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SEMICONDUCTOR PHYSICS

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SEMICONDUCTOR PHYSICS Powered By Docstoc
					  ECE, ME – I SEM


SEMICONDUCTOR
  PHYSICS AND
  TRANSISTORS
 WHAT IS A SEMICONDUCTOR?




•Many materials, such as most metals, allow electrical current to flow
through them
     •These are known as conductors
•Materials that do not allow electrical current to flow through them are
called insulators
•Pure silicon, the base material of most transistors, is considered a
semiconductor because its conductivity can be modulated by the
introduction of impurities
SEMICONDUCTORS, INSULATORS AND
METALS

           The electrical properties of metals and
           insulators are well known to all of us.

           Everyday experience        has already
           taught us a lot about the electrical
           properties of metals and insulators.

           But the same cannot be said about
           “semiconductors”.

            What happens when we connect a
           battery to a piece of a silicon;
               would it conduct well ? or
               would it act like an insulator ?
THE NAME “SEMICONDUCTOR” IMPLIES THAT IT CONDUCTS
SOMEWHERE BETWEEN THE TWO CASES (CONDUCTORS OR
INSULATORS)

              
 Conductivity :



 σmetals ~1010 /Ω-cm              The conductivity (σ) of
                                  a
              S/C
                                  semiconductor (S/C)
 σinsulators ~ 10   -22   /Ω-cm   lies
                                  between these two
                                  extreme cases.
CONDUCTORS AND SEMICONDUCTORS
 I                                             I  0

           VB                                             VB

        Conductor                                      I ntrinsic Silicon
     Holes (Ben Franklin)                                      Insulator
            Electrons


                            Bar of metal                                   Bar of silicon
 I                                              I

         VB                                               VB

     p-Type Silicon                                    n-Type Silicon
       Hole Conduction                                 Electron Conduction




                              Semiconductors
SEMICONDUCTORS

   A material whose properties are such that it is not
    quite a conductor, not quite an insulator
   Some common semiconductors
       elemental
           Si - Silicon (most common)
           Ge - Germanium
       compound
           GaAs - Gallium arsenide
           GaP - Gallium phosphide
           AlAs - Aluminum arsenide
           AlP - Aluminum phosphide
           InP - Indium Phosphide
  CRYSTAL STRUCTURE OF SILICON
Covalent Bond:                      View along a crystallographic
Two atoms Share
two electrons                       axis of silicon




Silicon atom: Four electrons/atom
CRYSTALLINE SOLIDS


   In a crystalline solid, the periodic arrangement of
    atoms is repeated over the entire crystal
   Silicon crystal has a diamond lattice
    CRYSTALLINE NATURE OF SILICON


   Silicon as utilized in integrated circuits is crystalline in nature
   As with all crystalline material, silicon consists of a repeating
    basic unit structure called a unit cell
   For silicon, the unit cell consists of an atom surrounded by four
    equidistant nearest neighbors which lie at the corners of the
    tetrahedron
WHAT’S SO SPECIAL ABOUT SILICON?
                     Cheap and abundant
                     Amazing mechanical, chemical and electronic
                     properties
                     The material is very well-known to mankind
                         SiO2: sand, glass




                     Si is column IV of the
                     periodic table
                     Similar to the carbon
                     (C) and the
                     germanium (Ge)
                     Has 3s² and 3p²
                     valence electrons
NATURE OF INTRINSIC SILICON


       Silicon that is free of doping impurities is
        called intrinsic
       Silicon has a valence of 4 and forms covalent
        bonds with four other neighboring silicon
        atoms
INTRINSIC SILICON
                             Temperature effects:
                             • At absolute zero all electrons are
                             bonded to neighbors. There is none
                             available for conduction.
                                  • Insulator
                             • At higher temperatures electrons
                             gain enough energy to escape the
                             bonds.
                                  • Some conduction but still
                                  basically an insulator




 Silicon: 4 electrons/atom
    PHOSPHORUS (N-TYPE) DOPED
    SILICON
• Replace silicon atom by a phosphorus atom which had five electrons
rather than four (silicon). One impurity atom for 106 to 1010 silicon atoms.
• Extra electron is free to move in crystal allowing conduction



Phosphorus
atom has net
(+) charge                                                   Extra electron (-)
which is fixed                                               which is mobile
in position



Phosphorous
atom
SIMPLIFIED REPRESENTATION OF N-TYPE
SILICON
• Charges are balanced: Number of (+) charges equals number of (-)charges

                        Charges due to one
                        impurity atom
                                                               Shows only impurity
                                                               atoms




 • Net positive charge of each             • Extra electron (- charge) for each
 phosphorus atom                           phosphorus atom
 • Fixed position in crystal               • Mobile: Moves around crystal due to
                                           E-field and diffusion
     p-Type Silicon
• Example with Boron doping atoms which has three electrons/atom.


                                Charges due to one
                                impurity atom




• Net negative charge of each             • Extra hole (+ charge) for each boron
boron atom                                atom
• Fixed position in lattice               • Mobile: Moves around crystal due to
                                          E-field and diffusion
MOTION OF CHARGES
Mobile charges move due to two effects:
  • Forces due to electric field: Force = charge x Electric Field
       • The resulting current is called “drift current”
  • Diffusion due to gradient in the charge density:
       • Charges move to be evenly distributed throughout space
            • Similar to perfume in room or heat in a solid.
       • The resulting current is called “diffusion current”
ELECTRICAL CONDUCTIVITY
  in order of conductivity: superconductors, conductors,
   semiconductors, insulators
  some representative resistivities ():
        R = L/A, R = resistance, L = length, A = cross section area; resistivity at 20o C
                                                                             resistivity in 
         m resistance(in )(L=1m, diam =1mm)
           aluminum 2.8x10-8               3.6x10-2
           brass                  8x10-8              10.1x10-2
           copper                 1.7x10-8             2.2x10-2
           platinum 10x10                  12.7x10-2
                             -8

           silver                 1.6x10-8              2.1x10-2
           carbon                 3.5x10-5             44.5
           germanium              0.45                 5.7x105
           silicon                 640                 6x108
           porcelain  1010 - 1012          1016 - 1018
           teflon                 1014                 1020
           blood                  1.5                  1.9x106
           fat                    24                   3x107
SEMICONDUCTOR CRYSTALLINE STRUCTURE
   Semiconductors have a regular
    crystalline structure
      for monocrystal, extends
       through entire structure
      for polycrystal, structure is
       interrupted at irregular
       boundaries
   Monocrystal has uniform 3-
    dimensional structure
   Atoms occupy fixed positions
    relative to one another, but
    are in constant vibration about
    equilibrium
SEMICONDUCTOR CRYSTALLINE STRUCTURE
   Silicon atoms have 4
    electrons in outer shell
        inner electrons are very
         closely bound to atom
   These      electrons are
    shared with neighbor
    atoms on both sides to
    “fill” the shell
      resulting structure is
       very stable
      electrons   are   fairly
       tightly bound
            no “loose” electrons
        at room temperature, if
         battery applied, very
         little electric current
         flows
    CONDUCTION IN CRYSTAL LATTICES
   Semiconductors (Si and Ge) have 4 electrons in their
    outer shell
      2 in the s subshell
      2 in the p subshell
   As the distance between atoms decreases the discrete
    subshells spread out into bands
   As the distance decreases further, the bands overlap
    and then separate
      the subshell model doesn’t hold anymore, and the electrons
       can be thought of as being part of the crystal, not part of the
       atom
      4 possible electrons in the lower band (valence band)
      4 possible electrons in the upper band (conduction band)
ENERGY BANDS IN SOLIDS:
   In solid materials, electron energy levels form bands of allowed energies,
    separated by forbidden bands
   valence band = outermost (highest) band filled with electrons (“filled” = all
    states occupied)
   conduction band = next highest band to valence band (empty or partly filled)
   “gap” = energy difference between valence and conduction bands, = width of
    the forbidden band
   Note:
      electrons in a completely filled band cannot move, since all states occupied
       (Pauli principle); only way to move would be to “jump” into next higher
       band - needs energy;
      electrons in partly filled band can move, since there are free states to move
       to.
   Classification of solids into three types, according to their band structure:
      insulators: gap = forbidden region between highest filled band (valence

       band) and lowest empty or partly filled band (conduction band) is very wide,
       about 3 to 6 eV;
      semiconductors: gap is small - about 0.1 to 1 eV;

      conductors: valence band only partially filled, or (if it is filled), the next

       allowed empty band overlaps with it
    .:: THE BAND THEORY OF SOLIDS ::.

                                             The electrons surrounding
                                Allowed       a nucleus have certain
                                 band         well-defined energy-levels.

                    Forbidden
                      band                   Electrons don’t like to have
                                              the same energy in the
                                Allowed       same potential system.
                                 band

                    Forbidden
                                             The most we could get
                       band                   together in the same
                                              energy-level   was   two,
                                Allowed       provided thet they had
                                 band         opposite spins. This is
                                              called   Pauli  Exclusion
1   2      4………………N
                                              Principle.
        Number of   atoms
   The difference in energy between each of these
    smaller levels is so tiny that it is more reasonable to
    consider each of these sets of smaller energy-levels as
    being continuous bands of energy, rather than
    considering the enormous number of discrete
    individual levels.

   Each allowed band is seperated from another one by
    a forbidden band.

   Electrons can be found in allowed bands but they can
    not be found in forbidden bands.
.:: SEMICONDUCTOR, INSULATORS, CONDUCTORS ::.




      Full band                            Empty band




All energy levels are               All energy levels are empty
occupied by electrons                    ( no electrons)




 Both full and empty bands do not partake in electrical conduction.
ENERGY BANDS IN SEMICONDUCTORS

   The space
    between the
    bands is the
    energy gap, or
    forbidden band
    ENERGY-BAND DIAGRAM
   A very important concept in the study of semiconductors
    is the energy-band diagram
   It is used to represent the range of energy a valence
    electron can have
   For semiconductors the electrons can have any one value
    of a continuous range of energy levels while they occupy
    the valence shell of the atom
       That band of energy levels is called the valence band
   Within the same valence shell, but at a slightly higher
    energy level, is yet another band of continuously variable,
    allowed energy levels
       This is the conduction band
    BAND GAP
   Between the valence and the conduction band is a range of
    energy levels where there are no allowed states for an electron
   This is the band gap E G
    In silicon at room temperature [in electron volts]:     EG  1.1eV
   Electron volt is an atomic measurement unit, 1 eV energy is
    necessary to decrease of the potential of the electron with 1 V.

                       1eV  1.602  10 19 joule
INSULATORS, SEMICONDUCTORS, AND
METALS
   This separation of the valence and conduction
    bands determines the electrical properties of the
    material
   Insulators have a large energy gap
      electrons can’t jump from valence to conduction
       bands
      no current flows
   Conductors (metals) have a very small (or
    nonexistent) energy gap
      electrons easily jump to conduction bands due to
       thermal excitation
      current flows easily
   Semiconductors have a moderate energy gap
        only a few electrons can jump to the conduction band
            leaving “holes”
        only a little current can flow
INSULATORS, SEMICONDUCTORS, AND METALS
(CONTINUED)


    Conductio
    n Band



    Valence
    Band

          Conductor Semiconductor Insulator
BAND STRUCTURE AND CONDUCTIVITY
HOLE - ELECTRON PAIRS
     Sometimes thermal energy is enough to cause an
      electron to jump from the valence band to the
      conduction band
         produces a hole - electron pair
     Electrons also “fall” back out of the conduction band
      into the valence band, combining with a hole




              pair elimination                 pair creation




                        hole                electron
    IMPROVING CONDUCTION BY DOPING
   To make semiconductors better conductors, add
    impurities (dopants) to contribute extra electrons or extra
    holes
     elements with 5 outer electrons contribute an extra electron to
      the lattice (donor dopant)
     elements with 3 outer electrons accept an electron from the
      silicon (acceptor dopant)
IMPROVING CONDUCTION BY DOPING
(CONT.)
   Phosphorus and arsenic
    are donor dopants
      if phosphorus is
       introduced into the
       silicon lattice, there is an
       extra electron “free” to
       move around and
       contribute to electric
       current
           very loosely bound to atom
            and can easily jump to
            conduction band
       produces n type silicon
           sometimes use + symbol to
            indicate heavier doping, so
            n+ silicon
       phosphorus becomes
        positive ion after giving
        up electron
IMPROVING CONDUCTION BY DOPING (CONT.)

    Boron has 3 electrons in its outer
     shell, so it contributes a hole if it
     displaces a silicon atom
       boron is an acceptor dopant
       yields p type silicon
       boron becomes negative ion
        after accepting an electron
    HOLE AND ELECTRON CONCENTRATIONS


   To produce reasonable levels of conduction doesn’t require much
    doping
       silicon has about 5 x 1022 atoms/cm3
       typical dopant levels are about 1015 atoms/cm3
   In undoped (intrinsic) silicon, the number of holes and number of free
    electrons is equal, and their product equals a constant
       actually, ni increases with increasing temperature
                               np = n 2i

   This equation holds true for doped silicon as well, so increasing the
    number of free electrons decreases the number of holes
INTRINSIC (PURE) SILICON

                                            At 0 Kelvin Silicon density is 5*10   ²³
                                            particles/cm   ³
                                            Silicon has 4 valence electrons, it
                                            covalently bonds with four adjacent
                                            atoms in the crystal lattice

Higher temperatures create free
charge carriers.
A “hole” is created in the absence of an
electron.

At 23C there are 10¹º particles/cm³ of
free carriers
                                         There are two types of doping
DOPING                                        N-type and P-type.




The N in N-type stands for negative.
A column V ion is inserted.
The extra valence electron is free to
move about the lattice


                                   The P in P-type stands for positive.
                                   A column III ion is inserted.
                                   Electrons from the surrounding
                                   Silicon move to fill the “hole.”
IMPURITIES
   Silicon crystal in pure form is
    good insulator - all electrons
    are bonded to silicon atom
   Replacement of Si atoms can
    alter electrical properties of
    semiconductor
   Group number - indicates
    number of electrons in
    valence level (Si - Group IV)
      IMPURITIES
   Replace Si atom in crystal with Group V atom
      substitution of 5 electrons for 4 electrons in outer shell
      extra electron not needed for crystal bonding structure
          can move to other areas of semiconductor
          current flows more easily - resistivity decreases
          many extra electrons --> “donor” or n-type material
   Replace Si atom with Group III atom
      substitution of 3 electrons for 4 electrons
      extra electron now needed for crystal bonding structure
          “hole” created (missing electron)
          hole can move to other areas of semiconductor if electrons continually
           fill holes
          again, current flows more easily - resistivity decreases
          electrons needed --> “acceptor” or p-type material
A LITTLE MATH

n= number of free electrons
p=number of holes
ni=number of electrons in intrinsic silicon=10¹º/cm³
pi-number of holes in intrinsic silicon= 10¹º/cm³
Mobile negative charge = -1.6*10-19 Coulombs
Mobile positive charge = 1.6*10-19 Coulombs
At thermal equilibrium (no applied voltage) n*p=(ni)2              (room
temperature approximation)
The substrate is called n-type when it has more than 10¹º free electrons (similar for
p-type)
Intrinsic silicon:




   DOPED SEMICONDUCTORS:
       “doped semiconductor”: (also “impure”, “extrinsic”) = semiconductor with small
        admixture of trivalent or      pentavalent atoms;
    N-TYPE MATERIAL
    donor (n-type) impurities:
      dopant with 5 valence electrons (e.g. P, As, Sb)

      4 electrons used for covalent bonds with surrounding Si atoms, one electron

       “left over”;
      left over electron is only loosely bound only small amount of energy

       needed to lift it into conduction band (0.05 eV in Si)
       “n-type semiconductor”, has conduction electrons, no holes (apart from

       the few intrinsic holes)
      example: doping fraction of 10 Sb in Si yields about 5x10
                                         -8                      16 conduction

       electrons per cubic centimeter at room temperature, i.e. gain of 5x106 over
       intrinsic Si.
N-TYPE MATERIAL



P-TYPE MATERIAL



   acceptor (p-type) impurities:
      dopant with 3 valence electrons (e.g. B, Al, Ga, In)  only 3 of the
       4 covalent bonds filled  vacancy in the fourth covalent bond 
       hole
      “p-type semiconductor”, has mobile holes, very few mobile electrons

       (only the intrinsic ones).
   advantages of doped semiconductors:
      can”tune” conductivity by choice of doping fraction

      can choose “majority carrier” (electron or hole)

      can vary doping fraction and/or majority carrier within piece of

       semiconductor
      can make “p-n junctions” (diodes) and “transistors”
P-TYPE MATERIAL
P-N JUNCTION



    Also known as a diode
    One of the basics of semiconductor technology -
    Created by placing n-type and p-type material in
     close contact
    Diffusion - mobile charges (holes) in p-type combine
     with mobile charges (electrons) in n-type
P-N JUNCTION
   Region of charges left behind (dopants fixed in
    crystal lattice)
     Group III in p-type (one less proton than Si- negative
      charge)
     Group IV in n-type (one more proton than Si - positive
      charge)
   Region is totally depleted of mobile charges -
    “depletion region”
     Electric field forms due to fixed charges in the depletion
      region
     Depletion region has high resistance due to lack of
      mobile charges
   p-n JUNCTION:
      p-n junction = semiconductor in which impurity changes abruptly from
       p-type to n-type ;
      “diffusion” = movement due to difference in concentration, from higher to

       lower concentration;
      in absence of electric field across the junction, holes “diffuse” towards

       and across boundary into n-type and capture electrons;
      electrons diffuse across boundary, fall into holes (“recombination of
       majority carriers”);  formation of a “depletion region”
                                    (= region without free charge carriers)
                                    around the boundary;
      charged ions are left behind (cannot move):

         negative ions left on p-side  net negative charge on p-side of the

          junction;
         positive ions left on n-side  net positive charge on n-side of the

          junction
          electric field across junction which prevents further diffusion.
DIODES (THE PN
JUNCTION)
DIODES (THE PN JUNCTION)
                           The free electrons in
                           the n region are
                           randomly drifting in all
                           directions. At the
                           instant of the pn
                           junction formation, the
                           free electrons near the
                           junction in the n region
                           begin to diffuse across
                           the junction into the p
                           region where they
                           combine with holes
                           near the junction. The
                           same is true for the p-
                           type material.
THE P-N JUNCTION
   p-n JUNCTION:
    
THE JUNCTION




       




               The “potential” or voltage across the silicon
               changes in the depletion region and goes from
               + in the n region to – in the p region
                        THINK OF THE DIODE
                        AS A SWITCH
BIASING THE P-N DIODE




Forward Bias              Reverse Bias
Applies - voltage         Applies + voltage to
to the n region           n region and –
and + voltage to          voltage to p region
the p region
                          NO CURRENT
CURRENT!
DIODES (THE DEPLETION REGION)
                          When the pn junction is
                          formed, the n region loses
                          free electrons as they diffuse
                          across the junction. This
                          creates a layer of positive
                          charges (ions) near the
                          junction. As the electrons
                          move across the junction,
                          the p region loses holes as
                          the electrons and holes
                          combine. This creates a layer
                          of negative charges (ions)
                          near the junction. These two
                          layers of positive and
                          negative charges form the
                          depletion region.
PN JUNCTION
    Formation of depletion region in pn-junction:
P-N JUNCTION – REVERSE BIAS
   positive voltage placed on n-type material
   electrons in n-type move closer to positive terminal,
    holes in p-type move closer to negative terminal
   width of depletion region increases
   allowed current is essentially zero (small “drift” current)
REVERSE BIASED DIODE

   Depletion region becomes wider,   barrier
    potential higher
DIODES (FORWARD BIAS )




Because like charges repel, the negative side of the bias-voltage source "pushes" the
free electrons, which are the majority carriers in the n region, toward the pn
junction. This flow of free electrons is called electron current. The negative side of
the source also provides a continuous flow of electrons through the external
connection (conductor) and into the n region as shown.
DIODES (FORWARD BIAS )




 •To bias a pn junction, apply an external dc voltage across it. Forward bias
 is the condition that allows current through a pn junction. The picture
 shows a dc voltage source connected by conductive material (contacts and
 wire) across a pn junction in the direction to produce forward bias.
 •This external bias voltage is designated as VBIAS. Notice that the negative
 side of VBIAS is connected to the n region of the pn junction and the
 positive side is connected to the p region. This is one requirement for
 forward bias. A second requirement is that the bias voltage, V BIAS, must be
 greater than the barrier potential (0.7V in silicon and 0.3 in germanium).
    P-N JUNCTION – FORWARD BIAS
   positive voltage placed on p-type material
   holes in p-type move away from positive terminal, electrons in n-
    type move further from negative terminal
   depletion region becomes smaller - resistance of device decreases
   voltage increased until critical voltage is reached, depletion
    region disappears, current can flow freely
FORWARD BIASED PN-JUNCTION
   Depletion region and potential barrier reduced
P-N JUNCTION - V-I
CHARACTERISTICS
 Voltage-Current relationship for a p-n junction (diode)
CURRENT-VOLTAGE
CHARACTERISTICS




      THE IDEAL DIODE


 Positive voltage yields finite
 current
 Negative voltage yields zero
 current                          REAL DIODE
THE IDEAL DIODE EQUATION

               qV        
     I  I 0  exp      1 ,
                  kT     
     where
     I 0  diode current with reverse bias
     q  1602  10 19 coulomb , the electronic ch arg e
          .
                     5   eV
     k  8.62  10           , Boltzmann' s cons tan t
                           K
    SEMICONDUCTOR DIODE - OPENED REGION

   The p-side is the cathode, the n-side is the anode
   The dropped voltage, VD is measured from the
    cathode to the anode

 Opened: VD  VF:
       VD = VF
ID = circuit limited, in our model the VD cannot exceed
  VF
SEMICONDUCTOR DIODE - CUT-OFF REGION

          Cut-off: 0 < VD < VF:
              ID  0 mA
SEMICONDUCTOR DIODE - CLOSED REGION


        Closed: VF < VD  0:
            VD is determined by the circuit, ID = 0 mA
        Typical values of VF: 0.5 ¸ 0.7 V
    ZENER EFFECT

   Zener break down: VD <= VZ:
        VD = VZ, ID is determined by the circuit.
   In case of standard diode the typical values of the
    break down voltage VZ of the Zener effect -20 ... -100
    V
   Zener diode
     Utilization of the Zener effect
     Typical break down values of VZ : -4.5 ... -15 V
THE ZENER DIODE
                                                                          +I
The zener diode exhibits a constant voltage                               F

drop when sufficiently reversed-biased.                   zener
                                                          point
This property allows the use of the zener
diode as a simple voltage regulator.                          -6   -3
                                                                                            +VF
                                                                                1   2   3
             +V

                                                              Constant
                                                              breakdown
                                                              voltage
                  R
                                  Here, Vr will be equal to the reverse breakdown voltage of
                                  the zener diode and should be constant. What is the purpose
                        Vr
                                  of the resistor in this circuit? Its job is to limit the current
                                  flowing through the zener diode:
                  D                                                              V  Vr
                                                                              I
                                                                                   R

   Kit Building Class
                             Page 70
        Lesson 3
              TRANSISTORS

 A transistor allows you to control the current, not just
  block it in one direction.
 A good analogy for a transistor is a pipe with an
  adjustable gate.
BIPOLAR JUNCTION TRANSISTORS (BJT)
                                                        collector
                                                                        collector



   Three Layers in a BJT                                    p
                                                                               n

       Collector
                                             base
       Base (very thin)                                     n
                                                                               p
           has fewer doping atoms
                                                             P+
       Emitter                                     i               i          n+


   Two Types of BJT’s
       PNP (figure on left)                            emitter
                                                                         emitter
           operates with outgoing base current
       NPN (figure on right)
           operates with incoming base current
BJT SCHEMATIC REPRESENTATION
                  collector

                                                 iB
                     p


   base              n
                              Corresponds to:

          i         P+




                  emitter

                  collector


                         n


                               Corresponds to:
                         p


              i       n+




                   emitter
THE PHYSICS OF TRANSISTORS
              A transistor is made from two
               p-n junctions back to back.
              An npn transistor has a p-type
               layer sandwiched between two
               n-type layers.
              A pnp transistor is the inverse.

              An n-type semiconductor is
               between two layers of p-type.
 BJT TRANSISTORS:

NPN
Transistor          Sandwiching a
                    P-type layer
                    between two n-
                    type layers.




PNP
Transistor          Sandwiching a
                    N-type layer
                    between two p-
                    type layers.
 THE BIPOLAR JUNCTION TRANSISTOR
The transistor is a versatile device usually configured to perform as a switch or as
an amplifier. The bipolar junction transistor (BJT) is the most common type and
has three leads:

                       3                                            3
                           Collector                                    Collector
              2                                           2
       Base                                        Base
                   1       Emitter                              1       Emitter

          PNP Transistor                              NPN Transistor




In a transistor, the flow of current from the collector to the emitter is controlled
by the amount of current flowing into the base of the transistor. If no current
flows into the base, no current will flow from the collector to the emitter (it acts
like an open switch). If current flows into the base, then a proportional amount
of current flows from the collector to the emitter (somewhat like a closed switch).
     HOW      A   “NPN” TRANSISTOR            WORKS?




The base-emitter diode
(forward) acts as a switch.
when v1>0.7 it lets the
electrons flow toward
collector. so we can control
our output current (Ic) with
the input current (Ib) by      C              B             E
using transistors.                 backward       Forward
           Transistors have three terminals:
                                 Collector

                   Base


                                Emitter

                                  Active: Always on
                                  Ic=BIb


Transistors work in 3 regions     Saturation :Ic=Isaturation
                                  On as a switch

                                   Off :Ic=0
                                   Off as a switch
TRANSISTOR      CURRENTS
                             -The arrow is always drawn
                              on the emitter

                             -The arrow always point
                              toward the n-type

                             -The arrow indicates the
                              direction of the emitter
                              current:
                                    pnp:E B
  IC=the collector current
                                    npn: B E
  IB= the base current
  IE= the emitter current
   By imaging the analogy of diode, transistor can be
    construct like two diodes that connetecd together.
   It can be conclude that the work of transistor is base on
    work of diode.
TRANSISTOR OPERATION
     The basic operation will be described using the pnp
      transistor. The operation of the pnp transistor is exactly
      the same if the roles played by the electron and hole
      are interchanged.
     One p-n junction of a transistor is reverse-biased,
      whereas the other is forward-biased.




        Forward-biased junction          Reverse-biased junction
           of a pnp transistor              of a pnp transistor
   Both biasing potentials have been applied to a pnp
    transistor and resulting majority and minority carrier
    flows indicated.
   Majority carriers (+) will diffuse across the forward-
    biased p-n junction into the n-type material.
   A very small number of carriers (+) will through n-type
    material to the base terminal. Resulting IB is typically in
    order of microamperes.
   The large number of majority carriers will diffuse across
    the reverse-biased junction into the p-type material
    connected to the collector terminal.
   Majority carriers can cross the reverse-biased
    junction because the injected majority carriers will
    appear as minority carriers in the n-type material.
   Applying KCL to the transistor :
                IE = IC + IB
   The comprises of two components – the majority
    and minority carriers
                IC = ICmajority + ICOminority
   ICO – IC current with emitter terminal open and is
    called leakage current.
    COMMON-BASE CONFIGURATION
   Common-base terminology is derived from the fact that
    the :
            - base is common to both input and output of the
              configuration.
            - base is usually the terminal closest to or at
              ground potential.
   All current directions will refer to conventional (hole)
    flow and the arrows in all electronic symbols have a
    direction defined by this convention.
   Note that the applied biasing (voltage sources) are such
    as to establish current in the direction indicated for each
    branch.
   To describe the behavior of common-base amplifiers
    requires two set of characteristics:
    - Input or driving point characteristics.
    - Output or collector characteristics

   The output characteristics has 3 basic regions:
    - Active region –defined by the biasing arrangements
    - Cutoff region – region where the collector current is 0A
    - Saturation region- region of the characteristics to the left of
      VCB = 0V
   The curves (output characteristics) clearly indicate that
    a first approximation to the relationship between IE
    and IC in the active region is given by
                    IC ≈IE
   Once a transistor is in the ‘on’ state, the base-emitter
    voltage will be assumed to be
                 VBE = 0.7V
   In the dc mode the level of IC and IE due to the
    majority carriers are related by a quantity called
    alpha
                           IC
                        = IE

                   IC = IE + ICBO
   It can then be summarize to IC = IE (ignore ICBO due
    to small value)
   For ac situations where the point of operation moves
    on the characteristics curve, an ac alpha defined by

                          IC
                            IE

   Alpha a common base current gain factor that shows
    the efficiency by calculating the current percent from
    current flow from emitter to collector.The value of  is
    typical from 0.9 ~ 0.998.
BIASING
   Proper biasing CB configuration in active region by
    approximation IC  IE (IB  0 uA)
TRANSISTOR   AS AN AMPLIFIER
SIMULATION   OF TRANSISTOR AS AN
AMPLIFIER
COMMON-EMITTER CONFIGURATION
    It is called common-emitter configuration since :
     - emitter is common or reference to both input and
       output terminals.
     - emitter is usually the terminal closest to or at
       ground
       potential.
    Almost amplifier design is using connection of CE due
     to the high gain for current and voltage.
    Two set of characteristics are necessary to describe the
     behavior for CE ;input (base terminal) and output
     (collector terminal) parameters.
                                   IB is microamperes compared
                                    to miliamperes of IC.
                                    IB will flow when VBE > 0.7V
                                    for silicon and 0.3V for
                                    germanium
                                   Before this value IB is very
                                    small and no IB.
                                    Base-emitter junction is
                                    forward bias
                                    Increasing VCE will reduce IB
                                      for different values.


  Input characteristics for a
common-emitter NPN transistor
                                          Output characteristics for a
                                            common-emitter npn
                                                    transistor




   For small VCE (VCE < VCESAT, IC increase linearly with
    increasing of VCE
   VCE > VCESAT IC not totally depends on VCE  constant IC
    IB(uA) is very small compare to IC (mA). Small increase in IB
    cause big increase in IC
   IB=0 A  ICEO occur.
   Noticing the value when IC=0A. There is still some value of
    current flows.
     BETA ()      OR AMPLIFICATION FACTOR
   The ratio of dc collector current (IC) to the dc base current
    (IB) is dc beta (dc ) which is dc current gain where IC and
    IB are determined at a particular operating point, Q-point
    (quiescent point).
   It’s define by the following equation:


              30 < dc < 300  2N3904




   On data sheet, dc=hFE with h is derived from ac hybrid
    equivalent cct. FE are derived from forward-current
    amplification and common-emitter configuration
    respectively.
   For ac conditions an ac beta has been defined as the
    changes of collector current (IC) compared to the
    changes of base current (IB) where IC and IB are
    determined at operating point.
   On data sheet, ac=hfe
   It can defined by the following equation:
EXAMPLE
 From output characteristics of common
 emitter configuration, find ac and dc with an
 Operating point at IB=25 A and VCE =7.5V.
Solution:
RELATIONSHIP   ANALYSIS BETWEEN Α AND Β
COMMON – COLLECTOR CONFIGURATION
   Also called emitter-follower (EF).
   It is called common-emitter configuration since both the
    signal source and the load share the collector terminal
    as a common connection point.
   The output voltage is obtained at emitter terminal.
   The input characteristic of common-collector
    configuration is similar with common-emitter.
    configuration.
   Common-collector circuit configuration is provided with
    the load resistor connected from emitter to ground.
   It is used primarily for impedance-matching purpose
    since it has high input impedance and low output
    impedance.
Notation and symbols used with the common-collector configuration:
               (a) pnp transistor ; (b) npn transistor.
Transistor as a Switch
     • Transistors can be used as switches.1




                 Transistor                       Switch

     •Transistors can either
                         conduct or not conduct current.2
     •ie, transistors can either be on or off.2
         A TRANSISTOR SWITCH
 In many electronic circuits a small voltage or current is
  used to switch a much larger voltage or current.
 Transistors work very well for this application because
  they behave like switches that can be turned on and off
  electronically instead of using manual or mechanical
  action.
           A TRANSISTOR SWITCH


   The resistance difference
    between “on” and “off” for a
    transistor switch is good
    enough for many useful
    circuits such as an
    indicator light bulb in a
    mechanical circuit.
BJT OPERATION REGIONS
Operation    IB or VCE     BC and BE   Mode
Region       Char.         Junctions
Cutoff       IB = Very     Reverse &   Open Switch
             small         Reverse
Saturation   VCE = Small   Forward &   Closed Switch
                           Forward
Active       VCE =         Reverse &   Linear
Linear       Moderate      Forward     Amplifier

Break-down VCE = Large     Beyond      Overload
                           Limits
LIMITS     OF     OPERATION
    Many BJT transistor used as an amplifier. Thus it is
     important to notice the limits of operations.
    At least 3 maximum values is mentioned in data sheet.
    There are:
          a) Maximum power dissipation at collector: PCmax
             or PD
          b) Maximum collector-emitter voltage: VCEmax
             sometimes named as VBR(CEO) or VCEO.
          c) Maximum collector current: ICmax
    There are few rules that need to be followed for BJT
     transistor used as an amplifier. The rules are:
          i) transistor need to be operate in active region!
          ii) IC < ICmax
          ii) PC < PCmax
Note:   VCE is at maximum and IC is at minimum (ICmax=ICEO) in the
        cutoff region. IC is at maximum and VCE is at minimum
        (VCE max = VCEsat = VCEO) in the saturation region. The transistor
        operates in the active region between saturation and cutoff.
Refer to the fig.
Step1:
The maximum collector
power dissipation,
PD=ICmax x VCEmax        (1)
    = 18m x 20 = 360 mW
Step 2:
At any point on the
characteristics the product of
and must be equal to 360 mW.
Ex. 1. If choose ICmax= 5 mA,
subtitute into the (1), we get
VCEmaxICmax= 360 mW
VCEmax(5 m)=360/5=7.2 V

Ex.2. If choose VCEmax=18 V,
subtitute into (1), we get
VCEmaxICmax= 360 mW
(10) ICmax=360m/18=20 mA
TRANSISTOR SPECIFICATION SHEET
TRANSISTOR TERMINAL IDENTIFICATION
TRANSISTOR TESTING
1. Curve Tracer
   Provides a graph of the characteristic curves.
2. DMM
   Some DMM’s will measure DC or HFE.
3. Ohmmeter
TRANSISTOR AS AN
AMPLIFIER
TRANSISTOR AMPLIFIER BASICS
 It is critical to understand the notation used for
  voltages and currents in the following discussion
  of transistor amplifiers.
 This is therefore dealt with explicitly ‘up front’.
 As with dynamic resistance in diodes we will be
  dealing with a.c. signals superimposed on d.c.
  bias levels.
TRANSISTOR AMPLIFIER BASICS
 We will use a capital (upper case) letter for a d.c.
  quantity (e.g. I, V).
 We will use a lower case letter for a time varying
  (a.c.) quantity (e.g. i, v)
TRANSISTOR AMPLIFIER BASICS
 These   primary quantities will also need a
  subscript identifier (e.g. is it the base
  current or the collector current?).
 For d.c. levels this subscript will be in
  upper case.
 We will use a lower case subscript for the
  a.c. signal bit (e.g. ib).
 And an upper case subscript for the total
  time varying signal (i.e. the a.c. signal bit
  plus the d.c. bias) (e.g. iB).This will be less
  common.
TRANSISTOR AMPLIFIER BASICS
 ib
                              0



 +

 IB



 =



 iB
TRANSISTOR AMPLIFIER BASICS
 It is convention to refer all transistor voltages to
  the ‘common’ terminal.
 Thus in the CE configuration we would write VCE
  for a d.c. collector emitter voltage and VBE for a
  d.c. base emitter voltage.
     A TRANSISTOR AMPLIFIER
 One of the most important uses of a transistor is to
  amplify a signal.
 In electronics, the word “amplify” means to make the
  voltage or current of the input signal larger without
  changing the shape of the signal.
Transistor as an amplifier:

    Transistors are often used as amplifiers to increase input
    signal in radios, televisions and some other applications .The
    circuit may be designed to increase the current or voltage
    level.
    The power gain is the product of current gain and voltage
    gain (P=V*I).
    A TRANSISTOR AMPLIFIER
 In an amplifier circuit,
  the transistor is not
  switched fully “on” like
  it is in a switching
  circuit.
 Instead, the transistor
  operates partially on
  and its resistance
  varies between a few
  hundred ohms and
  about 10,000 ohms,
  depending on the
  specific transistor.
    DC BIASING CIRCUITS
The ac operation of an                        +VCC
    amplifier depends on the
    initial dc values of IB, IC,
    and VCE.
                                                 RC
By varying IB around an initial
                                    RB
    dc value, IC and VCE are                                v out
    made to vary around their
    initial dc values.
DC biasing is a static
    operation since it deals v in                     vce
                                         ib
    with setting a fixed
    (steady) level of current
                                                      ic
    (through the device) with a
    desired fixed voltage drop
    across the device.

				
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