Semiconductors

1. Semi-conductorsSemi-conductors
Presented by:Presented by:
1.Rida Imtiaz1.Rida Imtiaz (Roll No:2172 )(Roll No:2172 )
2.Ishrat2.Ishrat (Roll No:2160 )(Roll No:2160 )
3.Kanza3.Kanza (Roll No:2165 )(Roll No:2165 )
Presented to:Presented to:
Miss xyzMiss xyz
Batch : FET 13, BS Electronics EngineeringBatch : FET 13, BS Electronics Engineering
International Islamic University Islamabad ,IIUIInternational Islamic University Islamabad ,IIUI

2. SemiconductorsSemiconductors
Overview:Overview:
Following are the topics which will be covered in this presentation.Following are the topics which will be covered in this presentation.
 Introduction to semiconductorsIntroduction to semiconductors
 Types of semiconductorsTypes of semiconductors
 N-type semiconductorsN-type semiconductors
 P-type semiconductorsP-type semiconductors
 DopingDoping
 N-type dopingN-type doping
 P-type dopingP-type doping
 BiasingBiasing
 Forward biasingForward biasing
 Reverse biasingReverse biasing
 DiodesDiodes
 Role of semiconductors in electronics engineeringRole of semiconductors in electronics engineering
 ApplicationsApplications
 ImportanceImportance
 ConclusionConclusion

3. Introduction to semiconductorsIntroduction to semiconductors
 SemiconductorsSemiconductors are materials that have properties in betweenare materials that have properties in between
normalnormal conductorsconductors (materials that allow electric current to pass, e.g. aluminium)(materials that allow electric current to pass, e.g. aluminium)
andand insulatorsinsulators (which block electric current, e.g. sulphur).(which block electric current, e.g. sulphur).
 Semiconductors fall into two broad categories.Semiconductors fall into two broad categories.
1.1. First, there areFirst, there are intrinsicintrinsic semiconductorssemiconductors
 These are composed of only one kind of material. Silicon and germanium are twoThese are composed of only one kind of material. Silicon and germanium are two
examplesexamples
 They are also called "undoped semiconductors" or "i-type semiconductors".They are also called "undoped semiconductors" or "i-type semiconductors".
2-2-ExtrinsicExtrinsic semiconductors.semiconductors.
 Which are made of intrinsic semiconductors that have had other substances added toWhich are made of intrinsic semiconductors that have had other substances added to
them to alter their properties.them to alter their properties.

4. Types of semiconductorsTypes of semiconductors
There are two types of semiconductorsThere are two types of semiconductors
 N-type semiconductorsN-type semiconductors
 A N-type material is one in which electrons are majority charge carriers i.e. they are negatively chargedA N-type material is one in which electrons are majority charge carriers i.e. they are negatively charged
materials (-----)materials (-----)
N-type has many free electrons in conduction band and few holes In valence band
Free Electron
15P
14P
14P 14P
14P
Phosphorous atom

5.  P-type semiconductors:P-type semiconductors:
 A P-type material is one in which holes are majority carriers i.e. they are positively charged materials (+++A P-type material is one in which holes are majority carriers i.e. they are positively charged materials (+++
+)+)
P-type has few free electrons in conduction band and many holes In valence
band
13P
14P
14P 14P
14P
Aluminum atomHole

6. Majority and minority carriersMajority and minority carriers
ElectronsElectrons areare
• MajorityMajority carriers incarriers in NN-type semiconductor-type semiconductor
• MinorityMinority carriers incarriers in PP-type semiconductor-type semiconductor
Holes are
• Majority carriers in P-type semiconductor
• Minority carriers in N-type semiconductor

7. DopingDoping
 The conductivity of semiconductors may easily be modified by introducing impuritiesThe conductivity of semiconductors may easily be modified by introducing impurities
into theirinto their crystal latticecrystal lattice..
 The process of adding controlled impurities to a semiconductor is known asThe process of adding controlled impurities to a semiconductor is known as dopingdoping..
 The amount of impurity, or dopant, added to anThe amount of impurity, or dopant, added to an intrinsicintrinsic (pure) semiconductor varies(pure) semiconductor varies
its level of conductivity.its level of conductivity.
 Doped semiconductors are referred to asDoped semiconductors are referred to as extrinsicextrinsic..
 By adding impurity to pure semiconductors, the electrical conductivity may be variedBy adding impurity to pure semiconductors, the electrical conductivity may be varied
by factors of thousands or millions.by factors of thousands or millions.

8. Types of DopingTypes of Doping
 n-dopingn-doping
 The 5-valent dopant has an outer electron more than the silicon atoms. Four outerThe 5-valent dopant has an outer electron more than the silicon atoms. Four outer
electrons combine with ever one silicon atom, while the fifth electron is free to moveelectrons combine with ever one silicon atom, while the fifth electron is free to move
and serves as charge carrier. This free electron requires much less energy to be liftedand serves as charge carrier. This free electron requires much less energy to be lifted
from the valence band into the conduction band, than the electrons which cause thefrom the valence band into the conduction band, than the electrons which cause the
intrinsic conductivity of silicon. The dopant, which emits an electron, is known as anintrinsic conductivity of silicon. The dopant, which emits an electron, is known as an
electron donor (donare, lat. = to give).electron donor (donare, lat. = to give).
 n-doping with phosphorusn-doping with phosphorus

9.  P-Type doping :P-Type doping :
 In contrast to the free electron due to doping with phosphorus, the 3-valent dopantIn contrast to the free electron due to doping with phosphorus, the 3-valent dopant
effect is exactly the opposite. The 3-valent dopants can catch an additional outereffect is exactly the opposite. The 3-valent dopants can catch an additional outer
electron, thus leaving a hole in the valence band of silicon atoms. Therefore theelectron, thus leaving a hole in the valence band of silicon atoms. Therefore the
electrons in the valence band become mobile. The holes move in the oppositeelectrons in the valence band become mobile. The holes move in the opposite
direction to the movement of the electrons. The necessary energy to lift an electrondirection to the movement of the electrons. The necessary energy to lift an electron
into the energy level of indium as a dopant, is only 1 % of the energy which is neededinto the energy level of indium as a dopant, is only 1 % of the energy which is needed
to raise a valence electron of silicon into the conduction band.to raise a valence electron of silicon into the conduction band.
 p-doping with boronp-doping with boron

10. BiasingBiasing
.. Forward bias:Forward bias:
In forward bias condition, higher or positive potential is applied at the anode andIn forward bias condition, higher or positive potential is applied at the anode and
negative or lower potential is applied at the cathode of a diode.negative or lower potential is applied at the cathode of a diode.
The positive potential at anode repels the holes in p-region towards n-region whileThe positive potential at anode repels the holes in p-region towards n-region while
negative potential at the cathode repels electrons in n-region towards p-region.negative potential at the cathode repels electrons in n-region towards p-region.
Thus, the height of the potential barrier reduces.Thus, the height of the potential barrier reduces.
The depletion region disappears when the applied voltage equals to the potential barrierThe depletion region disappears when the applied voltage equals to the potential barrier
and a large current flows through the diode.and a large current flows through the diode.
The voltage required to drive the diode into a state of conduction is called as the ‘CutThe voltage required to drive the diode into a state of conduction is called as the ‘Cut
in/Offset/Threshold/Firing voltage’.in/Offset/Threshold/Firing voltage’.
The current is of considerable magnitude as it is dominantly constituted by the majorityThe current is of considerable magnitude as it is dominantly constituted by the majority
charge currents that is the hole current in the p-region and the electron current in thecharge currents that is the hole current in the p-region and the electron current in the
n-region.n-region.

11. .. Reverse Bias:Reverse Bias:
In reverse bias condition, the higher or positive potential is applied at the cathode and negative In reverse bias condition, the higher or positive potential is applied at the cathode and negative
or lower potential is applied at the anode. or lower potential is applied at the anode.
The negative potential at anode attracts the holes in p-region that are away from the n-region The negative potential at anode attracts the holes in p-region that are away from the n-region
while positive potential at the cathode attracts electrons in n-region that are away from the p-while positive potential at the cathode attracts electrons in n-region that are away from the p-
region.region.
The applied voltage increases the height of the potential barrier.The applied voltage increases the height of the potential barrier.
The current flows dominantly due to the minority charge currents that is the electron current in The current flows dominantly due to the minority charge currents that is the electron current in
p-region and the hole current in n-region. p-region and the hole current in n-region.
Thus a constant current of negligible magnitude flows in the reverse direction which is called as Thus a constant current of negligible magnitude flows in the reverse direction which is called as
the ‘Reverse saturation current’.the ‘Reverse saturation current’.

12. DiodesDiodes
What is a Diode?What is a Diode?
A A DiodeDiode is the simplest two-terminal unilateral semiconductor device. is the simplest two-terminal unilateral semiconductor device.
It allows current to flow only in one direction and blocks the current that flows in It allows current to flow only in one direction and blocks the current that flows in
the opposite direction.the opposite direction.
The two terminals of the diode are called as anode and cathode. The two terminals of the diode are called as anode and cathode.
The The symbol of diodesymbol of diode is as shown in the figure below. is as shown in the figure below.

13. Characteristics of DiodeCharacteristics of Diode
Diode always conducts in one direction.Diode always conducts in one direction.
Diodes always conduct current when “Forward Biased” ( Zero resistance)Diodes always conduct current when “Forward Biased” ( Zero resistance)
Diodes do not conduct when Reverse BiasedDiodes do not conduct when Reverse Biased
(Infinite resistance)(Infinite resistance)
I-V characteristics of Ideal diodeI-V characteristics of Ideal diode

14. I-V Characteristics of Practical DiodeI-V Characteristics of Practical Diode

15. ApplicationsApplications
Amorphous semiconductors are promising electronic materials for wide range Amorphous semiconductors are promising electronic materials for wide range
of applications such as:of applications such as:
 Solar cellSolar cell
 Thin film transistors (TFT)Thin film transistors (TFT)
 Light sensorsLight sensors
 Optical memory devicesOptical memory devices
 Electro photographic applicationElectro photographic application
 X-ray image sensorsX-ray image sensors
 Eu-doped optical fiberEu-doped optical fiber
 DVD (digital video/versatile disc) DVD (digital video/versatile disc)
 Hard cover made from ta-C Hard cover made from ta-C

16. Electro photographic application:
one of the most common, everyday used application is electro photography or xerography
(Greek word, meaning is “dry writing”).
The first xerography was made by Carlson and Kornei in 1938(!) in Astoria NY (USA).
The really first experiment was made using sulfur, but later on Se was the basic
material. Recently a-Si:H films have been utilized instead.
Solar cells:
Potentially the most important application of the amorphous semiconductors a-Si:H is in
the direct conversion of sunlight to electric power.
This is a cheaper raw material than crystalline silicon. No structural damage!
For example: space shuttle use.

17.  The conversation of solar light to electric power is available renewaable sources ofThe conversation of solar light to electric power is available renewaable sources of
energies.energies.
 The basic physical principle involved is the absorption of photon resulting in theThe basic physical principle involved is the absorption of photon resulting in the
creation of electron-hole pairs; the excess electrons in the conduction band, and holescreation of electron-hole pairs; the excess electrons in the conduction band, and holes
in the valence band.in the valence band.
 Internal junction field separates them before recombination.Internal junction field separates them before recombination.

18. THETHE
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