Ημιαγωγοί

1 Νοε 2013 (πριν από 4 χρόνια και 8 μήνες)

85 εμφανίσεις

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

CO1

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

CO1

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

CO1

Three Categories of Material

Conduction
Band

Valance
Band

Electron Energy

Conductor

Semiconductor

Non
-
conductor

Overlap

Small Gap

Big Gap

CO1

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

CO1

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

CO1

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

CO1

Semiconductor

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

The examples are: Carbon, Silicon and
Germanium

CO1

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

CO1

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

CO1

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

CO1

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

CO1

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

CO1

N
-
type Semiconductor

Elements of Periodic Table Group V can be

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

CO1

N
-
type Semiconductor

CO1

N
-
type Semiconductor

4+

4+

5+

4+

4+

4+

4+

4+

4+

4+

4+

4+

4+

5+

4+

4+

5+

4+

extra free
electrons

CO1

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

CO1

P
-
type Semiconductor

Elements of Periodic Table Group III can be

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

CO1

P
-
type Semiconductor

CO1

P
-
type Semiconductor

4+

4+

3+

4+

4+

4+

4+

4+

4+

4+

4+

4+

4+

3+

4+

4+

3+

4+

extra holes

CO1

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

CO1

PN Junction

CO1

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

CO1

PN Junction

N

P

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

Depletion
Region

Electron migration

CO1

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

CO1

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
)

CO1

Diode

CO1

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

CO1

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

CO1

Reverse Biasing Diode

N

P

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

Depletion
Region
Increased

+

+

+

+

+

+

+

+

+

e
-

e
-

e
-

e
-

e
-

e
-

CO1

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

CO1

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

CO1

Forward Biasing Diode

N

P

+

+

+

+

+

+

+

+

+

Depletion
Region
Decreased

e
-

e
-

e
-

e
-

e
-

e
-

CO1

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

CO1

Forward Biasing Diode

N

P

e
-

e
-

e
-

e
-

e
-

e
-

> 0.75V

current flow

cathode

anode

CO1

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

CO1

I
-
V Characteristics

I

V

Reverse Bias

Forward Bias

Breakdown

V
D

= 0.75V

CO1

Ideal Model

I

V

Reverse Bias

Forward Bias

ON

(Forward Bias)

OFF

(Reverse Bias)

CO1

Offset Model

I

V

Reverse Bias

Forward Bias

+

V
D

V
D

ON

(Forward Bias)

OFF

(Reverse Bias)

+

V
D

CO1

Diode Circuits

CO1

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

CO1

Half
-
Wave Rectifier

v
L

v
S

+

+

R
L

v
S

v
L

t

t

CO1

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

CO1

Full
-
Wave Rectifier

v
L

v
S

+

+

R
L

v
S

v
L

t

t

v
S

+

CO1

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
experience the same voltage polarity through it

CO1

Bridge Rectifier

v
L

+

R
L

v
S

+

CO1

Bridge Rectifier

v
L

+

R
L

v
S

+

v
S

v
L

t

t

CO1

Bridge Rectifier

v
S

v
L

t

t

v
L

+

R
L

v
S

+

CO1

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

CO1