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ECA1212

Introduction to Electrical &

Electronics Engineering

Chapter 4: Basic Semiconductor and Diode


by Muhazam Mustapha, October 2011

Learning Outcome


Explain some basic theory about charge
transport in semiconductor


Explain diode circuit operation

By the end of this chapter students are
expected to:

Chapter Content


Physics of Semiconductor


PN Junction and Diode


Diode Circuits

Physics of Semiconductor


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Three Categories of Material


Based on their electrical conductivity, material
can be categorized into 3 groups:


Conductor


Non
-
conductor


Semiconductor


This conductivity property is determined by the
electronic structure in the outer most shell


Electronic structure in the outer most shell, in
turn, will determine the amount of energy
needed by the outer most electron to be freed
from the atom

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Three Categories of Material


In a system of a large number of atoms come
close together


in a compound or crystal, for
example


the energy level of the outer shells
will merge together to form BANDS


For a material to conduct electricity, its electron
in the outer band (VALENCE) must be able to
go up to the CONDUCTION band


The energy distance (gap) between the valence
band and the conduction band is what
determines the conductivity of the previous 3
categories of material

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Three Categories of Material

Conduction
Band

Valance
Band

Electron Energy

Conductor

Semiconductor

Non
-
conductor

Overlap

Small Gap

Big Gap

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Conductor


In a conductor, the conduction band and the
valance band are overlapping


This allows the electrons in outer most shell
(valance band) to freely move among the
system of atoms


This free movement of electron is permitted
without any external energy (or excitation)


Metals are the material that posses this kind of
conductivity

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Non
-
Conductor


In a non
-

conductor, the conduction band and
the valance band are far apart


This disallows the electrons in outer most shell
to freely move among the system of atoms


It is almost impossible to push the electrons up
to the conduction band without damaging the
material structure


Most of non
-
metallic material are non
-
conductor

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Semiconductor


In between conductor and non
-
conductor, there
exist a special type of material that possesses
an intermediate electronic structure property


The conduction band and the valance band are
not overlapping but not far apart either


This allows the electrons in its valance band to
jump into the conduction band if they acquire
enough energy


The source of such energy could be from heat,
electromagnetic rays, direct hit by another
electron, etc

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Semiconductor


The elements in the Periodic Table Group IV are
the most common semiconductors


The examples are: Carbon, Silicon and
Germanium

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Electron Transport in Semiconductor


We may view the crystalline structure of Group
IV elements as follows:

4+

4+

4+

4+

4+

4+

4+

4+

4+

4+

4+

4+

4+

4+

4+

4+

4+

4+

valance electron
bonding

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Electron Transport in Semiconductor


Some of the electrons in valance band may gain
some energy and become free

4+

4+

4+

4+

4+

4+

4+

4+

4+

4+

4+

4+

4+

4+

4+

4+

4+

4+

free electrons

holes

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Electron Transport in Semiconductor


The free electrons contribute to electric
conductivity in the semiconductor material


The covalent bond from where the electrons
come out will now be lacking of an electron and
become another electron transport medium
called HOLES


Holes made electron transport possible by
allowing an electron in a neighboring covalent
bond to jump into it and effectively create
electrical movement

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Electron Transport in Semiconductor


Holes transport phenomenon only exists in
semiconductor material


Under the influence of the same electric field,
holes make a net movement in an opposite
direction of electrons movement

Electric field

Free
electrons

Valance band
electrons

Holes

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Dopant


Pure semiconductor material is called intrinsic


In intrinsic semiconductor, the no. free electrons
and holes will be balanced


Dopant can be added to intrinsic semiconductor
to alter the no. one of the transport carrier


either free electrons or holes


Doped semiconductor is called extrinsic

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N
-
type Semiconductor


Elements of Periodic Table Group V can be
added as dopant (phosphorus, arsenic)


These elements have an extra electron that
cannot contribute to the covalent bond, hence it
is freed


These electrons do not need extra energy to be
freed, hence they behave like free electrons in
conductors


This type of semiconductor with more free
electrons than holes is called N
-
type
semiconductor

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N
-
type Semiconductor

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N
-
type Semiconductor

4+

4+

5+

4+

4+

4+

4+

4+

4+

4+

4+

4+

4+

5+

4+

4+

5+

4+

extra free
electrons

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N
-
type Semiconductor


In N
-
type semiconductor:


The majority carrier is free electrons


The atom (element) that contribute to the extra free
electron is called DONOR atom

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P
-
type Semiconductor


Elements of Periodic Table Group III can be
added as dopant (boron, gallium)


These elements lack an electron in the outer
shell hence cannot create a complete covalent
bond


These bonds are effectively created with holes


This type of semiconductor with more holes
than free electrons is called P
-
type
semiconductor

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P
-
type Semiconductor

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P
-
type Semiconductor

4+

4+

3+

4+

4+

4+

4+

4+

4+

4+

4+

4+

4+

3+

4+

4+

3+

4+

extra holes

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P
-
type Semiconductor


In P
-
type semiconductor:


The majority carrier is holes


The atom (element) that contribute to the extra holes
is called ACCEPTOR atom

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PN Junction


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PN Junction


What will happen if a P
-
type semiconductor is
fabricated next to an N
-
type semiconductor?


Will the extra free electrons in the N
-
type area
cross over into the P
-
type and neutralize the
extra holes?


As a matter of fact, they do


However, the crossing over causes charge
imbalance

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PN Junction

N

P



+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+



































Depletion
Region

Electron migration

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PN Junction


The N
-
type area that loses electron will have
more positive charge, and vice versa


This will then create a voltage difference at the
interface between the P
-

and N
-
type material in
the polarity that is the same direction to the
electrons migration from N to P


This voltage difference creates an electric field
that will then prevent further migration of
electrons to P
-
type when it reaches certain
voltage value

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PN Junction


The electron
-
hole neutralized area (but with
effective positive or negative charge), is called
depletion region


The width of the depletion region is very small
but it depends on the concentration of dopant


The voltage different across the junction is
around 0.75V


depletion voltage (
V
D
)

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Diode


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By creating a PN junction we basically create a
diode


Diode symbol in circuit:




Diode is mostly used in rectifier circuits (circuits
that allow current to flow only in 1 direction)


The direction of current flow is the same as the
direction of the triangle
(explained in
succeeding slides)

Diode

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Reverse Biasing Diode


Reverse biasing a diode means we are applying
voltage across its to make the depletion region
larger


This is done by applying a +ve potential to the N
side of the diode and

ve potential to the P side
of the diode


The +ve potential will then attract the electrons
in N
-
type side toward the terminal, and the

ve
potential will attract the holes in P
-
type side
toward the other terminal

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Reverse Biasing Diode

N

P



+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+



































Depletion
Region
Increased

+

+

+

+

+

+

+

+

+



















e
-

e
-

e
-

e
-

e
-

e
-

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Reverse Biasing Diode


Since the holes and electrons are attracted
toward the opposite terminals, the area of
depletion region increased


The whole process is like the process of
charging a capacitor because the electrons and
the holes attracted to the terminals are like the
charges that accumulated on capacitor plates


Hence, at final stage, there is no current flow
through the diode


just like capacitor

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Forward Biasing Diode


Forward biasing a diode means we are applying
voltage across its to make the depletion region
smaller


This is done by applying a

ve potential to the N
side of the diode and +ve potential to the P side
of the diode


The

ve potential will then repel the electrons in
N
-
type side toward the depletion region, and the
+ve potential will repel the holes in P
-
type side
toward the depletion region from the side

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Forward Biasing Diode

N

P

+

+

+

+

+

+

+

+

+



















Depletion
Region
Decreased

e
-

e
-

e
-

e
-

e
-

e
-

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Forward Biasing Diode


Even though the depletion region is smaller
now, but there is will still no current flowing until
the 0.75V voltage (barrier) due to the electron
migration is offset by the external potential


Once the voltage barrier is passed, the
depletion region vanishes, the current then
flows with very little resistance


Hence, when current is flowing through the
diode at forward biased, the diode is basically a
short circuit

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Forward Biasing Diode

N

P

e
-

e
-

e
-

e
-

e
-

e
-

> 0.75V

current flow

cathode

anode

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I
-
V Characteristics


On reverse bias voltage, there is zero current
flowing


On forward bias, there will be current flowing
after the 0.75V voltage barrier is overcome


At a very high reverse bias voltage, a junction
breakdown will take place and current will flow
in reverse direction


beyond the scope of this
course


Refer to the graph in the next slide

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I
-
V Characteristics

I

V

Reverse Bias

Forward Bias

Breakdown

V
D

= 0.75V

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Ideal Model

I

V

Reverse Bias

Forward Bias

ON

(Forward Bias)

OFF

(Reverse Bias)

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Offset Model

I

V

Reverse Bias

Forward Bias

+



V
D

V
D

ON

(Forward Bias)

OFF

(Reverse Bias)

+



V
D

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Diode Circuits


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Half
-
Wave Rectifier


Rectifier is a circuit that changes AC current to
DC


The process involves diodes as diodes only
allows current to flow in one direction


Half
-
wave rectifier only allows the positive (half)
part of an AC current to flow through it

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Half
-
Wave Rectifier

v
L

v
S

+



+



R
L

v
S

v
L

t

t

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Full
-
Wave Rectifier


Full
-
wave rectifier requires two diodes and a
transformer with a center tap


The diodes still allow only one way current
through them


one diode allows one half wave


The arrangement of the two diodes channels
the current to flow into the load in same
direction


Center tap makes the transferred power is half,
but since both half wave are allowed to flow, it
doubles back the power


making it the same
power as the half wave

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Full
-
Wave Rectifier

v
L

v
S

+



+



R
L

v
S

v
L

t

t

v
S

+

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Bridge Rectifier


Bridge rectifier is a full wave rectifier that
doesn’t require a center tapped transformer


Four diodes are arranged as shown in the next
slide


Only two diodes flowing current during each half
wave


The two half waves are channeled through the
load in the same direction making the load to
experience the same voltage polarity through it

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Bridge Rectifier

v
L

+



R
L

v
S

+



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Bridge Rectifier

v
L

+



R
L

v
S

+



v
S

v
L

t

t

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Bridge Rectifier

v
S

v
L

t

t

v
L

+



R
L

v
S

+



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Bridge Rectifier


Capacitor can be used to reduce ripple in
rectifiers

v
L

+



R
L

v
S

+



v
L

t

Capacitor
re
-
charge

Capacitor
discharge

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