Lecture 1 Introduction to Semiconductor 11123

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Transcript of Lecture 1 Introduction to Semiconductor 11123

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    1

    Chapter 1

    Diode Basics, Application and Special

    Diodes

    WEEK 1

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    2

    Chapter Outline

    1.1 Atomic Structure of Semiconductor

    1.2 Semiconductor Material and PN Junction

    1.3 Diode

    1.4 Diode Characteristics

    1.5 Special Purposes Diodes

    1.6 Diode in DC and AC Circuits

    1.7 Diode Applications

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    3

    Atomic Structure

    All matter is made from atom.

    All atoms consists of electrons (negative charge), protons(positive charge) and neurons (uncharged).

    An atom is the smallest particle of an element.

    Each type of atom has a certain number of electrons andprotons that distinguishes it from the atoms of all otherelements.

    The atomic number equals the number protons in the

    nucleus, which is the same as the number of electrons in anelectrically balanced (neutral) atom. (the positive chargescancel the negative charges and the atom has a net charge ofzero).

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    Atomic Structure

    Energy increases as the distance from the nucleus increases

    (electrons near nucleus have less energy than those in more

    distant orbits).

    These energy level known as shells, and each shell has a

    maximum number of electrons at permissible energy levels.

    There are 7 layer of shell that orbit the nucleus.

    The maximum electron, Ne are determine by, 2 x n2, where n is

    number of layer of shell.

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    Atomic Structure

    The Bohr model of an atom showing electrons in orbits around

    the nucleus, which consists of protons and neutrons

    (Nitrogen configuration)

    electron

    neuron

    proton

    http://education.jlab.org/qa/atom_model_03.gif

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    Atomic Structure

    Electrons that are in orbits farther from nucleus have higher

    energy and are less tightly bound to the atom than those

    closer to the nucleus.

    This is because the force of attraction between the positively

    charged nucleus and the negatively charged electron

    decreases with increasing distance from the nucleus.

    Electrons in the outermost shell have highest energy.

    The outermost shell is known as valence shell and electrons in

    the shell are called valence electrons.

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    Atomic Structure

    Atomic number for Aluminium is 13.

    i. What is electron configuration for Aluminium?

    ii. Draw a diagram of a Aluminium atom.

    iii. How many electrons valence for Aluminium atom?

    +13

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    Atomic Structure

    When an atom absorbs energy, the energies of the electronsare raised.

    The electron valence posses more energy and are moreloosely bound to the atom. So they can easily jump to higher

    orbits within the valence shell when external energy isabsorbed by the atom.

    If a valence electrons acquires a sufficient amount of energy,it can actually escape from the outer shell and atomsinfluence.

    The process of losing a valence electron is known asionization (the atom become positive charge with moreprotons than electrons).

    The escape valence electron is called a free electron.

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    Atomic Structure

    The chemical symbol for hydrogen is H. When a neutral

    hydrogen loses its valence electron and becomes a positive

    ion, it is designated H+.

    What is happened if the hydrogen atom receive electron?

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    Insulators

    An insulator is a material that does not conduct electrical

    current under normal conditions.

    Valence electrons are tightly bound to the atoms, therefore,

    there are very few free electrons in an insulator.

    Examples of insulators are rubber, glass, mica, etc.

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    11

    Conductors

    A conductor is a material that easily conducts electrical

    current.

    The best conductors are single-elements materials (copper,

    silver, gold), which are characterized by atoms with only one

    valence electron very loosely bound to the atom.

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    Semiconductors

    A semiconductor is a material that is between conductors and

    insulators in its ability to conduct electrical current.

    A semiconductor in its pure (intrinsic) state is neither a good

    conductor nor a good insulator.

    The most common single-element semiconductors are silicon,

    germanium and carbon (the single-element semiconductors

    are characterized by atoms with four valence electron).

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    Energy Bands

    When an electron acquires enough additional energy, it can

    leave the valence shell, become free electron, and exist in

    what is known as the conduction band.

    The different in energy between the valence band and the

    conduction band is called an energy gap.

    This is the amount of energy that a valence electron must

    have in order to jump from the valence band to the

    conduction band.

    Once in the conduction band, the electron is free to move

    throughout the material and is not tied to any given atom.

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    Energy Bands

    Valence band

    Conduction band

    Energy

    Valence band

    Conduction band

    Energy

    Valence band

    Conduction band

    Energy

    Overlap

    a) Insulator b) Semiconductor c) Conductor

    Energy diagrams for the three types of materials

    Energy gap

    Energy gap

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    Energy Bands

    The valence electron of metal are held loosely by all atom and

    free to move about. This electron holds the positive ions of

    the metal together, forming metallic bonding.

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    Comparison Between Conductor,

    Semiconductor and Insulator

    Characteristic Conductor Insulator Semiconductor

    Resistance

    Low resistance for

    easy current flow.

    High resistance so

    current cannot

    flow.

    Between

    conductor and

    insulator.

    Valence

    electron

    Atom is tend to

    release valence

    electron and it

    flow freely from

    one atom to

    another.

    Atom is tend to

    absorb valence

    electron to valence

    layer to make it

    stable and try

    avoid electrical of

    chemical activity.

    Difficult to free or

    accept valence

    electron from other

    atom.

    Energy band

    Conduction and

    valence band

    overlap, electron

    easily move

    The energy gap is

    big, so electron

    cannot easily

    move.

    Between

    conductor and

    insulator.

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    Silicon and Germanium

    Silicon is the most widely used material in diodes,

    transistors, integrated circuits and other semiconductors

    devices.

    Both silicon and germanium have the characteristic of four

    valence electrons.

    Silicon atom

    Germanium atom

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    Silicon and Germanium

    The valence electrons in germanium are in fourth shell while

    in silicon in the third shell, closer to the nucleus.

    Germanium valence electrons are at higher energy level than

    those in silicon (require a small amount of additional energy

    to escape from the atom).

    This property makes germanium more unstable at high

    temperature.

    This is why silicon is a more widely used as a semi-conductive

    material.

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    Covalent Bonds

    Atom with 4 valence electron is not stable and it try tocomplete it from 4 to 8 by sharing valence electron with other

    atom.

    The sharing valence electron is called covalent bond.

    This will make the atom is stable and the bond is strong. At absolute zero temperature all electron is in valence band

    and it will act as insulator.

    At room temperature many electron have sufficient energy to

    move to conduction band and it will act as conductor. Silicon absorb more heat than germanium before it act as

    conductor, so its more popular.

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    Silicon Covalent Bonds

    Figure below shows how each silicon atom positions itself withfour adjacent silicon atoms to form a silicon crystal.

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    Silicon Covalent Bonds

    This effectively creates eight shared valence electrons for

    each atom and produces a state of chemical stability.

    The centre silicon atom shares an electrons with each of four

    surrounding silicon atoms, creating a covalent bond with

    each. The surrounding atom are in turn bonded to other

    atoms and so on.

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    Current In Semiconductors-Conduction Electrons and Holes-

    When an intrinsic (pure) silicon crystal covalent bondinterrupted by heat, temperature, doping electron will releasefrom the atom bond and jump to conduction band becomingfree electrons.

    Free electrons are also called conduction electrons. Electronis negative charge and is called negative current carrier.

    A vacancy is left in the valence band and it is called hole(itspositive charge). It also called positive current carrier.

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    Hole

    Electron

    Heat energy

    For every electron free, there is one hole left in valence band

    and create electron-hole pair. Recombination occurs when a

    conduction band lost energy.

    Current In Semiconductors-Conduction Electrons and Holes-

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    Current In Semiconductors-Electrons and Holes Current-

    Electron current in intrinsic silicon is produced by the movement of

    thermally generated free electrons

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    Current In Semiconductors-Electrons and Holes Current-

    When a voltage is applied across a piece of intrinsic silicon,

    free electrons are generated in the conduction band (easily

    attracted toward the positive end).

    The movement of these free electrons is one type of current

    in a semiconductive materials, called electron current.

    Another type of current occurs in the valence band, where

    the holes created by the free electrons exist.

    Electrons remaining in the valence band are still attached to

    their atoms and are not free to move randomly as the free

    electrons.

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    Current In Semiconductors-Electrons and Holes Current-

    However, a valence electron can move into a nearby hole with

    little change in its energy electrons, thus leaving another hole

    where it came from.

    Effectively the hole has moved from one place to another in

    the crystal structure.

    This is called hole current even though the current in the

    valence band is produced by valence electrons.

    When a valence electron moves left to right to fill a hole while

    leaving another hole behind, the hole has effectively move

    from right to left.

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    N-Type and P-Type Semiconductor

    Doping is a process which impurity atoms are introduces to

    intrinsic semiconductor in order to alter the balance between

    holes and electron.

    There are two categories of impurities : n-type and p-type

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    N-Type semiconductor

    To increase the number of conduction-band electrons in

    intrinsic silicon, pentavalent atom (atoms with 5 valence

    electrons such as arsenic (As), antimony (Sb)) are added.

    Pentavalent impurity atom in a silicon crystal structure. An antimony (Sb)

    impurity atom is shown in the centre. The extra electron from the Sb atom

    becomes a free electron.

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    N-Type semiconductor

    Each pentavalent atom (antimony) forms covalent bond withfour adjacent silicon atoms.

    Four of the antimony atoms valence electron are used toform the covalent bonds with silicon atoms, leaving one extraelectron (this is conduction electron because not involved in

    bonding). The pentavalent atom also called as donor atom.

    The number of conduction electrons can be controlled by thenumber of pentavalent impurity atoms added to the silicon.

    The electrons are called the majority carriers in n-typematerials.

    There are also a few holes created when electron-hole pairsare thermally generated and holes in n-type material arecalled minority carriers.

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    P-Type semiconductor

    To increase the number of holes in intrinsic silicon, trivalent

    atom (atoms with 3 valence electrons such as boron (B),

    indium (In)) are added.

    Trivalent impurity atom in a silicon crystal structure. A boron (B)

    impurity atom is shown in the centre.

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    P-Type semiconductor

    Each trivalent atom (boron) forms covalent bond with fouradjacent silicon atoms.

    3 of the boron atoms valence electron are used in thecovalent bonds and since 4 electrons are required, a holeresults when each trivalent atom is added.

    The trivalent atom also called as acceptor atom (trivalentatom can take an electron).

    The number of holes can be controlled by the number oftrivalent impurity atoms added to the silicon.

    The holes are called the majority carriers in p-type materials.

    There are also a few conduction-band electrons created whenelectron-hole pairs are thermally generated and electrons inp-type material are called minority carriers.