EE 2109 Electronics-I · EE 2109 Electronics-I Credit: 3, Contact Hours: 3 Hrs/Week Syllabus...

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EE 2109 Electronics-I

Dr. Mostafa Zaman Chowdhury

Dept. of Electrical and Electronic Engineering, KUET

Presentation #1

EE 2109 Electronics-I

Credit: 3, Contact Hours: 3 Hrs/Week

Syllabus Introduction: Properties of Insulators, Semiconductors, and Metals; Conduction in

solids, Conventional current and electron flow, Drift and diffusion current, Mobility and Conductivity. The potential barrier; work function; contact potential. The Hall Effect and Hall devices.

Semiconductors: Intrinsic Semiconductors: Crystal and energy band diagram, Electrons and holes, conduction in semiconductors, Electron and hole concentration. Extrinsic semiconductors: n-type doping, p-type doping, and compensation doping, Temperature dependence of conductivity, Carrier Concentration Temperature Dependence, Degenerate and non-degenerate semiconductors. Diffusion and conduction equations, random motion and continuity equation, Time-dependent continuity equation, Steady-state continuity equation.

Semiconductor diode characteristics: Qualitative and Quantitative theory of the p-n junction as a diode; Ideal pn junction, pn junction band diagram, current components in p-n diode; Volt-ampere characteristics; Transition and diffusion capacitance, Dynamic resistance, Reverse breakdown; Avalanche and Zener breakdown; Zenerdiode, Rectifier Diode: controlled & uncontrolled rectification, Special-Purpose Diodes: Tunnel diode, varactor diode, and breakdown diode; Metal oxide semi-conductor diode, optical diode, PIN diode, Schottky diode, Current regulator diode.

Transistor: Transistor and its current components, transistor as an amplifier, BJT, Different transistor configurations, study of load lines, transistor switching times, detailed study of transistor biasing and thermal stabilization.

Dept. of Electrical and Electronic Engineering, KUET 2

References

Electronic Devices and Circuit Theory by Robert L.

Boylestad

Principle of Electronic Materials and Devices by S O

Kasap

Electronic Devices and Circuits by Jacob Millman

Principle of Electronics by V. K. Mehta

3Dept. of Electrical and Electronic Engineering, KUET

Semiconductor Materials


Materials commonly used in the development of semiconductor devices Silicon (Si)

Germanium (Ge)

Gallium Arsenide (GaAs)

Doping

The electrical characteristics of silicon and germanium are improved by adding materials in a process called doping.

There are just two types of doped semiconductor materials n-type

p-type

n-type materials contain an excess of conduction band electrons.

p-type materials contain an excess of valence band holes.



Majority and Minority Carriers

Two currents through a diode

Majority Carriers

− The majority carriers in n-type materials are electrons.

− The majority carriers in p-type materials are holes.

Minority Carriers

− The minority carriers in n-type materials are holes.

− The minority carriers in p-type materials are electrons.

p-n Junctions (1/2)

One end of a silicon or germanium crystal can be doped as a

p-type material and the other end as an n-type material.

The result is a p-n junction



p-n Junctions (2/2)

At the p-n junction, the excess conduction-band electrons on the n-type side are attracted to the valence-band holes on the p-type side.

The electrons in the n-type material migrate across the junction to the p-type material (electron flow).

The electron migration results in a negative charge on the p-type side of the junction and a positive charge on the n-type side of the junction.

The result is the formation of a

depletion region around the

junction.

Energy Levels


Extrinsic Materials (1/2)

A semiconductor material that

has been subjected to the doping

process is called an extrinsic

material

n-Type Material

The n-type is created by

introducing those impurity

elements that have five valence

electrons (pentavalent), such as

antimony, arsenic, and

phosphorus.


Extrinsic Materials (2/2)

p-Type Material

The p-type material is formed

by doping a pure germanium

or silicon crystal with impurity

atoms having three valence

electrons


Diodes


The diode is a 2-terminal

device. A diode ideally conducts in only one

direction.

The ideal diode, therefore, is a short circuit for the region of conduction

The ideal diode, therefore, is an open circuit in the region of

nonconduction.

Diode Characteristics

Conduction Region

The voltage across the diode is 0 V

The current is infinite

The forward resistance is defined

as RF = VF / IF

The diode acts like a short


Non-conduction Region

All of the voltage is across the

diode

The current is 0 A

The reverse resistance is

defined as RR = VR / IR

The diode acts like open

Diode Operating Conditions

A diode has three operating conditions

No bias

Forward bias

Reverse bias


Diode Operating Conditions- No Bias

No external voltage is applied: VD = 0 V

No current is flowing: ID = 0 A

Only a modest depletion region exists


Diode Operating Conditions- Reverse Bias

External voltage is applied across the p-n junction in the opposite

polarity of the p- and n-type materials.

The reverse voltage causes the depletion region to widen.

The electrons in the n-type material are attracted toward the positive

terminal of the voltage source.

The holes in the p-type material are attracted toward the negative

terminal of the voltage source.


Diode Operating Conditions- Forward Bias

External voltage is applied across the p-n junction in the

same polarity as the p- and n-type materials.

The forward voltage causes the depletion region to narrow

The electrons and holes are pushed toward the p-n junction

The electrons and holes have sufficient energy to cross the p-

n junction.


Actual Diode Characteristics

Note the regions for

no bias, reverse

bias, and forward

bias conditions.

Carefully note the

scale for each of

these conditions.


Zener Region

The Zener region is in the diode’s

reverse-bias region.

At some point the reverse bias voltage is

so large the diode breaks down and the

reverse current increases dramatically

The maximum reverse-bias potential

that can be applied before entering the

Zener region is called the peak inverse

voltage (referred to simply as the PIV

rating) or the peak reverse voltage

(denoted by PRV rating).

The voltage that causes a diode to enter

the zener region of operation is called

the zener voltage (VZ).


Forward Bias Voltage

The point at which the diode changes from no-bias

condition to forward-bias condition occurs when the

electrons and holes are given sufficient energy to cross the

p-n junction. This energy comes from the external voltage

applied across the diode.

The forward bias voltage required for a

gallium arsenide diode 1.2 V

silicon diode 0.7 V

germanium diode 0.3 V


Temperature Effects

As temperature increases it adds energy to the

diode.

It reduces the required forward bias voltage for

forward-bias conduction.

It increases the amount of reverse current in the

reverse-bias condition.

It increases maximum reverse bias avalanche voltage.

Germanium diodes are more sensitive to temperature

variations than silicon or gallium arsenide diodes.


Resistance Levels

Semiconductors react differently to DC and AC

currents.

There are three types of resistance DC (static) resistance

AC (dynamic) resistance

Average AC resistance


AC (Dynamic) Resistance

In the forward bias region

• The resistance depends on the amount of current (ID) in the diode.

• The voltage across the diode is fairly constant (26 mV for 25C).

• rB ranges from a typical 0.1 for high power devices to 2 for low power, general purpose

diodes. In some cases rB can be ignored.

In the reverse bias region


B

D

d rI

r mV 26

rd

Average AC Resistance

AC resistance can be calculated using the current and voltage values for

two points on the diode characteristic curve.


pt. to pt. d

dav

ΔI

ΔV r

Diode Equivalent Circuit


Diode Capacitance

In reverse bias, the depletion layer is very large. The diode’s strong positive

and negative polarities create capacitance, CT. The amount of capacitance

depends on the reverse voltage applied.

This is transition or depletion region capacitance

In forward bias storage capacitance or diffusion capacitance (CD) exists as the

diode voltage increases.


Reverse Recovery Time (trr)

Reverse recovery time is the time required for a diode to stop

conducting once it is switched from forward bias to reverse bias.


Diode Specification Sheets

1. Forward Voltage (VF) at a specified current and temperature

2. Maximum forward current (IF) at a specified temperature

3. Reverse saturation current (IR) at a specified voltage and

temperature

4. Reverse voltage rating, PIV or PRV or V(BR), at a specified

temperature

5. Maximum power dissipation at a specified temperature

6. Capacitance levels

7. Reverse recovery time, trr

8. Operating temperature range


Zener Diode

A Zener is a diode operated in reverse bias at the Zener

voltage (VZ).

Common Zener voltages are between 1.8 V and 200 V

Wireless Networks & Communications Lab. 29

Light-Emitting Diode (LED)

An LED emits photons when it is forward biased

These can be in the infrared or visible spectrum

The forward bias voltage is usually in the range of 2 V to 3 V.


Difference Between Zener Breakdown and

Avalanche Breakdown The Zener effect is a type of electrical breakdown in a reverse biased p-n diode in

which the electric field enables tunneling of electrons from the valence to the

conduction band of a semiconductor, leading to a large number of free minority

carriers, which suddenly increase the reverse current. Zener breakdown is employed

in a Zener diode.

Under a high reverse-bias voltage, the p-n junction's depletion region widens,

leading to a high strength electric field across the junction. A sufficiently strong

electric field enables tunneling of electrons across the depletion region of a

semiconductor leading to a large number of free charge carriers. This sudden

generation of carriers rapidly increases the reverse current and gives rise to the high

slope conductance of the Zener diode.



Avalanche Breakdown Zener breakdown:

1)heavily doped therefore have narrow depletion layer

2)strong electric field is developed across this narrow layer.

3)covalent bonds break due to very strong electric field so even a small

amount of reverse voltage is capable of producing large number of current

carriers.

zener breakdown voltage is less than avalanche breakdown.

Avalanche Breakdown needs collision of minority carriers and covalent

bonds to provide electrons and holes. This needs higher potential cos the

minority carriers need to be developed in sufficient numbers to get

sufficient current resulting in break down

Avalanche breakdown is a phenomenon that can occur in both insulating

and semiconducting materials

The voltage at which the breakdown occurs is called the breakdown

voltage.



Avalanche Breakdown Zener Breakdown

This occurs at junctions

which being heavily doped

have narrow depletion layers

This breakdown voltage sets a

very strong electric field

across this narrow layer.


Avalanche breakdown

This occurs at junctions which being

lightly doped have wide depletion layers.

Here electric field is not strong enough

to produce Zener breakdown.

Here minority carriers collide with semi

conductor atoms in the depletion region,

which breaks the covalent bonds and

electron-hole pairs are generated. Newly

generated charge carriers are

accelerated by the electric field which

results in more collision and generates

avalanche of charge carriers. This

results in avalanche breakdown.

E=V/d

* Do not confused that diode is

damaged with any of the

breakdowns. Damaged depends

on the current rating

EE 2109 Electronics-I · EE 2109 Electronics-I Credit: 3, Contact Hours: 3 Hrs/Week Syllabus...

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Transcript of EE 2109 Electronics-I · EE 2109 Electronics-I Credit: 3, Contact Hours: 3 Hrs/Week Syllabus...