Semiconductor theory

The following presentation is a part of the level 4 module -- Electrical and Electronic Principles. This resources is a part of the 2009/2010 Engineering (foundation degree, BEng and HN) courses from University of Wales Newport (course codes H101, H691, H620, HH37 and 001H). This resource is a part of the core modules for the full time 1st year undergraduate programme.
The BEng & Foundation Degrees and HNC/D in Engineering are designed to meet the needs of employers by placing the emphasis on the theoretical, practical and vocational aspects of engineering within the workplace and beyond. Engineering is becoming more high profile, and therefore more in demand as a skill set, in today’s high-tech world. This course has been designed to provide you with knowledge, skills and practical experience encountered in everyday engineering environments.

1. Semiconductor
Theory
Electrical and Electronic
Principles
© University of Wales Newport 2009 This work is licensed under a Creative Commons Attribution 2.0 License.

2. The following presentation is a part of the level 4 module -- Electrical and Electronic Principles. This resources is a
part of the 2009/2010 Engineering (foundation degree, BEng and HN) courses from University of Wales Newport
(course codes H101, H691, H620, HH37 and 001H). This resource is a part of the core modules for the full time 1
st
year undergraduate programme.
The BEng & Foundation Degrees and HNC/D in Engineering are designed to meet the needs of employers by placing
the emphasis on the theoretical, practical and vocational aspects of engineering within the workplace and beyond.
Engineering is becoming more high profile, and therefore more in demand as a skill set, in today’s high-tech world.
This course has been designed to provide you with knowledge, skills and practical experience encountered in
everyday engineering environments.
Contents
 Silicon
 Periodic Table of the Elements
 N-Type Impurity
 P-Type Impurity
 Creating A Diode
 Forward Bias
 Reverse Bias
 Diode Equation
 Rectification
 Credits
In addition to the resource below, there are supporting documents which should be used in combination with this
resource. Please see:
Green D C, Higher Electrical Principles, Longman 1998
Hughes E , Electrical & Electronic, Pearson Education 2002
Hambly A , Electronics 2
nd
Edition, Pearson Education 2000
Storey N, A Systems Approach, Addison-Wesley, 1998
Semiconductor Theory

3. Silicon is the most common metalloid. It is a chemical
element, which has the symbol Si and atomic number 14. A
tetravalent metalloid, it is less reactive than its chemical
analogue carbon.
Silicon is the eighth most common element in the universe
by mass, but very rarely occurs as the pure free element
in nature. It is more widely distributed in dusts, sands,
planetoids and planets as various forms of silicon dioxide
(silica) or silicates. In Earth's crust, silicon is the second
most abundant element after oxygen, making up 27.7% of
the crust by mass.
Silicon has many industrial uses. It is the principal
component of most semiconductor devices, most
importantly integrated circuits or microchips.
* The above text is taken from http://en.wikipedia.org/wiki/Silicon and is available under the
Creative Commons Attribution-ShareAlike License.
Silicon.

4. Silicon is widely used in semiconductors because it remains
a semiconductor at higher temperatures than the
semiconductor germanium and because its native oxide is
easily grown in a furnace and forms a better
semiconductor/dielectric interface than any other
material.
In the form of silica and silicates, silicon forms useful
glasses, cements, and ceramics. It is also a constituent of
silicones, a class-name for various synthetic plastic
substances made of silicon, oxygen, carbon and hydrogen,
often confused with silicon itself.
* The above text is taken from http://en.wikipedia.org/wiki/Silicon and is available under the
Creative Commons Attribution-ShareAlike License.

5. 5
B
Boron
6
C
Carbon
7
N
Nitrogen
13
Al
Aluminium
14
Si
Silicon
15
P
Phosphorus
31
Ga
Gallium
32
Ge
Germanium
33
As
Arsenic
Thew term tetravalent means that Carbon, Silicon and
Germanium have four electrons in their outermost
shell (orbital).
Silicon.

7. When pure Silicon crystallises it bonds with the four
atoms in its immediate vicinity. This means that each
electron is shared by two atoms and the result is shown
below.
Si
Si
Si
Si
Si
Si
Si
Si
Si
Semiconductor Theory

8. Metals conduct electricity well as they have electrons
which are not tied into the lattice – these are called
"free electrons“. Silicon crystals look like a metal but as
previously stated all the electrons are held firmly in
place. This means that pure silicon is an insulator and
does not allow the conduction of electricity. The sharing
of electrons between atoms is called covalent bonding.
Doping Silicon
If, instead of pure Silicon
you introduce a small
quantity of an impurity then
the Silicon changes its
nature.
5
B
Boron
6
C
Carbon
7
N
Nitrogen
13
Al
Aluminium
14
Si
Silicon
15
P
Phosphorus
31
Ga
Gallium
32
Ge
Germanium
33
As
Arsenic

9. There are two types of impurities:
N-type - By doping pure silicon with Group V elements such
as phosphorus, extra valence electrons are added that
become unbonded from individual atoms and allow the
compound to be an electrically conductive n-type
semiconductor. http://en.wikipedia.org/wiki/Silicon and is available under the
Creative Commons Attribution-ShareAlike License.
P
Si
Si
Si
Si
Si
Si
Si
Si
The extra (fifth) electron
has nothing to hold it in place
and so it is free to move
around and hence free to
carry current.
This is called n-type as the
electron is negatively
changed
Note the overall material is
not changed.

10. P-type - Doping with Group III elements, which are
missing the fourth valence electron, creates "broken
bonds" (holes) in the silicon lattice that are free to move.
The result is an electrically conductive p-type
semiconductor. http://en.wikipedia.org/wiki/Silicon and is available under the
Creative Commons Attribution-ShareAlike License.
B
Si
Si
Si
Si
Si
Si
Si
Si
The newly created hole
has nothing to hold it in
place and so it is free to
move around and hence
free to carry current.
This is called p-type as
the hole is positively
changed
Note, again, the overall
material is not changed.

11. A minute amount of either N-type or P-type doping
(typically one part per million) turns a silicon crystal from a
good insulator into a viable (but not great) conductor --
hence the name "semiconductor."
N-type and P-type silicon are not that amazing by
themselves; but when you put them together, you get some
very interesting behaviour at the junction.
Semiconductor Theory

12. Creating a Diode
When you connect electrically N-type and P-type silicon as
shown below, you find that the following happens and this
produces a useful effect.
p type n type
Negative Positive
The electrons in the N-type
will move across the
junction to recombine with
the holes in the P-type. This
would continue until the
regions were depleted of
carries (holes or electrons).
There are though two
factors which limit the
number which cross the
junction.

13. The N-type is loosing electrons therefore it becomes
positively charged which attracts the electrons and
reduces the chance of them crossing the barrier.

The P-type is gaining electrons making it negatively
charged. This repels the electrons wishing to cross into
the P-type.
If we were able to look at the charge along the PN device
we would see:
P-type N-type
Semiconductor Theory

14. The value of potential at which the system stabilises is
about 0.6 volts for Silicon.
For Germanium (which was the first semiconductor
material used for electronic devices) the value is about
0.2 volts.
Let us look at what happens if we now place a voltage
across the PN junction.
Semiconductor Theory

15. Forward Bias.
V
p n
The voltage source will
attempt to drive electrons
around the circuit
anticlockwise (opposite
direction to conventional
current flow).
If V is less than the junction potential (Vj) for Si,
electrons flowing into the N-type will see the negative
barrier and therefore current flow will be extremely
small.
V
Vj
e e e e

16. As V increases the barrier gets smaller and when V = Vj
the barrier disappears and current flow increases.
Any increase in V above Vj will result in damage to the
junction as the barrier becomes inverted and draws
electrons across it. If we do use this device we need to
ensure that we limit the current flowing through it.
V
Vj
e e e e
Semiconductor Theory

17. Reverse Bias.
V
p n
With the supply voltage
reversed the Junction
potential is modified as
follows:
V
Vj
e e e e
With the increased
barrier there will in
theory be no flow of
current. In practice
there will exist within
any crystal structure
doped or not carriers of
both types (holes and
electrons).
Semiconductor Theory

18. These are due to
temperature and are
called Thermally
Generated Hole Electron
Pairs. As the
temperature rises
electrons within the
lattice are given more
energy and it is possible
for them to break free.
This produces a hole in
the lattice and a free
electron. The number of
these is small but
increases with
temperature.
Si
Si
Si
Si
Si
Si
Si
Si
Si
Semiconductor Theory

19. These opposite polarity carriers mean that there will be a
small current flow and this is referred to as the Reverse
Leakage Current (Io).
In N-type Silicon –
The majority carriers are electrons
The minority carriers are holes
In P-type Silicon –
The majority carriers are holes
The minority carriers are electrons
The PN junction is referred to as a diode and its symbol
reflects its operation:
Shows that current
flows in this direction
Shows current does not
flow in this direction
Semiconductor Theory

20. The P-type side is called The N-type side is called
the Anode (A) the Cathode (K)
Semiconductor Theory

21. Diode equation
There exists a relationship between the voltage applied to
the diode and the current flowing through it. It has the
following form:
÷
ø
ö
ç
è
æ
-= 1
KT
Vq
oeII
where
I is the current flowing through the diode
Io is the reverse leakage current (typically 1 x 10
-10
A)
V is the applied voltage
q is the charge on an electron 1.602 x 10
-19
C
K is Boltzmann’s Constant 1.38 x 10
-23
JK
-1
T is the absolute temperature (ºC + 273)
Semiconductor Theory

22. For room temperature the constants can be combined to
give a single value:
When temperature is 20ºC.
This is normally quoted as 40.
Therefore the equation becomes:
Determine for the following values of V
6239.=
KT
q
( )1
40
-=
V
o
eII
V
e
40
V
0.05 7.39
0.1 54.6
0.2 2980
0.3 162754
0.4 8886110
V
e
40
From the table we can see
that as long as V is greater
than 0.1 the –1 in the
equation can be ignored.
The equation therefore
becomes:
( )
V
o
eII
40
=
Semiconductor Theory

23. If Io = 1 x 10
-10
A then determine the how the current
varies with applied voltage.
V I
0.05 739pA
0.1 5.46nA
0.2 298nA
0.3 16.3μA
0.4 889μA
0.5 48.5mA
0.6 2.65A
0.7 145A
The graph shows this as a
plot of Current I against
Applied Voltage V.
0
1
2
3
4
5
6
7
8
9
10
0.5 0.55 0.6 0.65
Voltage
Current
You can see that up to about 0.6
volts the current is relatively
small whilst above 0.6 the
current increases rapidly.

24. Rectification.
This is the process of converting an A.C. input into a D.C.
output.
A.C. – This is a signal that periodically changes polarity.
Examples include the mains voltage and signals generated
from acoustic sources.
AC mains signal
-400
-200
0
200
400
time
voltage
D.C. – This type
of signal never
changes polarity
and so it will be
either positive
or negative.
Batteries will
generate D.C.
outputs.
Semiconductor Theory

25. There are three basic rectifier circuits:
Half Wave Rectification
Vp
L
O
A
D
The diode will remove the
negative half cycle leaving
only the positive. Though
the voltage is fluctuating
it is D.C.
Half Wave Rextified
time
voltage
Vp

26. Full Wave Rectification
Vp
L
O
A
D
The diodes will remove the
negative half cycles from
the two waveforms, which
are 180º out of phase leaving
only the positive half cycles.
These are then summed to
give the output.
Full Wave Rectified
time
voltage
Vp/2
This
produces a
smoother
output but
with a
reduced
voltage.

27. Bridge Rectification
Vp
L
O
A
D
The diodes direct
the flow to the
load differently
depending upon the
polarity of the
input A.C.
If the input is + on the top,
diodes 2 and 3 conduct.
+
-
+
-
If + on the bottom diodes
1 and 4 conduct.
This ensures that the polarity at the load never changes.
Semiconductor Theory

28. Bridge Rectified
time
voltage
Vp
Note that at any moment in time two diodes are
conducting which means that the output peak is
actually Vp – 1.2v (2 x 0.6v)
Semiconductor Theory

29. Semiconductor Theory
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© 2009 University of Wales Newport
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