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

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

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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