1. Electronics Sections Dr. Azza
Semiconductor
P-N Junction
Diode , Zener Diode
BJT Transistor
MosFet Transistor
Some Applications Using Transistor
Slide 1

2. Electronics Sections Dr. Amen Nasar
Op-Amp
Some Applications Using Op-Amp
Slide 2

3. 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

4. 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

5. 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

6. The Silicon, Si, Atom
Silicon has a valency of 4 i.e. 4 electrons in its outer shell
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
This picture shows the shared electrons
Slide 6

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. Intrinsic Conduction If more heat is applies the process continues…
More current…
Less resistance…
The silicon is acting as a thermistor
Its resistance decreases with temperature
Slide 15

16. 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

17. 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

18. 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
Thus, its resistance is reduced
LDR Symbol
Slide 18

19. 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

20. 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

21. 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

22. 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

23. Extrinsic Conduction – n-type Silicon
A current will flow
Note:
The negative electrons move towards the positive terminal
Slide 23

24. N-type Silicon This type of silicon is called n-type
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

25. 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

26. 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

27. 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

28. 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

29. P-type Silicon This type of silicon is called p-type
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

30. 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

31. 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

32. 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

33. 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

34. The Reverse Biased P-N Junction
Thus, the depletion layer ( INSULATOR ) is widened and no current flows through the 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
Slide 34

35. 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

36. 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

37. 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

38. 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 arrow shows the direction in which it conducts current
The p-n junction is called a DIODE and is represented by the symbol…
Slide 38

39. 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

40. 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

41. 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

42. 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

43. The pn Junction Diode Schematic diagram Circuit symbol ID + VD –
p-type n-type
net acceptor
concentration NA
net donor
concentration ND
+ VD –
cross-sectional area AD
Physical structure:
(an example) ID + VD –
metal
SiO2
SiO2
p-type Si
For simplicity, assume that
the doping profile changes
abruptly at the junction.
n-type Si
metal

44. Charge Density Distribution
Charge is stored in the depletion region.
acceptor ions
donor ions – + – +
p n – + – + – +
quasi-neutral p region
depletion region
quasi-neutral n region
charge density (C/cm3)
distance

45. Two Governing Laws
Gauss’s Law describes the relationship of charge (density) and electric field.
Poisson’s Equation describes the relationship between electric field distribution and electric potential

46. Depletion Approximation 1
Gauss’s Law p n

47. Depletion Approximation 2
E0(x) n
-xpo
xno x s no d po a x qN E e - = ) (
Poisson’s Equation
f0(x) qN qN d x 2 + a x 2 2 e no 2 e po s s
n=1017
p=105
P=1018
n=104
-xpo
xno x

48. Depletion Approximation 3

49. Depletion Approx. – with VD<0 reverse bias
E0(x) n
-xpo
xno
-xp xn x s no d po a x qN E e - = ) (
Higher barrier and few holes in n-type lead to little current!
p=105
f0(x) qN
-xp xn qN
fbi-qVD
Built-in potential fbi= d x 2 + a x 2 2 e no 2 e po
n=1017 s s
P=1018
fbi
n=104
-xpo
xno x

50. Depletion Approx. – with VD>0 forward bias
E0(x) n
-xpo
xno
-xp xn x s no d po a x qN E e - = ) (
Poisson’s Equation
Lower barrier and large hole (electron) density at the right places lead to large current!
f0(x) qN qN
Built-in potential fbi= d x 2 + a x 2 2 e no 2 e po
P=1018 s s
n=1017
p=105
fbi
n=104
fbi-qVD
-xpo
-xp xn
xno x

51. Forward Bias
As VD increases, the potential barrier to carrier diffusion across the junction decreases*, and current increases exponentially.
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.
VD > 0 – + – +
p n – + – + – +
ID (Amperes)
VD (Volts)
* Hence, the width of the depletion region decreases.

52. Reverse Bias
As |VD| increases, the potential barrier to carrier diffusion across the junction increases*; thus, no carriers diffuse across the junction.
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.
VD < 0 – + – +
p n – + – + – +
ID (Amperes)
VD (Volts)
* Hence, the width of the depletion region increases.

53. Reference 1.Copyright © Declan O’Keeffe Ard Scoil na nDéise,
Dungarvan @
2. EE40 Lecture 32 Prof. Chang-Hasnain
Slide 53

54. The End
Slide 54