Electronics Sections
Dr. Azza
1.
Semiconductor
2.
P-N Junction
3.
Diode , Zener Diode
4.
BJT Transistor
5.
MosFet Transistor
6.
Some Applications Using Transistor
Slide 1
Electronics Sections
Dr. Amen Nasar
1.
Op-Amp
2.
Some Applications Using Op-Amp
Slide 2
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
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 6
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 7
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 8
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 9
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 10
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 11
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 12
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 13
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 14
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 15
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 16
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 17
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 18
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
LDR Symbol
Slide 19
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 20
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 21
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 22
Extrinsic Conduction – n-type Silicon
A current will
flow
Note:
The negative
electrons move
towards the
positive
terminal
Slide 23
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 24
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 25
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 26
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 27
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 28
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 29
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 30
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 31
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 32
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 33
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 34
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 35
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 36
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 37
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 38
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 39
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 40
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 41
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 42
The pn Junction Diode
Schematic diagram
p-type
net acceptor
concentration NA
Circuit symbol
ID
n-type
net donor
concentration ND
cross-sectional area AD
Physical structure:
(an example)
+
+ ID
VD
metal
SiO2
For simplicity, assume that
the doping profile changes
abruptly at the junction.
VD
SiO2
p-type Si
n-type Si
–
–
metal
Charge Density Distribution
Charge is stored in the depletion region.
acceptor ions
p
quasi-neutral p region
donor ions
–
–
–
–
–
+
+
+
+
+
n
depletion region
quasi-neutral n region
charge density (C/cm3)
distance
Two Governing Laws
Gauss’s Law describes the relationship of charge (density) and
electric field.
1
 E  dA      dV 
S
dE 

dx 
V
Qencl

E ( x)  E ( x0 ) 
1

x
x0
 ( x)dx
Poisson’s Equation describes the relationship between electric
field distribution and electric potential
d 2 ( x)
dE ( x)
 ( x)


2
dx
dx

x
 ( x)   ( x0 )    E ( x)dx
x0
Depletion Approximation 1
 qN  x  x  0
 0 x   
a
 qN d
p0
0  x  xn0 
and  0 x   0 x   x p 0 , x  xn 0 
ρo(x)
p
qNd
n
x
E0 ( x) 
qN a
s
E0 ( x)  
xno
x
E0 ( x) 
-xpo
( x  x po )
x
( x po  x  0)
-qNa
0 ( x)
qN d
dx  E0 ( xno ) 
( xno  x)  0
s
s
qN d
s
(0  x  xno )
xno
( x  xno )
Gauss’s Law
E0(x)
p
n
-xpo
xno
x
x
E0 (0) 
 qN a x po
s

 qN d xno
s
Depletion Approximation 2
E0(x)
p
-xpo
qN
d
2s
xno2 
xno
E0 (0) 
Poisson’s Equation
qN
a
2s
n
 qNa x po
s
x

 qNd xno
s
0(x)
xpo2
n=1017
p=105
P=1018
n=104
-xpo
xno
x
Depletion Approximation 3
0 ( x)  
x
 x po
 E0 ( x)dx  0 ( x po )  
qN a
x
s
 x po
( x  x po )dx  0
x
qN a  x


x
dx

x
dx

 xpo po 
 s   x po
qN a
0 ( x) 
( x  x po ) 2
2 s
(  x po  x  0)
x
x
0
0
0 ( x)    E0 ( x)dx  0 (0)   

qN d
s
 
x
0
x dx  
x
0

qN d
s
qN a
( x  xno )dx 
(0  x po ) 2
2 s
qN a
xno dx 
x po 2
2 s
qN d
qN a
2
0 ( x) 
x(2 xno  x) 
x po 2
2 s
2 s
(0  x  xno )
Depletion Approx. – with VD<0 reverse bias
p
E0(x)
-xp -xpo
n
xno x
x
n
E0 (0) 
 qNa x po
s

 qNd xno
s
Higher barrier and few holes in n-type
lead to little current!
p=105
0(x)
qN
qN
bi-qVD
a
d
2
2

Built-in potential bi=
xno
xpo


2s
2s
n=1017
P=1018
bi
n=104
-xp -xpo
xnoxn
x
Depletion Approx. – with VD>0 forward bias
E0(x)
p
-xpo
-x
p
n
xnxno
E0 (0) 
 qNa x po
x

 qNd xno
s
s
Poisson’s Equation
Lower barrier and large hole (electron) density
at the right places lead to large current!
0(x)
qN
qN
a
d
Built-in potential bi=
xno2 
xpo2
2s
2s
18
P=10
n=1017
p=105
bi
bi-qVD
n=104
-xp
-xpo
xnxno
x
Forward Bias

As VD increases, the potential barrier to carrier
diffusion across the junction decreases*, and current
increases exponentially.
VD > 0
p
–
–
–
–
–
+
+
+
+
+
I D  I S (e
The carriers that diffuse across the
junction become minority carriers in
the quasi-neutral regions; they then
recombine with majority carriers,
“dying out” with distance.
ID (Amperes)
n
qVD kT
 1)
VD (Volts)
* Hence, the width of the depletion region decreases.
Reverse Bias

As |VD| increases, the potential barrier to carrier
diffusion across the junction increases*; thus, no
carriers diffuse across the junction.
VD < 0
p
–
–
–
–
–
+
+
+
+
+
n
A very small amount of reverse
current (ID < 0) does flow, due to
minority carriers diffusing from the
quasi-neutral regions into the depletion
region and drifting across the junction.
ID (Amperes)
VD (Volts)
* Hence, the width of the depletion region increases.
Reference
1.Copyright © Declan O’Keeffe
Ard Scoil na nDéise,
Dungarvan @ http://physics.slss.ie/forum
2. EE40
Lecture 32
Prof. Chang-Hasnain
Slide 53
The End
Slide 54