another boron atom
As more holes are
available for
conduction we have
increased the
conductivity of the
material
Boron is the dopant
in this case
If we now apply a potential difference
across the silicon…
Slide 31
Extrinsic Conduction – p-type silicon
A current will
flow – this time
carried by
positive holes
Note:
The positive
holes move
towards the
negative terminal
Slide 32
P-type Silicon




From now
on p-type
will be
shown like
this.
This type of silicon is called p-type
This is because the majority charge carriers are positive
holes
A small number of minority charge carriers – electrons –
will exist due to electrons-hole pairs being created in the
silicon atoms due to heat
The silicon is still electrically neutral as the number of
protons is equal to the number of electrons
Slide 33
Typical Donor and Acceptor Dopants for Si
For Silicon:
 Donors (n type):
– P, As, Sb
 Acceptors (p type):
– B, Al, Ga, In
The p-n Junction
Suppose we join a piece of p-type silicon to a piece
of n-type silicon
We get what is called a p-n junction
Remember – both pieces are electrically neutral
Slide 35
The p-n Junction
When initially joined
electrons from the
n-type migrate into the
p-type – less electron
density there
When an electron
fills a hole – both the
electron and hole
disappear as the gap
in the bond is filled
This leaves a region with no free charge carriers – the depletion
layer – this layer acts as an insulator
Slide 36
The p-n Junction
0.6 V
As the p-type has
gained electrons – it
is left with an overall
negative charge…
As the n-type has
lost electrons – it is
left with an overall
positive charge…
Therefore there is a voltage across the junction – the junction
voltage – for silicon this is approximately 0.6 V
Slide 37
The Reverse Biased P-N Junction
Take a p-n junction
Apply a voltage
across it with the
p-type negative
n-type positive
Close the switch
The voltage sets
up an electric
field throughout
the junction
The junction is said to be reverse – biased
Slide 38
The Reverse Biased P-N Junction
Negative electrons
in the n-type feel
an attractive force
which pulls them
away from the
depletion layer
Positive holes in
the p-type also
experience an
attractive force
which pulls them
away from the
depletion layer
Thus, the depletion layer ( INSULATOR ) is
widened and no current flows through the
p-n junction
Slide 39
The Forward Biased P-N Junction
Take a p-n junction
Apply a voltage
across it with the
p-type postitive
n-type negative
Close the switch
The voltage sets
up an electric
field throughout
the junction
The junction is said to be
forward – biased
Slide 40
The Forward Biased P-N Junction
Negative electrons
in the n-type feel a
repulsive force
which pushes
them into the
depletion layer
Positive holes in
the p-type also
experience a
repulsive force
which pushes them
into the depletion
layer
Therefore, the depletion layer is eliminated
and a current flows through the p-n junction
Slide 41
The Forward Biased P-N Junction
At the junction
electrons fill holes
Both disappear
as they are no
longer free for
conduction
They are
replenished by the
external cell and
current flows
This continues as long as the external voltage
is greater than the junction voltage i.e. 0.6 V
Slide 42
The Forward Biased P-N Junction
If we apply a
higher voltage…
The electrons feel
a greater force
and move faster
The current will
be greater and
will look like
this….
The p-n junction is called a DIODE
and is represented by the symbol…
The arrow shows the
direction in which it
conducts current
Slide 43
The Semiconductor Diode
The semiconductor diode is a p-n
junction
 In reverse bias it does not conduct

In forward bias it conducts as long
as the external voltage is greater
than the junction voltage
 A diode should always have a
protective resistor in series as it
can be damaged by a large current

Slide 44
The Semiconductor Diode




The silver line drawn on one side of the
diode represents the line in its symbol
This side should be connected to the
negative terminal for the diode to be
forward biased
Diodes are used to change alternating
current to direct current
Diodes are also used to prevent damage in
a circuit by connecting a battery or power
supply the wrong way around
Slide 45
The Light Emitting Diode (LED)





Some diodes emit light as they conduct
These are called LED’s and come in various colours
LED’s have one leg longer than the other
The longer leg should be connected to the positive
terminal for the LED to be forward biased
LED’s are often used as power indicators on radios,
TV’s and other electronic devices
Symbol
Slide 46
LED cont.
Diode Fundamentals
Light Emitting Diodes (LEDs) are solidstate semiconductor devices that convert
electrical energy directly into light. LED
"cold" generation of light leads to high
efficacy because most of the energy
radiates within the visible spectrum.
Because LEDs are the solid-state
devices, they can be extremely small and
durable; they also provide longer lamp
life than other sources.
LEDs are made of various semiconducting compounds, depending on the desired
colour output. Infrared and red LEDs generally use a gallium, aluminum, and
arsenide compound. Orange and yellow LEDs most often use gallium, aluminum,
and either indium or phosphorus. Green and blue LEDs typically use either
47
silicon and carbon, or gallium and nitrogen.
LEDs cont.
Diode Fundamentals
Light Emitting Diodes (LEDs)
48
The Characteristic Curve of a Diode





Diodes do not obey Ohm’s Law
A graph of CURRENT vs
VOLTAGE for a diode will not
be a straight line through the
origin
The curve will look like this one
Note how the current increases
dramatically once the voltage
reaches a value of 0.6 V approx.
i.e. the junction voltage
This curve is known as the
characteristic curve of the diode
Slide 49
Introduction to Transistors

A transistor is a device with three separate layers of
semiconductor material stacked together
– The layers are made of n–type or p–type material in the order
pnp or npn
– The layers change abruptly to form the pn or np junctions
– A terminal is attached to each layer
(The Art of Electronics, Horowitz
and Hill, 2nd Ed.)
(Introductory Electronics, Simpson, 2nd Ed.)
Modern Transistors
BJT (Bipolar Junction Transistor)
•Three terminal device
•Three semiconductor regions, above is “pnp”
•E: Emitter, B: Base, C: Collector
•Voltage between two terminals to control current
Use as Amplifier or Switch
53
NPN Transistor
•Current flow in an npn transistor biased to operate in the
active mode
•Forward bias of Emitter-Base Junction: current flows to
emitter, electrons move towards base, holes to emitter
•Reverse bias of Base-Collector Junction: IC independent of 54
VCB
PNP Transistor
•Current in PNP mainly due to holes injected from emitter to
base
55
Transistor types
MOS - Metal Oxide Semiconductor
 FET - Field Effect Transistor
 BJT - Bipolar Junction Transistor

Moore’s Law
It’s an observation made by Gordon E. Moore, in
which he predicted that the number of transistors,
inside an Integrated Circuit, could be doubled
every 24 months.
 At the density that also minimized the cost of a
transistor.

http://upload.wikimedia.org/wikipedia/commons/0/06/Moore_Law_diagram_%282004%29.png
Transistor problems

Power density increased

Device variability

Reliability

Complexity

Leakage

Power dissipation limits device density

Transistor will operate near ultimate limits of size and quality
– eventually, no transistor can be fundamentally better
The Future of transistors







"Photo: National Research Council of Canada.“
http://www.nrc-cnrc.gc.ca/multimedia/picture/
fundamental/nrc-nint_moleculartransistor_e.html
Molecular electronics
Carbon nanotubes transistors
Nanowire transistors
Quantum computing
CMOS devices will add functionality
to CMOS non-volatile memory, optoelectronics, sensing….
CMOS technology will address new
markets macroelectronics, bio-medical
devices, …
Biology may provide inspiration for
new technologies bottom-up
assembly, human intelligence
Pictorial History of Transistors
http://www.bellsystemmemorial.com/belllabs_transistor.html
The End
© Declan O’Keeffe
Slide 62