1. Chapter 1 INTRODUCTION to SEMICONDUCTORS
By:
Syahrul Ashikin Azmi
School of Electrical System Engineering
UNIMAP

2. Objectives Discuss basic structures of atoms
Discuss properties of insulators,
conductors, and semiconductors
Discuss covalent bonding
Describe the conductions in semiconductor
Discuss N-type and P-type semiconductor
Discuss the diode
Discuss the bias of a diode

3. LECTURE’S CONTENT 1.1 Atomic structure
1.2 Semiconductor, conductors and insulators
1.3 Covalent bonding
1.4 Conduction in semiconductors
1.5 N-type and P-type semiconductors
1.6 Diode
1.7 Biasing the diode
1.8 Voltage-current characteristic of a diode
1.9 Diode models
1.10 Testing a diode

4. REVIEW

5. REVIEW

6. WHY WE USE ELECTRONICS? - Easy to move/control electrons than real
Electronics are easy to move/control
- Easy to move/control electrons than real
physical stuff
Fig. 1-32a & b forward and reverse bias schematic diagram
Move information not things
- Phone, fax, internet
- Takes much less energy and money

7. History Of Semiconductor Devices
Fig. 1-32a & b forward and reverse bias schematic diagram

8. 1.1 Atomic Structure

9. ATOM 1.1 Atomic Structure Atomic number Basic structure
Electron shells
Figure 1-1 Bohr Model
ATOM
Valence electron
Free electron
Ionization

10. 1.1 Atomic Structure (cont.)
smallest particle of an element contain 3 basic particles:
Electrons
(negative charge)
ATOM
Protons
(positive charge)
Figure 1-1 Bohr Model
Nucleus
(core of atom)
Neutrons
(uncharged)

11. 1.1 Atomic Structure (cont.)
This model was proposed by Niels Bohr in 1915.
-electrons circle the nucleus that consists of protons and neutrons.
Figure 1-1 Bohr Model
Figure Bohr model of an atom

12. 1.1 Atomic Structure (cont.)
Atomic Number
- Element in periodic table are arranged according to atomic number
- Atomic number = number of protons in nucleus
Electron Shells and Orbits
Electrons near the nucleus have less energy than those in more
distant orbits.
Each distance (orbits) from the nucleus corresponding to a
certain energy level.
In an atom, the orbits are group into energy bands – shells
Diff. in energy level within a shell << diff. in energy between
shells.
Figure 1-1 Bohr Model
Valence Electrons
Electrons with the highest energy levels exist in the outermost
shell and loosely bound to the atom. The outermost shell – valence shell.
Electron in the valence shell called valence electrons.

13. 1.1 Atomic Structure (cont.)
Ionization
When atoms absorb energy (e.g heat source) – losing valence
electrons called ionization.
Escape electron called free electron.
The Number of Electrons in Each Shell
The maximum number of electrons (Ne) in each shell is calculated using formula below:
n = number of shell
Example for 2nd shell
Figure 1-1 Bohr Model

14. 1.2 Semiconductors, conductors and insulators

15. 1.2 Semiconductors, Conductors, and Insulators
Atom can be represented by the valence shell and a core
A core consists of all the inner shell and the nucleus
Carbon atom:
-valence shell – 4 e
-inner shell – 2 e
Nucleus:
-6 protons
-6 neutrons
+6 for the nucleus
and -2 for the two
inner-shell electrons
(net charge +4)

16. 1-2 Semiconductors, Conductors, and Insulators (cont.)
material that easily conducts electrical current.
The best conductors are single-element material (e.g copper, silver, gold, aluminum)
Only one valence electron very loosely bound to the atom- free electron
Insulators
material does not conduct electrical current
valence electron are tightly bound to the atom – very few free electron
Semiconductors
material between conductors and insulators in its ability to conduct electric current
in its pure (intrinsic) state is neither a good conductor nor a good insulator
most common semiconductor- silicon(Si), germanium(Ge), and carbon(C) which contains four valence electrons.

17. 1.2 Semiconductors, Conductors, and Insulators (cont)

18. 1.2 Semiconductors, Conductors, and Insulators (cont.)
Energy Bands

19. 1-2 Semiconductors, Conductors, and Insulators (cont.)
Energy Bands
Energy gap-the difference between the energy levels of any two orbital shells
Band-another name for an orbital shell (valence shell=valence band)
Conduction band –the band outside the valence shell where it has free electrons.

20. 1-2 Semiconductors, Conductors, and Insulators (cont.)
Energy Bands
at room temperature
25°
eV (electron volt) – the energy absorbed by an electron when it is
subjected to a 1V difference of potential

21. 1-2 Semiconductors, Conductors, and Insulators (cont.)
Comparison of a Semiconductor Atom & Conductor Atom
A Silicon atom:
4 valence electrons
A semiconductor
Electron conf.: 2:8:4
A Copper atom:
Only 1 valence electron
A good conductor
Electron conf.:2:8:18:1
Fig. 1-6 Copper and Silicon atoms
14 protons
14 nucleus
10 electrons
in inner shell
29 protons
29 nucleus
28 electrons in
inner shell

22. 1-3 Covalent Bonding

23. 1-3 Covalent Bonding 1-3 Covalent Bonding
Covalent bonding – holding atoms together by sharing valence electrons
sharing of valence electron
produce the covalent bond
To form Si crystal
Fig. 1-8 Covalent bonding

24. 1-3 Covalent Bonding (cont.)
Result of the bonding:
The atom are held together forming a solid substrate.
The atoms are all electrically stable, because
their valence shells are complete.
The complete valence shells cause the silicon to act as an insulator-intrinsic (pure) silicon.
In other word, it is a very poor conductor.
Fig. 1-8 Covalent bonding

25. 1-3 Covalent Bonding (cont.)
Covalent bonding in an intrinsic or pure silicon crystal. An intrinsic crystal has no impurities.
Fig. 1-9 Intrinsic Silicon
Covalent bonds in a 3-D silicon crystal

26. 1-4 Conduction in Semiconductor

27. 1-4 Conduction in Semiconductor Conduction Electrons and Holes
Fig. 1-9 Intrinsic Silicon
Figure Energy band diagram for a pure (intrinsic) silicon crystal with unexcited (no external energy such as heat) atoms. There are no electrons in the conduction band. This condition occurs only at a temperature of absolute 0 Kelvin.

28. 1-4 Conduction in Semiconductor (cont.) Conduction Electrons and Holes
Absorbs enough energy (thermal energy)
to jumps
Fig. 1-9 Intrinsic Silicon
a free electron and
its matching valence
band hole –
electron-hole pair
Recombination-when a conduction electron
loses energy and fall back into hole in
valence band
Figure 1-11 Creation of electron-hole pairs in a silicon crystal. Electrons in the conduction band are free (also called conduction electrons).

29. 1-4 Conduction in Semiconductor (cont.) Conduction Electrons and Holes
Fig. 1-9 Intrinsic Silicon
Figure 1-12 Electron-hole pairs in a silicon crystal. Free electrons are being generated continuously while some recombine with holes.

30. 1-4 Conduction in Semiconductor (cont.) Electrons and Holes Current
Electron current
free
electrons
Fig. 1-9 Intrinsic Silicon
Apply voltage
When a voltage is applied, free electrons are free to move randomly
and attracted toward +ve end. The movement of electrons is one type of current
in semiconductor and is called electron current.
Figure 1-13 Electron current in intrinsic silicon is produced by the movement of thermally generated free electrons.

31. 1-4 Conduction in Semiconductor (cont.) Electrons and Holes Current
Fig. 1-9 Intrinsic Silicon
movement
of holes
Figure Hole current in intrinsic silicon.

32. 1-5 N-type and P-type Semiconductors

33. 1-5 N-type and P-type Semiconductors
Doping
- The process of creating N and P type materials
- By adding impurity atoms to intrinsic Si or Ge to improve the
conductivity of the semiconductor
- Two types of doping – trivalent (3 valence e-) & pentavalent (5 valence e-)
p-type material – a semiconductor that has added trivalent impurities
n-type material – a semiconductor that has added pentavalent
impurities
Fig & 16 Pentavalent and Trivalent
Trivalent Impurities:
Aluminum (Al)
Gallium (Ga)
Boron (B)
Indium (In)
Pentavalent Impurites:
Phosphorus (P)
Arsenic (As)
Antimony (Sb)
Bismuth (Bi)

34. 1-5 N-type and P-type Semiconductors (cont.)
N-type semiconductor:
Pentavalent impurities are added to Si or Ge, the result is an increase of
free electrons
1 extra electrons becomes a conduction electrons because it is not
attached to any atom
No. of conduction electrons can be controlled by the no. of impurity atoms
Pentavalent atom gives up an electron -call a donor atom
Current carries in n-type are electrons – majority carriers
Holes – minority carriers (holes created in Si when generation of electron-
holes pair.
Fig & 16 Pentavalent and Trivalent
Sb impurity atom
Pentavalent impurity atom in a Si crystal

35. 1-5 N-type and P-type Semiconductors (cont.)
- Trivalent impurities are added to Si or Ge to increase number of holes.
Boron, indium and gallium have 3 valence e- form covalent bond with 4
adjacent silicon atom. A hole created when each trivalent atom is added.
The no. of holes can be controlled by the no. of trivalent impurity atoms
- The trivalent atom can take an electron- acceptor atom
- Current carries in p-type are holes – majority carries
electrons – minority carries (created during electron-holes pairs
generation).
Fig & 16 Pentavalent and Trivalent
B impurity atom
Trivalent impurity atom in a Si crystal

36. 1-6 The Diode

37. 1-6 The Diode
- Diode is a device that conducts current only in one direction.
- n-type material & p-type material become extremely useful when
joined together to form a pn junction – then diode is created
- before the pn junction is formed -no net charge (neutral) since no of proton and electron is equal in both n-type and p-type.
p region: holes (majority carriers), e- (minority carriers)
-n region: e- (majority carriers), holes (minority carriers)
Fig 1-18 a & b depletion region

38. 1-6 The Diode (cont.) The Depletion Region
Fig 1-18 a & b depletion region

39. 1-6 The Diode (cont.) The Depletion Region
Summary:
When an n-type material is joined with a p-type material:
A small amount of diffusion occurs across the junction.
When e- diffuse into p-region, they give up their energy and fall into the holes near the junction.
Since the n-region loses electrons, it creates a layer of +ve charges (pentavalent ions).
p-region loses holes since holes combine with electron and will creates layer of –ve charges (trivalent ion). These two layers form depletion region.
Depletion region establish equilibrium (no further diffusion) when total –ve charge in the region repels any further diffusion of electrons into p-region.
Fig 1-18 a & b depletion region

40. 1-6 The Diode Barrier Potential
In depletion region, many +ve and –ve charges on opposite sides of pn junction.
The forces between the opposite charges form a “field of forces "called an electric field.
This electric field is a barrier to the free electrons in the n-region, need more energy to move an e- through the electric field.
The potential difference of electric field across the depletion region is the amount of voltage required to move e- through the electric field. This potential difference is called barrier potential. [ unit: V ]
Depends on: type of semicon. material, amount of doping and temperature. (e.g : 0.7V for Si and 0.3 V for Ge at 25°C).

41. 1-6 The Diode (cont.) Energy Diagram of the PN Junction and Depletion Region
Fig 1-18 a & b depletion region
Overlapping

42. 1-6 The Diode (cont.) Energy Diagram of the PN Junction and Depletion Region
Energy level for n-type (Valence and Cond. Band) << p- type material (difference in atomic characteristic : pentavalent & trivalent) and significant amount of overlapping.
Free e- in upper part conduction band in n-region can easily diffuse across junction and temporarily become free e- in lower part conduction band in p-region. After crossing the junction, the e- loose energy quickly & fall into the holes in p-region valence band.

43. 1-6 The Diode (cont.) Energy Diagram of the PN Junction and Depletion Region
As the diffusion continues, the depletion region begins to form and the energy level of n-region conduction band decreases due to loss of higher-energy e- that diffused across junction to p-region.
Soon, no more electrons left in n-region conduction band with enough energy to cross the junction to p-region conduction band.
Figure (b), the junction is at equilibrium state, the depletion region is complete diffusion has ceased (stop). Create an energy gradient which act as energy ‘hill’ where electron at n-region must climb to get to the p-region.
The energy gap between valence & cond. band – remains the same

44. 1-7 Biasing The Diode

45. 1.7 Biasing The Diode
No electron move through the pn-junction at equilibrium state.
Bias is a potential applied (dc voltage) to a pn junction to obtain a desired mode of operation – control the width of the depletion layer.
Two bias conditions : forward bias & reverse bias
Fig. 1-22b depletion region forward & Fig depletion region reverse
The relationship between the width of depletion layer & the junction current
Depletion Layer Width
Junction Resistance
Junction Current
Min
Max

46. 1.7 Biasing The Diode (cont.) Forward bias
1. Voltage source or bias connections are + to the p region and – to the n region.
2. Bias voltage must be greater than barrier potential (0 .3 V for Germanium or 0.7 V for Silicon).
The depletion region narrows.
R – limits the current which can prevent damage to the diode
Fig. 1-22b depletion region forward & Fig depletion region reverse
Diode connection

47. Flow of majority carries and the voltage across the depletion region
1.7 Biasing The Diode (cont.) Forward bias
The negative side of the bias voltage push the free electrons in the n-region -> pn junction. Flow of free electron is called electron current.
Also provide a continuous flow of electron through the external connection into n-region.
Bias voltage imparts energy to the free e- to move to p-region.
Electrons in p-region loss energy-combine with holes in valence band.
Flow of majority carries and the voltage across the depletion region

48. 1.7 Biasing The Diode (cont.) Forward bias
Since unlike charges attract, positive side of bias voltage source attracts the e- left end of p-region.
Holes in p-region act as medium or pathway for these e- to move through the p-region.
e- move from one hole to the next toward the left.
The holes move to right toward the junction. This effective flow is called hole current.
Flow of majority carries and the voltage across the depletion region

49. 1. 7 Biasing The Diode (cont
1.7 Biasing The Diode (cont.) The Effect of Forward bias on the Depletion Region
Fig. 1-22b depletion region forward & Fig depletion region reverse
As more electrons flow into the depletion region, the no. of +ve ion is reduced.
As more holes flow into the depletion region on the other side of pn junction, the no. of –ve ions is reduced.
Reduction in +ve & -ve ions – causes the depletion region to narrow.

50. 1. 7 Biasing The Diode (cont
1.7 Biasing The Diode (cont.) The Effect of the Barrier Potential During Forward Bias
Electric field between +ve & -ve ions in depletion region creates “energy hill” that prevent free e- from diffusing at equilibrium state -> barrier potential
When apply forward bias – free e- provided enough energy to climb the hill and cross the depletion region.
Electron got the same energy = barrier potential to cross the depletion region.
An add. small voltage drop occurs across the p and n regions due to internal resistance of material – called dynamic resistance – very small and can be neglected
Fig. 1-22b depletion region forward & Fig depletion region reverse

51. 1.7 Biasing The Diode (cont.) Reverse bias
Fig. 1-22b depletion region forward & Fig depletion region reverse
Diode connection
Reverse bias - Condition that prevents current through the diode
Voltage source or bias connections are – to the p material and + to the n material
Current flow is negligible in most cases.
The depletion region widens than in forward bias.

52. 1-7 Biasing The Diode Shot transition time immediately after reverse bias voltage is applied
+ side of bias pulls the free electrons in the n-region away from pn junction cause add. +ve ions are created, widening the depletion region.
In the p-region, e- from – side of the voltage source enter as valence electrons e- and move from hole to hole toward the depletion region, then created add. –ve ions.
As the depletion region widens, the availability of majority carriers decrease.

53. 1.7 Biasing The Diode (cont.) Reverse Current
Extremely small current exist – after the transition current dies
out caused by the minority carries in n & p regions that are produced by
thermally generated electron hole pairs.
Small number of free minority e- in p region are “pushed toward the pn
junction by the –ve bias voltage.
e- reach wide depletion region, they “fall down the energy hill” combine
with minority holes in n -region as valence e- and flow towards the +ve bias
voltage – create small hole current.
The cond. band in p region is at higher energy level compare to cond. band in
n-region e- easily pass through the depletion region because they require no
additional energy.
Fig. 1-22b depletion region forward & Fig depletion region reverse

54. 1-8 Voltage-Current Characteristic Of A Diode

55. 1.8 Voltage-Current Characteristic of a Diode V-I Characteristic for Forward Bias
-When a forward bias voltage is applied, there is current called forward current, IF .
-In this case with the voltage applied is less than the barrier potential so the diode for all practical purposes is still in a non-conducting state. Current is very small.
-Increase forward bias voltage – current also increase.
Fig 1-26a measurements with meters
FIGURE Forward-bias measurements show general changes in VF and IF as VBIAS is increased.

56. 1. 8 Voltage-Current Characteristic of a Diode (cont
1.8 Voltage-Current Characteristic of a Diode (cont.) V-I Characteristic for Forward Bias
- With the applied voltage exceeding the barrier potential (0.7V), forward current begins increasing rapidly.
- But the voltage across the diode increase only gradually above 0.7 V. this is due to voltage drop across internal dynamic resistance of semicon material.
Fig. 1-26b measurements with meters
FIGURE Forward-bias measurements show general changes in VF and IF as VBIAS is increased.

57. dynamic resistance r’d decreases as you move up the curve
1.8 Voltage-Current Characteristic of a Diode (cont.) V-I Characteristic for Forward Bias
-Plot the result of measurement in Figure 1-26, you get the V-I characteristic curve for a forward bias diode
Increase to the right
increase upward
After 0.7V, voltage remains at 0.7V but IF increase rapidly.
Normal operation for a forward-biased diode is above the knee of the curve.
dynamic resistance r’d decreases as you move up the curve
Fig. 1-26b measurements with meters
zero
bias
Below knee, resistance is
greatest since current increase
very little for given voltage,
Resistance become smallest above
knee where a large change in current
for given change in voltage.

58. 1. 8 Voltage-Current Characteristic of a Diode (cont
1.8 Voltage-Current Characteristic of a Diode (cont.) V-I Characteristic for Reverse Bias
VR increase to the left along x-axis while IR increase downward along y-axis.
When VR reaches VBR , IR begin to increase rapidly.
Breakdown voltage, VBR.
not a normal operation of pn junction devices.
the value can be vary for typical Si.
Cause overheating and possible damage to diode.
Fig. 1-26b measurements with meters
Reverse Current

59. Combine-Forward bias & Reverse bias  CompleteV-I characteristic curve
1.8 Voltage-Current Characteristic of a Diode (cont.) The Complete V-I Characteristic Curve
Combine-Forward bias & Reverse bias  CompleteV-I characteristic curve
Fig. 1-26b measurements with meters

60. 1. 8 Voltage-Current Characteristic of a Diode (cont
1.8 Voltage-Current Characteristic of a Diode (cont.) Temperature Effects on the Diode V-I Characteristic
Forward biased diode : for a given value of
Barrier potential decrease as T increase.
For reverse-biased, T increase, IR increase.
Reverse current breakdown – small & can be neglected
Fig. 1-26b measurements with meters

61. 1-9 Diode Models

62. 1-9 Diode Models Diode Structure & Symbol
Direction of current
cathode
anode
Fig ideal diode curve

63. 1-9 Diode Models (cont.) DIODE MODEL The Ideal Diode Model
The Practical Diode Model
DIODE
MODEL
Fig ideal diode curve
The Complete Diode Model

64. 1-9 Diode Models (cont.) The Ideal Diode Model
Ideal model of diode- simple switch:
Closed (on) switch -> FB
Open (off) switch -> RB
Barrier potential, dynamic resistance and reverse current all neglected.
Assume to have zero voltage across diode when FB.
Forward current determined by Ohm’s law
Fig ideal diode curve

65. 1-9 Diode Models (cont.) The Practical Diode Model
Adds the barrier potential to the ideal switch model
‘ is neglected
From figure (c):
The forward current [by applying Kirchhoff’s voltage law to figure (a)]
By Ohm’s Law:
Equivalent to close switch in series with a small equivalent voltage source equal to the barrier potential 0.7V
Represent by produced across the pn junction
Open circuit, same as ideal diode model.
Barrier potential doesn’t affect RB
Fig ideal diode curve

66. 1-9 Diode Models (cont.) The Complete Diode Model
Complete model of diode consists:
Barrier potential
Dynamic resistance,
Internal reverse resistance,
The forward voltage consists of barrier potential & voltage drop across r’d :
The forward current:
acts as closed switch in series with barrier potential and small
acts as open switch in parallel with the large
Fig ideal diode curve

67. 1-9 Diode Models (cont.) Example 1
(1) Determine the forward voltage and forward current [forward bias] for each of the diode model also find the voltage across the limiting resistor in each cases. Assumed rd’ = 10 at the determined value of forward current.
Fig ideal diode curve
1.0kΩ
1.0kΩ 5V
10V

68. 1-9 Diode Models (cont.) Example 1
Ideal Model:
Practical Model:
(c) Complete model:
Fig ideal diode curve

69. 1-9 Diode Models (cont.) Typical Diodes
Diodes come in a variety of sizes and shapes. The design and structure is
determined by what type of circuit they will be used in.
Fig ideal diode curve

70. 1-10 Testing A Diodes By Digital Multimeter
- Testing a diode is quite simple, particularly if the multimeter used has a diode check function. With the diode check function a specific known voltage is applied from the meter across the diode.
- With the diode check function a good diode will show approximately 0.7 V or 0.3 V when forward biased.
Fig 1-38 DMM check w/electrode labels
- When checking in reverse bias, reading based on meter’s internal voltage source. 2.6V is typical value that indicate diode has extremely high reverse resistance.
K A
A K

71. 1-10 Testing A Diodes (By Digital Multimeter)
When diode is failed open, open
reading voltage is 2.6V or “OL”
indication for forward and reverse
bias.
If diode is shorted, meter reads 0V
in both tests. If the diode exhibit a
small resistance, the meter reading
is less than 2.6V.
Fig 1-38 DMM check w/electrode labels

72. 1-10 Testing A Diodes By Analog Multimeter – OHMs Function
Select OHMs range
Good diode:
Forward-bias:
get low resistance reading (10 to 100 ohm)
Reverse-bias:
get high reading (0 or infinity)
Fig 1-38 DMM check w/electrode labels

73. Summary
Diodes, transistors, and integrated circuits are all made of semiconductor material.
P-materials are doped with trivalent impurities
N-materials are doped with pentavalent impurities
P and N type materials are joined together to form a PN junction.
A diode is nothing more than a PN junction.
At the junction a depletion region is formed. This creates barrier which requires approximately 0.3 V for a Germanium and 0.7 V for Silicon for conduction to take place.

74. Summary
A diode conducts when forward biased and does not conduct when reverse biased
The voltage at which avalanche current occurs is called reverse breakdown voltage. Reverse breakdown voltage for diode is typically greater than 50V.
There are three ways of analyzing a diode. These are ideal, practical, and complete. Typically we use a practical diode model.

75. Your destiny is in your hand
There once was a wise man that was known throughout the land for his wisdom. One day a young boy wanted to test him to prove that the wise man a fake.
He thought to himself, “I will bring one live bird to test the old man. I will ask him whether the bird in my hand is dead or alive. If he says that it is alive, I will squeeze hard to kill the bird to prove that he is wrong.
On the other hand if he says that it is dead, I will let the bird fly off, proving that he is wrong. Either way the wise man will be wrong.”

76. With that idea in mind, he approached the wise man and asked, “Oh wise man, I have a bird in my hand. Can you tell me if the bird is dead or alive?”.
The wise man paused for a moment and replied, “Young man, you indeed have a lot t learn. That which you hold in your hand, it is what you make of it. The life of the bird is in your hand.
If you wish it to be dead, then it will die. On the other hand if you desire it to live, it will surely live”. The young boy finally realized that the answer given was indeed that of a man of wisdom.

77. Success principles
Our dreams are very fragile, just like the little bird. It is our own decision, if we decide to kill it, or allow others to steal it away from us. However, it is also our own choice to nurture it and let it grow to fruition. Success comes to those who allow their dreams to fly high, just like the little bird, which will soar into the sky if the young boy released it from his grasp.

78. Energy increases as the distance from the nucleus increases

79. Basic diode structure at the instant of junction formation showing only majority and minority carriers.