1. CHAPTER 1 Semiconductors

2. Introduction to Semiconductors Chapter Outline :
1.1 Atomic Structures
1.2 Semiconductors, Conductors, and Insulators
1.3 Covalent Bonds
1.4 Conduction in Semiconductor
1.5 N-Type and P-Type Semiconductor
1.6 The Diode
1.7 Biasing the Diode
1.8 Voltage Current Characteristic of a Diode
1.9 Diode Models
1.10 Testing a Diode

3. Introduction to Semiconductor - Chapter Objectives :
Discuss basic operation of a diode
Discuss the basic structure of atoms
Discuss properties of insulators, conductors and semiconductors
Discuss covalent bonding
Describe the properties of both p and n type materials
Discuss both forward and reverse biasing of a p-n junction

4. 1.1 Atomic Structures History of Semiconductor

5. 1.1 Atomic Structures Atomic number basic structure Electron shells
Valence electron
Figure 1-1 Bohr Model
Free electron
ionization

6. 1.1 Atomic Structures
smallest particle of an element contain 3 basic particles:
Neutrons
(uncharged)
Protons
(positive charge)
Electrons
(negative charge)
Nucleus
(core of atom)
Figure 1-1 Bohr Model
ATOM

7. 1.1 Atomic Structures Atomic Number
Element in periodic table are arranged according to atomic number
Atomic number = number of protons in nucleus
Figure 1-1 Bohr Model

8. 1.1 Atomic Structures Electron Shells and Orbits
In an atom, the orbits are group into energy bands – shells
Diff. in energy level within a shell << diff. an energy between shells
Energy increases as the distance from the nucleus increases.
Figure 1-1 Bohr Model

9. 1.1 Atomic Structures Valence Electrons
Electrons with the highest energy levels exist in the outermost shell.
Electron in the valence shell called valence electrons.
The term valence is used to indicate the potential required to removed any one
of these electrons.
Figure 1-1 Bohr Model

10. 1.1 Atomic Structures Bohr model of an atom
This model was proposed by Niels Bohr in 1915.
electrons circle the nucleus.
nucleus made of:
i) +protons
ii) Neutral:neutron
Figure 1-1 Bohr Model

11. 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

12. 1.2 Semiconductors, Conductors and Insulators
material that easily conducts electrical current.
The best conductors are single-element material (copper, silver, gold, aluminum)
One valence electron very loosely bound to the atom- free electron
Insulators
material does not conduct electric current
valence electron are tightly bound to the atom – less free electron

13. 1.2 Semiconductors, Conductors and Insulators
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 commonly use semiconductor ; silicon(Si), germanium(Ge), and carbon(C).
contains four valence electrons

14. 1.2 Semiconductors, Conductors and Insulators

15. 1.2 Semiconductors, Conductors and Insulators
Energy Bands

16. 1.2 Semiconductors, Conductors and Insulators
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

17. 1.2 Semiconductors, Conductors and Insulators
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

18. 1.2 Semiconductors, Conductors and Insulators
Comparison of a Semiconductor Atom & Conductor Atom
A Copper atom:
only 1 valence electron
a good conductor
Electron conf.:2:8:18:1
A Silicon atom:
4 valence electrons
a semiconductor
Electron conf.: 2:8:4
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

19. 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

20. 1.3 Covalent Bonding The 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 is a very poor conductor
Fig. 1-8 Covalent bonding

21. 1.3 Covalent Bonding
Certain atoms will combine in this way to form a crystal structure. Silicon and Germanium atoms combine in this way in their intrinsic or pure state.
Fig. 1-9 Intrinsic Silicon
Covalent bonds in a 3-D silicon crystal

22. 1.4 Conduction in Semiconductor (Conduction Electron and holes)
Fig. 1-9 Intrinsic Silicon
FIGURE Energy band diagram for a pure (intrinsic) silicon crystal with unexcited atoms. There are no electrons in the conduction band.

23. 1.4 Conduction in Semiconductor (Conduction Electron and holes)
Absorbs enough energy (thermal energy)
to jumps
Fig. 1-9 Intrinsic Silicon
a free electron and
its matching valence
band hole
FIGURE Creation of electron-hole pairs in a silicon crystal. Electrons in the conduction band are free.

24. 1.4 Conduction in Semiconductor (Conduction Electron and holes)
Fig. 1-9 Intrinsic Silicon
FIGURE Electron-hole pairs in a silicon crystal. Free electrons are being generated continuously while some recombine with holes.

25. 1.4 Conduction in Semiconductor (Electron and holes currents)
Electron current
free
electrons
Fig. 1-9 Intrinsic Silicon
Apply voltage
FIGURE Electron current in intrinsic silicon is produced by the movement of thermally generated free electrons.

26. 1.4 Conduction in Semiconductor (Electron and holes currents)
movement
of holes
Fig. 1-9 Intrinsic Silicon
FIGURE Hole current in intrinsic silicon.

27. 1.5 N-types and P-types Semiconductors (Doping)
Doping -the process of creating N and P type materials
-by adding impurity atoms to intrinsic Si or Ge to imporove 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
Trivalent Impurities:
Aluminum (Al)
Gallium (Ga)
Boron (B)
Indium (In)
Pentavalent Impurites:
Phosphorus (P)
Arsenic (As)
Antimony (Sb)
Bismuth (Bi)
Fig & 16 Pentavalent and Trivalent

28. 1.5 N-types and P-types Semiconductors
N-type semiconductor:
- Pentavalent impurities are added to Si or Ge, the result is an
increase the free electrons
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 carries
Holes – minority carries
Sb impurity atom
Fig & 16 Pentavalent and Trivalent
Pentavalent impurity atom in a Si crystal

29. 1.5 N-types and P-types Semiconductors
P-type semiconductor:
- Trivalent impurities are added to Si or Ge to create a deficiency of
electrons or hole charges
The holes created by doping process
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
B impurity atom
Fig & 16 Pentavalent and Trivalent
Trivalent impurity atom in a Si crystal

30. 1.6 The Diode
-n-type material & p-type material become extremely useful when
joined together to form a pn junction – then diode is created
-p region- holes (majority carriers), e- (minority carriers)
-n region- e- (majority carriers), holes (minority carriers)
-before the pn junction is formed -no net charge (neutral)
Fig 1-18 a & b depletion region

31. 1.6 The Diode (The Depletion Region)
Fig 1-18 a & b depletion region

32. 1.6 The Diode (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 in the
valance band covalent bonds.
Since the n-region have lost an electron, they have an overall +ve charge.
Since the p-region have gained an electron, they have an overall –ve charge.
The difference in charges on the two sides of the junction is called the barrier potential.
(typically in the mV range)
Barrier Potential:
The buildup of –ve charge on the p-region of the junction and of +ve charge on the
n-region of the junction-therefore difference of potential between the two sides of the
junction is exist.
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 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. [ unit: V ]
Depend on: type of semicon. material, amount of doping, temperature. (e.g : 0.7V for Si
and 0.3 V for Ge at 25°C)
Fig 1-18 a & b depletion region

33. 1.6 The Diode (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)
After cross the junction, the e- lose energy & fall into the holes in p-region valence band.
As the diffusion continues, the depletion region begins to form and the energy level of
n-region conduction band decrease.
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 –energy ‘hill’ – electron at n-region must climb
to get to the p-region.
The energy gap between valence & cond. band – remains the same
Fig 1-18 a & b depletion region

34. 1.7 Biasing The Diode (Bias)
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
The relationship between the width of depletion layer & the junction current
Fig. 1-22b depletion region forward & Fig depletion region reverse
Depletion Layer Width
Junction Resistance
Junction Current
Min
Max

35. 1.7 Biasing The Diode ( Forward Bias)
Flow of majority carries and the voltage across the depletion region
Diode connection
The negative side of the bias voltage push
the free electrons in the n-region -> pn
junction
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- positive side
of bias voltage source attracts the e- left the p-
region
Holes in p-region act as medium or pathway
for these e- to move through the p-region
Voltage source or bias connections are + to
the p material and – to the n material
Bias must be greater than barrier potential
(0 .3 V for Germanium or 0.7 V for Silicon
diodes)
The depletion region narrows.
R – limits the current to prevent damage
for diode
Fig. 1-22b depletion region forward & Fig depletion region reverse

36. 1.7 Biasing The Diode ( The Effect of Forward Bias on the Depletion Region)
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 – the no. of –ve
ions is reduced.
Reduction in +ve & -ve ions – causes the depletion region to narrow
Fig. 1-22b depletion region forward & Fig depletion region reverse

37. 1.7 Biasing The Diode ( The Effect of the Barrier Potential during Forward Bias)
Electric field between +ve & -ve ions in depletion region creates “energy hill”
-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

38. 1.7 Biasing The Diode ( ReverseBias)
Shot transition time immediately after reverse bias voltage is applied
Diode connection
+ 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- 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
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
Fig. 1-22b depletion region forward & Fig depletion region reverse

39. 1.7 Biasing The Diode ( 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- (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
Fig. 1-22b depletion region forward & Fig depletion region reverse

40. dynamic resistance r’d decreases as you move up the curve
1.8 Voltage-Current Characteristic of a Diode ( 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
dynamic resistance r’d decreases as you move up the curve
zero
bias
Fig. 1-26b measurements with meters

41. 1.8 Voltage-Current Characteristic of a Diode ( V-I Characteristic for Reverse bias)
Breakdown voltage
not a normal operation of pn junction devices
the value can be vary for typical Si
Fig. 1-26b measurements with meters
Reverse Current

42. 1.8 Voltage-Current Characteristic of a Diode ( Complete V-I Characteristic curve)
Combine-Forward bias
& Reverse bias  Complete
V-I characteristic curve
Fig. 1-26b measurements with meters

43. 1.8 Voltage-Current Characteristic of a Diode ( Temperature effect on the diode V-I Characteristic)
Forward biased dioed : for a given value of
For a given
Barrier potential decrease as T increase
Reverse current breakdown – small & can be neglected
Fig. 1-26b measurements with meters

44. 1.9 Diode Models ( Diode structure and symbol)
Directional of current
cathode
anod
Fig ideal diode curve

45. 1.9 Diode Models ( 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

46. 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 .7 V or .3 V when forward biased.
Fig 1-38 DMM check w/electrode labels
When checking in reverse bias the full applied testing voltage will be seen on the display.
K A
A K

47. 1.10 Testing A Diodes ( By Digital multimeter)
Fig 1-38 DMM check w/electrode labels
NG DIODE

48. 1.10 Testing A Diodes ( By Analog multimeter – ohm 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

49. 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 .3 V for a Germanium and .7 V for Silicon for conduction to take place.

50. Summary
A diode conducts when forward biased and does not conduct when reverse biased
When reversed biased a diode can only withstand so much applied voltage. The voltage at which avalanche current occurs is called reverse breakdown voltage.