01-PN Junction and Diodes

8/18/2019 01-PN Junction and Diodes

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EE

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112 Basic Electronics Dr. Faraz Akram


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• Electronic Devices: Conventional Current

Version, by Thomas L. Floyd


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Insulators:

• Material that does not conduct electrical current under normal condition•

Valence electrons are tightly bound to the atoms; therefore very few free electrons

•

e.g. rubber, plastic, glass, wood

Conductors:

• Material

that

easily conducts

electrical

current

• Valence electrons are

loosely

bound to the

atoms

• Copper, silver,

g

old, aluminum•

One valance electron, very loosely bound to atom

Semiconductors:

•

Material

that is between conductors and

insulators in

its ability to conduct electricalcurrent

•

Neither

a good conductor nor a good

insulator in its pure state

• Silicon, germanium•

Four valance electrons


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Insulator Semiconductor Conductor

(a) (b) (c)


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Bohr diagram of the silicon (Semiconductor) and copper(Conductor) atoms

• The valence electron in the silicon atom “feels” an attractive force of +4

• The valence electron in the copper atom “feels” an attractive force of +1

Core has a net charge of

+4 (14p

-

10e)

net charge +

1

(29p

-

28e)


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EE-112 Basic Electronics Dr. Faraz Akram

•

Intrinsic Semiconductor:• Pure semiconductor containing no impurities

• Extrinsic Conductors

– Impure semiconductors

» P-type

» N-type


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• Both silicon and germanium have the characteristic four valence electrons.

• The valence electrons in germanium are in the fourth shell while those in silicon

are in the third shell, closer to the nucleus.

• This means that the germanium valence electrons require a smaller amount of

additional energy to escape from the atom.


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Energy band diagram for an unexcited (no external energy such as heat) atom in a

pure silicon crystal. This condition occurs only at a temperature of absolute 0 Kelvin.


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An intrinsic (pure) silicon crystal at room temperature has sufficient

heat (thermal) energy for some valence electrons to jump the gap from

the valence band into the conduction band, becoming free electrons.

Free electrons are also called conduction electrons .


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Dopping

Two categories of impurities

• n-type Adding Group V dopant such as arsenic (As), phosphorus

(P), bismuth (Bi), and antimony (Sb)

•p-type Adding Group III dopant such as boron(B), Indium (In),

gallium (Ga)

*Doping: Adding impurities to intrinsic semiconductors

Doping increases the number of current carriers (Electronsor holes)


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N-Type Material

N-Type Material:

An n-type material is produced when extra

valence electrons are introduced into a

material (silicon) by putting impurities or

dopants (Group V elements) into thesilicon.

The diagram shows the extra electron that

will be present when a Group V dopant is

introduced to a material such as silicon.This extra electron is very mobile.

+4+4

+5

+4

+4+4+4

+4+4


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P-Type Material

P-Type Material:

P-type material is produced when the dopant

that is introduced is from Group III. Group

III elements have only 3 valence electrons

and therefore there is an electron missing.

This creates a hole (h+), or a positive charge

that can move around in the material.

The diagram shows the hole that will be

present when a Group III dopant is

introduced to a material such as silicon.This hole is quite mobile in the same way the

extra electron is mobile in a n-type material.

+4+4

+3

+4

+4+4+4

+4+4


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p-n junction is the arrangement obtained when a p-type semiconductor is

joined to a n-type semiconductor

P-N Junction

Note:

P has a surplus of holes.

N has a surplus of electrons.


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As soon as p-n junction is formed electrons from n type material and

holes from p-type material diffuse into p-type and n-type material

respectively.

P-N Junction

Electrons and holes combine at junction.

Each recombination eliminates as electron and a hole.


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As a result

• n-region loses free electrons, this creates a layer of positive charge

near the junction.

• p-region loses holes as the electrons and hole combine. This

creates a layer of negative charge near the junction.

Depletion region:

The region of the p-n junction containing the uncompensated

receptor and donor ions is known as depletion region

The Width of

depletion region isExaggerated for

illustration

Note


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P N

+

+

+-

-

-

-

-

- +

+

+

Diode

Diodes are electronic components functions as a one-way valve

it means it allow current to flow in one direction.


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DiodeA diode is made from a small piece of semiconductor material,

usually silicon, in which half is doped as a p-region and half is

doped as an n-region with a PN junction and depletion region inbetween.

The p region is called the anode and is connected to a

conductive terminal. The n region is called the cathode and is

connected to a second conductive terminal


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Forward BiasPositive terminal of battery to P region

Negative terminal of the battery to N region

+

+

+-

-

-

-

-

- +

+

+

+

+

+

+

+

+

+

+

+ -

-

-

-

-

-

-

-

-

N-typeP-type

+ -

+ -

Vapplied


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Forward Bias

Effect of forward bias on depletion region

+

+

+-

-

-

-

-

- +

+

+

+

+

+

+

+

+

+

+

+ -

-

-

-

-

-

-

-

-

N-typeP-type

+ -

+ -

Positive terminal

repels holes

Negative terminal

repels electrons

Vapplied

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Forward Bias

Due to this repulsion, the depletion region narrows down

+

+

+-

-

-+

+

+

+

+

+

+

+

+ -

-

-

-

-

-

-

-

-

N-typeP-type

+ -

+ -

+

+

+ -

-

-

Vapplied

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Forward BiasIf the voltage is above a specified range, electrons in the N-region

drifts through the junction and migrates to the P region. And

holes from p region migrate to N- region.

+

+

+-

-

-+

+

+

+

+

+

+

+

+ -

-

-

-

-

-

-

-

-

N-typeP-type

+ -

+ -

+

+

+ -

-

-

Now the current flows across the circuit

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Reverse Bias

Positive terminal of battery to N region

Negative terminal of the battery to P region

+

+

+-

-

-

-

-

- +

+

+

+

+

+

+

+

+

+

+

+ -

-

-

-

-

-

-

-

-

N-typeP-type

- +

- +

Vapplied

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Reverse Bias

+

+

+-

-

-

-

-

- +

+

+

+

+

+

+

+

+

+

+

+ -

-

-

-

-

-

-

-

-

N-typeP-type

- +

- +

Holes get attracted to

Negative terminal of the

battery

Electrons get attracted to

the positive terminal of

the battery

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Reverse Bias

As a result, the depletion region increases

+

+

+-

-

-

-

-

- +

+

+

+

+

+

+

+

+

+

+

+ -

-

-

-

-

-

N-typeP-type

- +

- +-

-

-

Now the PN junction acts as an insulator and will not allow

any current to flow in the circuit

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Forward Bias vs Reverse Bias

Reverse BiasForward Bias

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• What is an Ideal Diode?

An ideal diode is a diode that acts like a perfect conductor when

voltage is applied forward biased and like a perfect insulator

when voltage is applied reverse biased.

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Voltage Current (v-i) Characteristics

of a Diode

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1. Ideal Diode Model2. Practical Diode Model

3. Complete Diode Model

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1

Ideal Diode Model


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1-Ideal Diode Model

The Ideal Diode

Model• Least accurate approximation and can be

represented by a simple switch.

• In many situations, using the ideal diodeapproximation is acceptable.

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Example: Ideal Diode Model


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Example: Ideal Diode Model

Example: Assume the diode in the circuit below is ideal. Determine the

value of ID ifa) VA = 5 volts (forward bias) and

b) VA = -5 volts (reverse bias)

+

_ VA

ID

RS = 50

a) With VA > 0 the diode is in forward bias

and is acting like a perfect conductor so:

ID = VA /RS = 5 V / 50 = 100 mA

b) With VA

< 0 the diode is in reverse bias

and is acting like a perfect insulator,

therefore no current can flow and ID = 0.

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2

-

Practical Diode Model


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2-Practical Diode Model

The Ideal Diode with

Barrier PotentialThis model is more accurate than the simple

ideal diode model because it includes the

approximate barrier potential voltage.Remember the barrier potential voltage is the

voltage at which appreciable current starts to

flow.

+

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Example: Practical Diode Model


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Example: Practical Diode Model

Example: To be more accurate than just using the ideal diode model

include the barrier potential. Assume VF = 0.3 volts (typical for agermanium diode) Determine the value of ID if VA = 5 volts (forward bias).

With VA > 0 the diode is in forward bias and

is acting like a perfect conductor so write a

KVL equation to find ID:

0 = VA – IDRS - VF

=

−

=

.7

= 94

+

_ VA

ID

RS = 50

VF

+

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3

-

Complete

Diode Model


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3-Complete Diode Model

• This model is the most accurate of the three. It Includes

• Barrier potential

• Small forward dynamic resistance ( )

• Large internal reverse resistance ( )

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Example:

Complete

Diode Model


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Example: Complete Diode Model

Example: Assume the diode with a forward resistance value of 5 ohms.

The barrier potential voltage is still: VF

= 0.3 volts. Determine the value

of ID if VA = 5 volts.

+

_ VA

ID

RS = 50

VF

+

r d

Once again, write a KVL equation

for the circuit:

0 = VA – IDRS - VF - IDr d

ID = VA - VF = 5 – 0.3 = 85.5 mA

RS + r d 50 + 5

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Diode Circuit Models


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Diode Circuit Models

Values of ID for the Three Different Diode Circuit Models

Ideal Diode

Model

Practical Diode

Model

Complete Diode

Model

ID 100 mA 94 mA 85.5 mA

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Example


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Example

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01-PN Junction and Diodes

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