ECE101 Modules 1& 2 Semiconductor Theory Diode

8/4/2019 ECE101 Modules 1& 2 Semiconductor Theory Diode

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Semiconductor Theory &

Modules 1 & 2

By

Dr. Bernie Redoña

Reference: Electronic Devices and Circuit Theory by Boylestad & Nashelsky, 10th Ed



Have you seen a semiconductordevice???

1



Semiconductor Diode

ESD Protectiondiode in compact

2

2.0 x 1.6 x 1.6 mmSurface MountDevice (SMD)

Package



Semiconductor ComponentCircuitry

Semiconductor

3

circuitrycompared to

strand of hair.



1. Discuss the concept of atomic theory, and the

subatomic particles of the atom such as the

electron, proton, and neutron.

LEARNING OUTCOMES:

2. Identify and differentiate conductors,

semiconductors and insulators.

3. Discuss the crystal structure of the commonsemiconductor materials and ions formed from

covalent bonding.



4. Explain the general characteristics of three

important semiconductor materials (Ge, Si, andGaAs.

LEARNING OUTCOMES:

.

semiconductors using electron and hole theory.

6. Differentiate and describe the difference between

n-type and p-type materials.

7. Explain what happens in a diode during no bias,

forward bias, and reverse bias conditions.



ATOMIC THEORY:Discovery of the nucleus: The goldfoil experimentExpected results: alpha particles passing

through the plum pudding model of the atomwith negligible deflection.

Observed results: a small ortion of the

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particles were deflected by the concentratedpositive charge of the nucleus.

Thomson's plum pudding model wasdisproved in 1909 by one of his formerstudents, Ernest Rutherford, who discoveredthat most of the mass and positive charge ofan atom is concentrated in a very smallfraction of its volume, which he assumed to be

at the very center.



ATOMIC THEORY:In the gold foil experiment, alpha particles wereshot at a thin sheet of gold, measuring their

deflection with a fluorescent screen.

Given the very small mass of the electrons, thehigh momentum of the alpha particles and the

unconcentrated distribution of positive charge ofthe plum pudding model, the experimenters

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expec e a e a p a par c es o pass roug egold sheet without significant deflection.

To their astonishment, a small fraction of thealpha particles experienced heavy deflection.

This led Rutherford to propose a planetary modelin which a cloud of electrons surrounded a small,

compact nucleus of positive charge. Only such aconcentration of charge could produce the electricfield strong enough to cause the heavy

deflection.

[12]



ATOMIC THEORY:The Bohr model of the atom

Quantum theory revolutionized physics at the beginningof the 20th century, when Max Planck and Albert Einstein

postulated that light energy is emitted or absorbed indiscrete amounts known as quanta.

In 1913, Niels Bohr incorporated this idea into his Bohrmodel of the atom, in which an electron could only orbit

8

t e nuc eus n part cu ar c rcu ar or ts w t xe angu ar

momentum and energy, its distance from the nucleus(i.e., their radii) being proportional to its energy.

Under this model an electron could not spiral into thenucleus because it could not lose energy in a continuousmanner; instead, it could only make instantaneous

“quantum leaps” between the fixed energy levels.

When this occurred, light was emitted or absorbed at afrequency proportional to the change in energy (hencethe absorption and emission of light in discrete spectra).



NOTE: The construction of every discrete (individual) solid-

state (hard crystal structure) electronic device or

integrated circuit begins with a semiconductor material

SEMICONDUCTOR MATERIALS:Ge, Si and GaAs

.

9

RECALL: Semiconductors are a

special class of elements

having a conductivity between

that of a good conductor and

that of an insulator.



SEMICONDUCTOR MATERIALS:Ge, Si and GaAs

Two (2) General Classes of Semiconductor Materials:

Single crystal – like Germanium (Ge) and Silicon having a

repetitive crystal structure

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Compound – like Galium Arsenide (GaAs)), Cadmium Sulfide(CdS), Galium Nitride (GaN) and Galium Arsenide Phosphide

(GaAsP) constructed of two or more semiconductor materials

of different atomic structure

NOTE: The three semiconductors used most frequently

in the construction of electronic devices are Ge, Si

and GaAs.



SILICON CRYSTAL STRUCTURE

11



SEMICONDUCTOR MATERIALS:

Ge, Si and GaAs

Discovery

of Diode

1939 1970

First Silicon

Transistor

1954

Development of

GaAs Transistor

Discovery

of Transistor

1947

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• easy to find• available in fairly large quantities• sensitive to changes in temperature

Aluminum as Material of Choice

• less temperature sensitive• abundant material on earth

GaAs developed • demand for increased speed• speed 5X of Si but more

expensive



Covalent Bonding and Intrinsic Materials

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+ +

Silicon Germanium




14

Gallium Arsenic



It is still possible for the valence electrons to absorb sufficient kinetic energy from external natural

causes to break the covalent bond and assume the “free” state. (Room Temp = 1.5 X 10E15 free carriers


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energy (photons) and thermal energy (heat) from the surrounding medium.

The term intrinsic is applied to any semiconductor

material that has been carefully refined to reduce the number of impurities to a very low level – essentially as pure as can be made available thru modern technology.



The free electrons in a material due only to external causes are referred to as intrinsic carriers.

The number of carriers in the intrinsic form is important, but other characteristic such as relative mobility of the free carriers to move


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t roug out t e mater a , are more s gn cant n determining its use in the field.

SemiconductorIntrinsic Carriers(per cubic centimeter)

GaAs 1.7 x 106

Si 1.5 x 1010

Ge 2.5 x 1013

SemiconductorMobility Factor

(cm2 / V.s)

Si 1500

Ge 3900

GaAs 8500



Semiconductors and conductors differ in their

reaction to heat.

For conductors, the resistance increases with anincrease in heat, hence having a positive temperature


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coefficient.

Semiconductor materials exhibit an increased level ofconductivity with the application of heat, having a

negative temperature coefficient.



Energy Levels The farther an electron is from the nucleus, the higher is the energy state,

and any electron that has left its parent atom has a higher energy state than

any electron in the atomic structure.

Energy Energy Energy

SEMICONDUCTOR CONDUCTOR INSULATOR

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Valence Band

-- - -

Conduction Band

Valence Band

-- - -

--

--

Eg > 5 eV

Conduction Band

Valence Band

-- - -

-- - -Conduction Band



INSULATORS, SEMICONDUCTORS,CONDUCTORS

The separation of the valence and conduction bands determines the

electrical properties of the material

Insulators have a large energy gap,electrons can’t jump from

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Conductors (metals) have a very small (or nonexistent) energy

gap, electrons easily jump to conduction bands due tothermal excitation, current flows easily

Semiconductors have a moderate energy gap, only a few

electrons can jump to the conduction band leaving “holes”only a little current can flow



Conduction Band

-- - -

Energy

Eg = 0.67 eV (Ge)

Eg = 1.1 eV (Si)

Eg = 1.43 eV (GaAs)

Energy Levels

21

An electron in the valence band of Silicon must absorb more energy than

one in the valence band of Germanium to become a free carrier. Similarly,an electron in the valence band of Galium Arsenide must gain more energy than one in Silicon or Germanium to enter the conduction band.

Valence Band

-- - -

SEMICONDUCTOR



QUESTION: If I am designing the following, what

semiconductor material shall I use???

1. Photodetectors sensitive to light

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2. Security systems sensitive to heat

3. Transistor networks requiring stability

SILICON

GERMANIUM

GERMANIUM



The characteristics of a semiconductor material can be alteredsignificantly by the addition of specific impurity atoms to

the relatively pure semiconductor material. This process iscalled doping .

A semiconductor material that has been sub ected to the

Extrinsic Materials: n-Type and p-Type

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doping process is called an extrinsic material .

There are two extrinsic materials of immeasurable importanceto semiconductor device fabrication: n-type and p-type

materials.

Both n-type and p-type materials are formed by adding apredetermined number of impurity atoms to a silicon base.



Extrinsic Materials: n-Type

Conduction Band

Energy

Eg for intrinsic

Eg = considerablyless than for intrinsic

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Valence Band

-- - -

-- - -materials Donor energy level

Effect of donor impurities on the energy band structure



The p-type material is formed by doping a pure germanium or silicon crystalwith impurity atoms having three valence electrons (trivalent) such asboron, gallium and indium.

SiSi Si- -- - --

- --

Extrinsic Materials: p-Type

Diffused impurities withthree valence electronsare called acceptor

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Si B Si

Si Si Si --

-

--

--

-

-

-

-

- -

-

-

-

-

-

-

-

- --

- --

Void( + or O)

Boron (B)impurity



- --

Electron versus Hole Flow

-

If a valence electron acquires enough kinetic energy to break its covalent bondand fills the void created by a hole, then a vacancy or a hole will becreated in the covalent bond.

There will be transfer of holes to the left and electrons to the right.

The conventional flow is the direction of the hole flow.

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In an n-type, the electron is the majority carrier and the hole the minority carrier.

In a p-type, the hole is the majority carrier and the electron the minority carrier.

SiSi Si

Si B Si

- -

-

- -

--

-

-

-

-

+ -

-

-

--

- --

Hole flowSi

Si

-

-

-

-

-

-

-

-

Electron flow



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Semiconductor Diode:With the availability of n-type and p-type materials, the first solid-state

electronic device can be constructed, the SEMICONDUCTOR DIODE.

Created by simply joining an n-type and a p-type material together.

Can be found in numerous applications.

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SEMICONDUCTOR DIODE:

The number of uncovered positive ions in the depletion region of the n-typematerial will increase due to the large number of free electrons drawn tothe positive potential of the applied voltage.

Similarly, the number of uncovered negative ions will increase in the p-typematerial.

The net effect is the widening of the depletion region. This will establish toogreat a barrier for the majority carriers to overcome, effectively reducing

Reverse-Bias Condition (VD < 0 V)

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the majority carrier flow to zero.

I S I S

V D

+-

p n

-

-

-

-

-+

+

+

+

-

-

---

--

+++

++

---

--

---

--

+++

++

+++

++

+

++

+

++

-

-

-

-

-

-+ +

Depletion region

I S Minority-carrier flow

- +

I majority ~= 0 A

The current thatexists under reverse-

bias conditions iscalled the reversesaturation current and

is represented by I s .

SEMICONDUCTOR DIODE




The reverse saturation current is seldom more than a few microamperes,except for high power devices, and typically in the nanoampere rangefor silicon devices.

The term saturation comes from the fact that it reaches its maximum levelquickly and does not change significantly with increases in the reverse-bias potential.

Reverse-Bias Condition (VD < 0 V)

34

I S I S

V D

+-

p n

--

-

-

-+

+

+

+

-

-

-----

+++++

-----

-----

+++++

+++++

+

+

+

+

++

-

-

-

-

-

-++

Depletion region

I S Minority-carrier flow

- +

I majority ~= 0 A

SEMICONDUCTOR DIODE




Also known as “on” condition, and is established by applying the positivepotential to the p-type material and the negative potential to the n-type material.

Forward-Bias Condition (VD > 0 V)

- -

I S

I majority

I D = I majority - I S

-

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I D I D

V D

+ -

p n

--

-

-

-+

+

+-

-

-

-

----

++++

+

++ +

+-

-

-

-

-

-+

Depletion region

-+

D

I D

-++ +

+

+

++

+ -

n p

(Similar)

SEMICONDUCTOR DIODE




The application of a forward-bias potential VD will “pressure” electrons in then-type material and holes in the p-type material to recombine with theions near the boundary and reduce the width of the depletion region.

The resulting minority carrier flow of electrons from the p-type material to then-type material (and of holes from the n-type material to the p-type

material) has not changed in magnitude, but the reduction in the width ofthe de letion re ion has resulted in a heav ma orit flow across the


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junction.

I D I D

V D + -

p n

-

-

-

-

-+

+

+

+-

-

-

-

-

-----

+++++

++

+

+

+

+-

-

-

-

-

-+

Depletion region

I S

-+

I majority


++ +

+

+

+

-

+

-

SEMICONDUCTOR DIODE




An electron of the n-type material now “sees” a reduced barrier at the junctiondue to the reduced depletion region and a strong attraction for thepositive potential applied for the p-type material.

As the applied bias increases in magnitude, the depletion region will continueto decrease in width until a flood of electrons can pass through the junction, resulting in an exponential rise in current.


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I D I D

V D + -

p n

-

-

-

-

-+

+

+

+-

-

-

-

-

-----

+++++

++

+

+

+

+-

-

-

-

-

-+

Depletion region

I S

-+

I majority


++ +

+

+

+

-

+

-

SEMICONDUCTOR DIODE




Shockley’s Equation: (forward- and reverse-bias regions)


I D = I S ( e V D / nV T - 1 )where:

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I s is the reverse saturation current

V D is the applied forward-bias voltage across the diode

n is the ideality factor, which is the function of the

operating conditions and physical construction; it has

a range between 1 and 2 depending a wide varietyof factors

V T is called the thermal voltage






V T =

The thermal voltage VT is determined by:

kT q

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Where:

k is Boltzmann’s constant = 1.38 X 10-23 J/K

T is the absolute temperature in kelvins = 273 +

temperature in°C

q is the magnitude of electronic charge = 1.6 x 10-19 C





Forward-Bias Condition (VD > 0 V)EXAMPLE: At a temperature of 27 C (common temperature forcomponents in an enclosed operating system), determine the thermal

voltage VT.

SOLUTION: Substituting to the equation, we obtain

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V T = 25.875 mV

(1.38 x 10 -23 J/K) (300 K)

1.6 x 10 -19 C

T = 273 + °C = 273 + 27 = 300 K

V T =



SUMMARY: A semiconductor is a material that has a conductivity level somewherebetween that of a good conductor and that of an insulator.

A bonding of atoms, strengthened by the sharing of electrons betweenneighboring atoms, is called covalent bonding.

Increasing temperatures can cause a significant increase in the numberof free electrons in a semiconductor material.

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Most semiconductor materials used in the industry have negativetemperature coefficients, i.e., the resistance drops with an increase intemperature.

Intrinsic materials are those semiconductors that have very low level of

impurities, whereas extrinsic materials are semiconductors that have beenexposed to a doping process.

An n-type material is formed by adding donor atoms that have fivevalence electrons – the electron is the majority carrier and the hole is the

minority carrier.



SUMMARY:

A p-type material is formed by adding acceptor atoms with three valenceelectrons – the hole is the majority carrier and the electron is the minoritycarrier.

The region near the junction of a diode that has very few carriers iscalled the depletion region.

In the absence of any externally applied bias, the diode current is zero.

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The characteristics of an ideal diode are a close match with those of asimple switch except for the important fact that an ideal diode can conductin only one direction.

The ideal diode is a short in the region of conduction and an open circuit

in the region of non-conduction.

In the forward-bias region the diode current increases exponentially withincrease in voltage across the diode.



SUMMARY:

In the reverse-bias region the diode current is the very small reversesaturation current until Zener breakdown is reached and current will flow inthe opposite direction through the diode.

The threshold voltage for silicon diodes is about 0.7 V, and forgermanium diodes is 0.3 V.

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Realize that life is a school and you Realize that life is a school and you Realize that life is a school and you Realize that life is a school and you are here to learn… Problems areare here to learn… Problems areare here to learn… Problems areare here to learn… Problems are

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simply part of the curriculum that simply part of the curriculum that simply part of the curriculum that simply part of the curriculum that appear and fade away like Algebra appear and fade away like Algebra appear and fade away like Algebra appear and fade away like Algebra

class but the lessons you learn will class but the lessons you learn will class but the lessons you learn will class but the lessons you learn will last a lifetime…last a lifetime…last a lifetime…last a lifetime…



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ECE101 Modules 1& 2 Semiconductor Theory Diode

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Transcript of ECE101 Modules 1& 2 Semiconductor Theory Diode