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ELECTRONICS 1

EEE 231 (3+1)

Dr. Naeem Shehzad

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Grading Policy

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� Assignments 10%

� Minimum 3

� Quizzes (scheduled/surprised) 15%

� Minimum 3

� Midterms 25%

� Sessional 1 10%

� Sessional 2 15%

� Final exam 50%

Electronics ?

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Electronics ?

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Objectives

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1. Basic knowledge of semiconductor devices

2. To analyze a given circuit

3. To design an optimized circuit according to

given requirements

4. To be familiar with the commonly used

configurations

Advantages of semiconductor Devices

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1. Small in size

2. Low power consumption

3. Long life

4. Low operating voltages

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Overview

� Introduction to semiconductors

� What are P-type and N-type semiconductors?

� What are Diodes?

� Forward Bias & Reverse Bias

� Characteristics Of Ideal Diode

� Shockley Equation

� I – V Characteristics of Diodes

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Introduction

� The goal of electronic materials is to generate and control the flow of an electrical current.

� Electronic materials include:� Conductors: have low resistance which allows

electrical current flow

� Insulators: have high resistance which suppresses electrical current flow

� Semiconductors: can allow or suppress electrical current flow

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Atomic Structure

� The highest energy band completely filled with

electrons (at T = 0 K) is called the Valence Band

� The next band is called the Conduction Band

� The energy difference between the bottom of the Conduction and the top of the Valence bands is called the Band Gap

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Insulators, Semiconductors, and Metals

The Band Gap determines the electrical properties of the material

� Insulators have a large energy gap (>5eV)� electrons can’t jump from valence to conduction bands� no current flows

� Conductors (metals) have a very small (or nonexistent) energy gap� electrons easily jump to conduction bands due to thermal

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

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Valence and Conduction Bands� The band structures of insulators and semiconductors

resemble each other qualitatively.

� Both in insulators and semiconductors a filled energy band (referred to as the valence band) separated from the next higher band (referred to as the conduction band) by an energy gap.

� If this gap is at least several electron volts, the material is an insulator. It is too difficult for an applied field to overcome that large energy gap, and thermal excitations lack the energy to promote sufficient numbers of electrons to the conduction band.

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Conductor Atomic Structure

� Good conductors have low resistance so electrons flow through them with ease.

� The atomic structure of good conductors usually includes only one electron in their outer shell.

� It is called a valence electron.

� It is easily striped from the atom, producing current flow.

Copper Atom

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Why Semiconductors?

Resistivity vs temperature�

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Commonly used Semiconductors

� Germanium (Ge)

� Good availability, Easy to refine it But Low level

of reliability and sensitive to temperature

� Silicon (Si)

� Less sensitive to temperature and abundantly

available But refining is complex

� Gallium Arsenide (GaAs)

� High speed (5 times that of Si) But costly and

temperature sensitive

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Semiconductor Valence Orbit

� The main characteristic

of a semiconductor

element is that it has

four electrons in its

outer or valence orbit.

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Crystal Lattice Structure

� The semiconductor atoms link together to form a physical structure called a crystal lattice.

� The atoms link together with one another sharing their outer electrons.

� These links are called covalent bonds. 2D Crystal Lattice Structure

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Intrinsic and extrinsic semiconductor

� Intrinsic = Pure semiconductor

� Extrinsic = Impure or doped semiconductor

“Doping means mixing a pure semiconductor with

impurities to increase its electrical conductivity”

Impurities change the conductivity of the material

so that it can be fabricated into a device

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N-Type (Doping with Penta-valent atoms)

� An impurity, or element like arsenic, antimony has 5 valence electrons.

� Adding arsenic (doping) will allow four of the arsenic valence electrons to bond with the neighboring silicon atoms.

� The one electron left over for each arsenic atom becomes available to conduct current flow.

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P-Type (Doping with Tri-valent atoms)

� Doping with an atom such as boron, gallium that has only 3 valence electrons.

� The 3 electrons in the outer orbit do form covalent. But one electron is missing from the bond.

� This place where a fourth electron should be is referred to as a hole.

� The hole assumes a positive charge so it can attract electrons from some other source.

� Holes become a type of current carrier like the electron to support current flow.

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Majority and Minority carrier

� In N-type

� Electrons are majority carriers

� Holes are minority carriers

� In P-type

� Holes are majority carriers

� Electrons are minority carriers

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Donner and Acceptor ions

� Group V impurities are called Donors, since they “donate” electrons into the Conduction Band

� Donor atom becomes an ion with +ve charge D+

� Group III impurities are called Acceptors since they “accept” an electron in valence band

� Acceptor atom becomes an ion with -ve charge A-

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Summary upto here� In its pure state, semiconductor is a poor conductor

� The commonly used semiconductor material is silicon.

� Semiconductor materials can be doped with other atoms to add or subtract electrons.

� An N-type semiconductor material has extra electrons.

� A P-type semiconductor material has a shortage of electrons with vacancies called holes.

� The heavier the doping, the greater the conductivity or the lower the resistance.

� By controlling the doping of silicon the semiconductor material can be made as conductive as desired.

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Diodes

� A diode is formed by putting a N-type and P-type

of semiconductor together

N typeP type

Anode Cathode

P-N Junction

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Diodes

N typP type

Anode

P-N Junction

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Diode

� Migration of holes from P to N and electrons from

N to P causes a formation of depletion layer

P type N type+

+

+

+

-

-

-

-

Anode Cathode-

-

-

-

+

+

+

+

This gives rise to barrier potential (Eγ) preventing

further migration of holes and electrons

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Energy bands in a unbiased diode

Energy

PN

Depletion layer

Conduction band

Valence band

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Forward biased diode

P type N type

+

+

+

+

-

-

-

-

+ -

R

VB

Anode Cathode

+ - Vγ

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Energy bands of a forward biased diode

Energy

PN

Smaller depletion layer

Conduction band

Valence band

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Forward Biased diode

� The diode behaves like a ‘ON’ switch in this

mode

� Resistance R and diode’s body resistance

limits the current through the diode

� VB has to overcome Vγ in order for the diode

to conduct

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Reverse Biased diode

P type N type

+-

+

+

+

+

+

+

+

+

+

+

+

+

-

-

-

-

-

-

-

-

-

-

-

-

Larger depletion layer

Anode Cathode

VB

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Energy bands in a reverse biased diode

Energy

PN

Larger Depletion layer

Valence band

Conduction band

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Reverse Biased diode

•The diode behaves like a ‘OFF’ switch in this mode

• If we continue to increase reverse voltage VB

breakdown voltage of the diode is reached

• Once breakdown voltage is reached diode conducts

heavily causing its destruction

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Forward Bias and Reverse Bias

� Forward Bias : Connect positive of the Diode

to positive of supply…negative of Diode to

negative of supply

� Reverse Bias: Connect positive of the Diode

to negative of supply…negative of diode to

positive of supply.

Diode Biasing

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• To forward bias a diode, the anode must be more positive than the cathode or LESS NEGATIVE

• To reverse bias a diode, the anode must be less positive than the cathode or MORE NEGATIVE

A Diode Puzzle

� Which lamps are alight

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Characteristics of Diode

� Diode always conducts in one direction.

� Diodes always conduct current when

“Forward Biased” ( Zero resistance)

� Diodes do not conduct when Reverse Biased

(Infinite resistance)

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Example from a daily life

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I-V characteristics of Ideal diode

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I-V Characteristics of Practical Diode

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Breakdown� Diode breakdown is caused by thermally generated

electrons in the depletion region

� When the reverse voltage across diode reaches breakdown voltage these electrons will get sufficient energy to collide and dislodge other electrons

� The number of high energy electrons increases ingeometric progression leading to an avalanche effect causing heavy current and ultimately destruction of diode

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Shockley Equation

−

= 1exp

T

DsD

nV

VIi q

kTVT =

Is is the saturation current ~10 -14

VD is the diode voltage

n – emission coefficient (varies from 1 - 2 )

k = 1.38 × 10–23 J/K is Boltzmann’s constant

q = 1.60 × 10–19 C is the electrical charge of an electron.

At a temperature of 300 K, we have

mV 26≅TV

The forward bias current is closely approximated by

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Shockley Equation

� For no bias situation VD = 0 ,

� For reverse voltage across diode,

� For forward voltage across diode

0)1)0(exp( =−= SD II

SD II −=

=

T

DsD

nV

VII exp

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Zener Diode

� Same as ordinary diode but

it is placed in the circuit in

reverse bias and operates in

reverse breakdown.

� Forward biased

Characteristics are same

� Available in range of 1.8 to

200 V breakdown voltages

� Break down voltage

depends on doping

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Zener Diode

� It maintains a specific voltage across its terminals

� Used for providing a stable reference voltage for use in power

supplies and other equipment

This particular zener circuit will work to maintain 10 V across the load.

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Light Emitting Diode (LED)

� A light emitting diode (LED) is essentially a PN

junction opto-semiconductor that emits a

monochromatic (single color) light when operated

in a forward biased direction.

� When the electron falls down from conduction band

and fills in a hole in valence band, there is an

obvious loss of energy.

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Light Emitting Diode (LED)

� The bandwidth of qaunta of light energy released is

approximately proportional to the band gap of the

semiconductor.

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Light Emitting Diode (LED)

� In Si and Ge diode, the energy is emitted in form of

heat and is insignificant.

� In GaAs diode, the emitted light is in infrared zone

(invisible light) .

� In GaN, GaP etc emitt visible light of different

colors and at different voltages when forward bias.

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Temperature Effects

� In forward bias region the characteristic of

silicon diode shift to left at rate of 2.5mv/oC

� In reverse bias, IS Silicon diode is doubles

after every 10 oC

� The reverse breakdown voltage depends on

the variation in temperature

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Types of resistances

� As the operating point of a diode moves from one region to

another the resistance of the diode will also change due to the

nonlinear shape of the characteristic curve

� The type of applied voltage or signal will define the

resistance level of interest

� Three different types of Diode resistances according to

applied voltage

� DC or Static Resistance

� AC or Dynamic Resistance

� Average AC Resistance

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DC Resistance of Diode

� The application of a dc voltage to

a circuit containing a

semiconductor diode will result in

an operating point on the

characteristic curve that will not

change with time

� The resistance of the diode at the

operating point can be found

simply by finding the

corresponding levels of VD and ID

� The lower current through a diode

the higher the dc resistance level

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Example DC Resistance

� Determine the dc

resistance

a) ID = 2 mA

b) ID = 20 mA

c) VD = -10 V

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Solution

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AC or Dynamic Resistance

� The varying input will move

the instantaneous operating

point up and down a region

of the characteristics and

thus defines a specific

change in current and

voltage

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AC resistance

� A straight line drawn tangent to the

curve through the Q-point. It will define

a particular change in voltage and current

that can be used to determine the ac or

dynamic resistance for this region of the

diode characteristics

� In equation = ∆Vd/ ∆Id

� In general, the lower the Q-point of

operation (smaller current or lower

voltage) the higher the ac resistance.

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Example

� For the characteristics

given curve

� Determine the ac

resistance at ID = 2 mA.

� Determine the ac

resistance at ID = 25

mA.

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Average AC Resistance� If the input signal is sufficiently

large to produce a broad swing such as indicated, the resistance associated with the device for this region is called the average ac

resistance

� The average ac resistance is, by definition, the resistance deter-mined by a straight line drawn between the two intersections established by the maximum and minimum values of input voltage

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Diode SpecificationsDiode data sheets contain standard information, making cross-matching

of diodes for replacement or design easier.

� Forward Voltage (VF) at a specified current and temperature

� Maximum forward current (IF) at a specified temperature

� Reverse saturation current (IR) at a specified voltage and temperature

� Reverse voltage rating, PIV or PRV at a specified temperature

� Maximum power dissipation at a specified temperature

� Reverse recovery time, trr

� Operating temperature range