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

SemiconductorsSemiconductorsOverview: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 semiconductors P-type semiconductors

Doping Doping N-type doping N-type doping P-type dopingP-type doping

Biasing Biasing Forward biasing Forward biasing Reverse biasing Reverse biasing

DiodesDiodes Role of semiconductors in electronics engineering Role of semiconductors in electronics engineering Applications Applications Importance Importance Conclusion Conclusion

Introduction to semiconductorsIntroduction to semiconductors

SemiconductorsSemiconductors are materials that have properties in between are materials that have properties in between normal normal conductorsconductors (materials that allow electric current to pass, e.g. aluminium) (materials that allow electric current to pass, e.g. aluminium) and and 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 are First, there are intrinsicintrinsic semiconductors semiconductors

These are composed of only one kind of material. Silicon and germanium are two These 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 to Which are made of intrinsic semiconductors that have had other substances added to them to alter their properties.them to alter their properties.

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 charged A 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

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 atom Hole

Majority and minority carriersMajority and minority carriers

Electrons Electrons areare• MajorityMajority carriers in carriers in NN-type semiconductor-type semiconductor• Minority Minority carriers in carriers in PP-type semiconductor-type semiconductor

Holes are

• Majority carriers in P-type semiconductor• Minority carriers in N-type semiconductor

DopingDoping

The conductivity of semiconductors may easily be modified by introducing impurities The conductivity of semiconductors may easily be modified by introducing impurities into their into their crystal latticecrystal lattice..

The process of adding controlled impurities to a semiconductor is known as The process of adding controlled impurities to a semiconductor is known as dopingdoping..

The amount of impurity, or dopant, added to an The 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 as Doped semiconductors are referred to as extrinsicextrinsic..

By adding impurity to pure semiconductors, the electrical conductivity may be varied By adding impurity to pure semiconductors, the electrical conductivity may be varied by factors of thousands or millions.by factors of thousands or millions.

Types of Doping Types of Doping

n-dopingn-doping The 5-valent dopant has an outer electron more than the silicon atoms. Four outer The 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 move electrons 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 lifted and 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 the from 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 an intrinsic 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

P-Type doping :P-Type doping : In contrast to the free electron due to doping with phosphorus, the 3-valent dopant In 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 outer effect 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 the electron, 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 opposite electrons 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 electron direction 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 needed into 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

Biasing Biasing

.. Forward bias:Forward bias:

In forward bias condition, higher or positive potential is applied at the anode and In 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 while The 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 barrier The 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 ‘Cut The 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 majority The 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 the charge currents that is the hole current in the p-region and the electron current in the n-region.n-region.

..            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’.

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. 

  

Characteristics of DiodeCharacteristics of DiodeDiode 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

I-V Characteristics of Practical Diode I-V Characteristics of Practical Diode

ApplicationsApplicationsAmorphous 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   

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.

The conversation of solar light to electric power is available renewaable sources of The 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 the The 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 holes creation 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.

THE THE

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