Chapter 1 Fundamental Solid-State Principles by yurtgc548

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									       Chapter 1
  Fundamental Solid-
    State Principles

Pictures are redrawn (with some modifications) from
  Introductory Electronic Devices and Circuits
                         By
                  Robert T. Paynter




                                                      1
           Objectives (1)
• Describe the relationship between the number of
  valence electrons and conductivity properties.
• Describe the relationship between conduction and
  temperature.
• Contrast trivalent and pentavalent elements.
• List the similarities and differences between n-
  type and p-type semiconductors.
• Describe diffusion current.
• Describe how a depletion layer is formed around
  a pn junction.


                                                     2
           Objectives (2)
• Explain the source of barrier potential, and list
  the barrier potential values for Si and Ge.
• Define bias.
• Desbribe the different methods of forward and
  reverse biasing a pn junction.
• Explain why Si is used more commonly than Ge
  in the production of solid state devices.




                                                      3
Fig. 1.1 Bohr Model of the atom.

                                 Orbital shells



                                                                M
                                                            L
                                                        K




                              Valence shell


             (a)                                  (b)
Orbital shells are identified using the letters K through Q.

                                                                    4
Relationship between Valence
 Electrons and Conductivity
The conductivity decreases with an increase
in the number of valence electrons.

1 valence electron    nearly perfect conductor
8 valence electrons   insulator
 (Max = 8)




                                                 5
Fig. 1.2 Semiconductor atoms.

 4 valence electrons        semiconductor




  Silicon (Si)   Germanium (Ge)   Carbon (C)


                                               6
  Electrons in Orbital Shells
• Electrons travels only in orbital shells.
• Each orbital shells relates to a specific energy
  range.
• An electron can jump from one orbital shell to
  another that has higher energy level if the
  electron absorbs energy equal to the energy
  difference between the two orbital shells.
• After jumping to a higher energy shell, the
  electron will eventually give up the energy and
  return to a lower-energy shell.


                                                     7
Fig. 1.3 Silicon energy gaps and
         levels.
                             Energy
           Conduction band                     e4 = 1.8 eV
 Valence
                                  Energy gap
 band
                                               e3 = 0.7 eV
                                               e2
                                               e1




                               1.8 eV – 0.7 eV = 1.1 eV




                                                             8
Fig. 1.4 Silicon covalent bonding.

                             Intrinsic (pure) silicon is
                   Si        a very poor conductor.

    Si
              Si
                                   Energy gap of Si:
                        Si          single atom = 0.05 eV
                                    crystal = 0.7 eV

         Si



                                                            9
Fig. 1.5 Generation of an
         electron-hole pair.
                            Energy


                  Si                 Conduction band
                                                       Electrons
   Si
             Si                        Valence band
                       Si                              Holes

        Si




                                                                   10
Conduction vs Temperature
• At room temperature, thermal energy
  (hear) causes the constant creation of
  electron-hole pair, with their subsequent
  recombination.

• Conductivity in a semiconductor varies
  directly with temperature.




                                              11
Table 1.1 Commonly used
          doping elements.
 Trivalent Impurities    Pentavalent Impurities
       (p-Type)                (n-Type)
   Aluminum (Al)            Phosphorus (P)

    Gallium (Ga)             Arsenic (As)

      Boron (B)             Antimony (Sb)

     Indium (In)             Bismuth (Bi)

 (Acceptor impurities)      (Donor impurities)
                                                  12
Fig. 1.6 n-type material and
         its energy diagram.

                                                      Energy
                            Excess covalent
                  Si        bond electron
                                                               Conduction band
   Si                                    Electrons
                                (majority carriers)
             As
                       Si                                        Valence band
                                            Holes
                                (minority carriers)
        Si




 Conductivity of n-type material is increased
 due to more free-electrons.
                                                                                 13
Fig. 1.8 p-type material and
         its energy diagram.
                                                      Energy

                  Si        Covalent bond hole
                                                               Conduction band
   Si                                    Electrons
                                (minority carriers)
             Al
                       Si                                        Valence band
                                            Holes
                                (majority carriers)
        Si




 Conductivity of p-type material is increased
 due to more holes in valence band.
                                                                                 14
      Doping Density
1 impurity atom per 105 to 108 Si atoms
     and about 1022 Si atoms/cm3

    1017 to 1014 impurity atom/cm3
  (much more than heat-rupture electrons)




                                            15
 Effect of Doping on Conductivity

• At the rate 1 donor atom per 108 Si atoms, the
  conductivity at 30°C is multiplied by a factor of
  24,100.

• Conductivity in doping semiconductor is less
  dependent on temperature.




                                                      16
Fig. 1.11 pn-junction initial
          energy levels.
                                                                    Junction


                n                     p                   n           p
Energy




                                                 Energy
                               Conduction band
         Conduction band


                                Valence band
          Valence band

                         (a)                                  (b)

                                                                               17
Fig. 1.12 The forming of the
          depletion layer.
         n   p            n                 p


                          Depletion layer
Energy




                 Energy




                                                18
Fig. 1.13 Depletion Layer Charges.

       n                                       p
                     +4                                  +4

  +4                                      +4
                +5
                               Electric             +3
                                field
                          +4                                  +4

           +4                                  +4

                               Junction

  Total (+) = 21                           Total (+) = 19
  Total (-) = 20                           Total (-) = 20
 Net charge = +1                          Net charge = -1

                                                                   19
    Things to Remember
• Depletion layer (or region) is the
  area around a pn junction that is
  depleted of charge carriers.

• Barrier potential is the natural
  potential across a pn junction.
  (Barrier potential is typically in the
  millivolt range.)

                                           20
Depletion Layer Width vs
  Junction Resistance


 Depletion     Junction    Junction
layer width   resistance    current
 Minimum       Minimum     Maximum
Maximum       Maximum      Minimum




                                      21
                   Bias
• Applying the potential (bias) to a pn
  junction, we can adjust the width of the
  depletion layer.

• Forward bias is a potential to reduce the
  depletion layer width and junction
  resistance as a result.
• Reverse bias is a potential to increase the
  depletion layer width and junction
  resistance as a consequence.

                                                22
Fig. 1.14 The effect of forward bias.

              n       p                    n              p


                  V                             V
    SW1                              SW1



 (a) An unbiased pn junction      (b) Charge motion at the
                                      moment SW1 is closed

                                            n             p
          n           p

                                       Rn            Rp
                  V
    SW1
                                                Rb

(c) Conduction increases as the        (d) Bulk resistance
    depletion layer becomes
    narrower                                                  23
Fig. 1.15 Some forward-
          biased pn junction.
           +V     -V




           p      n


           n      p




                                24
Fig. 1.16 The effect of reverse bias.

         n            p                         n         p


               V                                    V
   SW1                                    SW1


(a) A conducting pn junction            (b) A depletion layer forms
                                            when there is no current


                                n          p


                                    V
                          SW1


         (c) When the bias is reversed, the depletion
             layer widens as charge carriers move away
             from the junction
                                                                       25
Fig. 1.17 Some reverse-
          biased pn junction.
           +V     -V




           n      p


           p      n




                                26
  Fig. 1.18 pn-junction biasing.
            Forward bias                   Reverse bias
            +V             -V              -V               +V


                 p     n                        p       n

                 n     p                        n       p




Bias Type            Junction Polarities            Junction Resistance
Forward     n-type is more (-) than p-type            Extremely low
Reverse     p-type is more (-) than n-type            Extremely high


                                                                          27
          A Final Note
Si is preferred to Ge:
• Si is more tolerant of heat.
• Germanium oxide is water soluble – make
  it difficult to process.
• A Ge device allows more leakage current
  than that of Si.




                                            28
               Summary
•   Semiconductor valence shell.
•   n-type and p-type doping.
•   pn junction.
•   Forward and reverse bias.
•   Why Si is preferred to Ge.




                                   29

								
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