introduction to electronics

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1Introduction To Electronics BEB 11103

Topic 1:Introduction To

Semiconductor Diodes

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Lecture 1

http://sites.google.com/site/amirunikl/[email protected]. Mohd Amir Abas

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Topic Outline

Atomic StructureElectron Shells and Orbits Semiconductors, Conductors, InsulatorComparison of a Semiconductor AtomCovalent BondsN-Type and P-Type Semiconductors

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Atomic Structure

All matter is made of atoms; and all atoms are made of electrons, protons and neutron.

An atom is the smallest particles of an element.Bohr Model -Atom has planetary type of structure that consists of nucleus (positively charge particles called proton and uncharged particles called neutron) and surrounded by orbiting electron (negatively charged particles).Each atom has certain number of electrons and protons.

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Atomic Number

The atomic number = the number of the protons in the nucleus= the number of the electrons in neutral state.

Bohr Model

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Atomic Number

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Slide 3 Electron Shells and Orbits

Energy LevelsIn atom, the orbits are group into energy bands known as shells. Each shell has a fixed maximum number of electrons.

The shells are designated as K, L, M, N and so on.

Energy levels increase as the distance from the nucleus increase.

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ELECTRON SHELLS AND ORBITS

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Valence Electrons

Electron that are in orbits farther from the nucleus have higher energy and are less tightly bound to the atom than those closer to the nucleus.

The outermost shell is known as the valence shell and the electrons in this shell are called valence electron , which contributing to chemical reactions to determine its electrical properties.

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Ionization

When an atom absorbs energy from heat or light, the energy level of the electrons are raised and more loosely bound to atom than inner electrons.

If valence electron acquires sufficient energy, it can escape from the outer shell.

The atom now is excess pf positive charge. Process of loosing valence electron known as ionization, i.e resulting positively charged atom so called positive ion. The escape electron is called free electron.

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The Number Of Electrons in Each Shell

The maximum number of electrons (Ne) that can exist in each shell of an atom can be calculated by the formula

Ne = 2n2

Where n is the number of shell. The innermost (K) shell is number 1, the L shell is number 2, the M shell is number 3, and so on.

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Example:

The maximum number of electrons that can exist in the innermost shell is

Ne = 2n2 = 2(1)2 = 2

The maximum number of electrons that can exist in the second shell is

Ne = 2n2 = 2(2)2 = 8

The maximum number of electrons that can exist in the third shell is

Ne = 2n2 = 2(3)2 = 18

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Number Of Electrons in Each Shell

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Materials classification

Conductor, Semiconductor, and Insulator.

(based on their electrical properties)

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Materials classification

ConductorIs a material that easily conducts electrical current such as copper, silver, gold and aluminum.They have only one valence electron and very loosely bound to the atom.These electron will be the free electrons and when they are moving in the same direction, produce the current.

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InsulatorIs a material that does not conduct electrical current under normal conditions because valence electrons are tightly bound to atoms. no free electrons in an insulator.

SemiconductorA semiconductor in its pure (intrinsic) state is neither a good conductor nor a good insulator such as silicon, germanium and carbon. They have 4 valence electrons.

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Energy BandsAn electron in the valence shell is said to have energy corresponding to the valence band of energy or simply valence band.

However, as a result of acquiring a specific amount of additional energy, an electron in the valence shell becomes free of the nucleus, with its new energy, an electron is characterized as being in conduction band of energy or simply, conduction band.

Differentiation among materials can be made on the basis of the amount of energy needed to liberate a single valence electron from the influence of the nucleus.

The amount of energy between the highest energy in the valence band, Ev, and the lowest energy in the conduction band, Ec, is a characteristic of the material and is called the energy gap, Eg.

Thus Eg.= Ec.- Ev

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Energy Bands

The difference energy between the valence band and the conduction band is called an energy gap.

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Semiconductor

The principle semiconductor material used in electronics is silicon (Si) with Germanium (Ge) as secondary material. An atom of pure (intrinsic) silicon has 14 electrons, and its electron shell configuration is 2, 8, 4. The third shell is incomplete. Each silicon atom shares electron with each of its four nearest neighbors, this unique sharing is known as covalent bonding.

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Atomic structure: (a) germanium; (b) silicon.

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COVALENT BOND

The atoms within the crystal structure are held together by covalent bond (interaction of the valence electron of the atom).

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Covalent bonds in a three-dimensional silicon crystal.

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Doping

The electrical characteristics of Silicon and Germanium are improved by adding materials (impurities) in a process called doping.

The additional materials are in two types:• n-type• p-type

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N – Type Semiconductors

N-type semiconductors are formed by adding small amounts of pentavalent impurities to the intrinsic semiconductor crystal.Typically, such impurities are chemical elements of phosphorus (P), arsenic (As), bismuth (Bi) and antimony (Sb). These are atoms with five (5) valence electrons.Each impurity atom contributes one electron to each of four covalent bonds, and each has one excess electron that is not taking part in a covalent bond.This excess electron is weakly bound to the core and usually hasenough energy to be considered as free electron.Pentavalent impurities are called donor atoms which increase the free electrons population in a semiconductor.Therefore, the essential characteristic of N-type semiconductor is that electrons are more.

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N-type semiconductor

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P – Type Semiconductors

P-type semiconductors are formed by adding small amounts of trivalent impurities to the intrinsic semiconductor crystal.Typically, such impurities are chemical elements of boron (B), gallium (Ga) and indium (In). These are atoms with three (3) valence electrons.The trivalent atoms cannot satisfy all four covalent bounds around them. One covalent bond near each of P-type impurity atoms is incomplete. It introduce hole. “The absence of an electron in a covalent bond is known as hole”.Trivalent impurities are called acceptor atoms because each can accept one electron into its incomplete bond.Therefore, the essential characteristic of P-type semiconductor is that holes are more and they are termed majority carries.

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27P-type semiconductor

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n-type materials make the Silicon (or Germanium) atoms more negative.p-type materials make the Silicon (or Germanium) atoms more positive.

Join n-type and p-type doped Silicon (or Germanium) to form a p-n junction.

n-type versus p-type

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p-n junction

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p-n junction

•When the materials are joined, the negatively charged atoms of the n-type doped side are attracted to the positively charged atoms of the p-type doped side.

•The electrons in the n-type material migrate/diffuse across the junction to the p-type material (electron flow).

•The loses of electrons in n-type region creates a layer of positive charges near the n-junction. The excessive of electrons creates a layer of negative charges near the p-junction.

•The result is the formation of a depletion layer around the junction.

•This region has expanded to a point where equilibrium is established and there is no further diffusion of electrons across the junction. Thus the depletion region acts as barrier to avoid further movement of electrons across the junction.

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p-n junction

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Slide 16 Barrier Potential

The forces between the opposite charges form a “field of force”called electric field.This electric field is a barrier to the free electrons in the n-type region.The electric field is also called barrier potential and its expresses in volts.The typical barrier potential for silicon is 0.7V and for germanium is 0.3V at 25˚ C.

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Slide 17 Operating Conditions of pn-junction

• No Bias

• Forward Bias

• Reverse Bias

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Slide 18

No external voltage is applied: VD = 0V and no current is flowing ID = 0A.

Only a modest depletion layer exists.

No Bias Condition

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Forward Bias Condition

External voltage is applied across the p-n junction in the same polarity of the p- and n-type materials.

The negative side of the Vbias " pushes " the free electrons toward the PN junction.

The free electrons have sufficient energy to overcome the barrier potential of the depletion region and move on through into the P region.As more electrons flow into the depletion region, the number of positive ions is reduced. Causes the depletion region to narrow.

Thus permits the current to flow through the diode.

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Reverse Bias Condition

External voltage is applied across the p-n junction in the opposite polarity of the p- and n-type materials.

• The electrons in the n-type material are attracted towards the positive terminal and the ‘holes’ in the p-type material are attracted towards the negative terminal. Causes the depletion region to widen.

• Thus prevent the current to flow through the diode.

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REVERSE CURRENT

The extremely small current that exists in reverse bias after the transition current dies out is caused by the minority carriers in the N region and P regions that are produced by thermally generated.The minority electrons in P region are pushed toward the PN junction and fall down the energy hill and combine with minority holes as valences electrons and flow toward the positive bias voltage, creating a small current.

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REVERSE BREAKDOWN

If the external voltage is increased to a value called the breakdown voltage, the reverse current will drastically increase.The high reverse voltage imparts energy to the free minority electrons so that as they speed through the P region, they collide with atoms with enough energy to knock valence electrons out of orbit and into the conduction band.This results in a very high reverse current that can damage the PN structure.

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I-V CHARACTERISTIC FOR FORWARD BIAS