FUNDAMENTAL OF ELECTRONICS

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T3

FUNDAMENTAL OF ELECTRONICS

INDIAN RAILWAYS INSTITUTE OF

SIGNAL ENGINEERING & TELECOMMUNICATIONS

SECUNDERABAD - 500 017

Issued in October 2008 VISION:

MISSION:

TO MAKE IRISET AN INSTITUTE OF INTERNATIONAL REPUTE, SETTING ITS OWN STANDARDS AND BENCHMARKS

TO ENHANCE QUALITY AND INCREASE PRODUCTIVITY OF SIGNALLING &

TELECOMMUNICATION PERSONNEL THROUGH TRAINING

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T3

FUNDAMENTAL OF ELECTRONICS

Contents

S.No. Chapter Page No.

1. Theory of semiconductors 01

2. Types of Semiconductor Materials 07

3. PN Junction 13

4. Transistor Operation 19

5. Transistor Current Configurations 23

6. The Common Emitter 25

7. The Common Base 29

8. The Common Collector 31

9. Comparison of Transistor Configurations 33

10. Characteristic Curves 36

11. Operation Limit of Transistors 40

12. Specifications of Transistors 42

13. Field Effect Transistor 50

14. The Zener Diode 57

15. Silicon Controlled Rectifiers 63

16. Uni Junction Transistors 67

17. Special Devices 70

18. Semiconductor Microwave Devices 75

http://www.iriset.ac.in

Prepared by S.S. Muralidharan IMP-1 Checked by S.N. Pal, Asst. Professor-Tele

Approved by S.K.Biswas, Sr. Prof. Tele (Nov. 2005) DTP and Drawings K.Srinivas, JE II(D)

Date of Issue Nov. 2005

Edition No 01

First Reprint October 2008 No. of Pages 93

No.of Sheets 48

© IRISET

“This is the Intellectual property for exclusive use of Indian Railways. No part of this publication may be stored in a retrieval system, transmitted or reproduced in any way, including but not limited to photo copy, photograph, magnetic, optical or other record without the prior agreement and written permission of IRISET, Secunderabad, India”

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THEORY OF SEMICONDUCTORS

CHAPTER 1 THEORY OF SEMICONDUCTORS

1.1 Matter is anything that has weight. Atomic theory describes all matter, whether it is solid, liquid or gas as being composed of atoms. The atom contains a central nucleus in which exist neutrons and protons. Protons are positively charged particles and neutrons are neutral particles both being approximately 1840 times as heavy as an electron. Electrons which are negatively charged particles are arranged in ' orbits around the nucleus in a manner similar to the arrangement of planets around the sun.

Thus the electrons in atoms are frequently called planetary electrons. Representative drawing of an atom in shown in Fig. 1.1.

1.2. Atoms of different elements are found to have a different number of protons, neutrons and electrons. In order to classify or identify the various atoms a number which indicates the no. of protons in the nucleus of a given atom, has been assigned to the atoms of each known element. This number is called the atomic number of the element. The atomic numbers for some of the elements associated with the study of semiconductors are as shown below:

Element Symbol Atomic No.

Germanium Ge 32

Silicon Si 14

Antimony Sb 51

Arsenic As 33

Indium In 49

Gallium Ga 31

Boron B 5

The normal atom has an equal no. of protons and planetary electrons to maintain its net charge at zero. In the case of Germanium 32 protons and 32 planetary electrons comprise the atom.

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IRISET 2 T3 - FUNDAMENTAL OF ELECTRONICS

1.3 The orbits of planetary electrons are grouped around the nucleus in rings or shells with specific number of electrons permitted in each ring. A discrete energy may then be associated with each ring or shell. The rings or shells are numbered starting with the first ring nearest to the nucleus as No. 1 the second ring from nucleus as No. 2 etc. The maximum number of electrons permitted in each ring is as follows:

Ring No.1 2 electrons Ring No.2 8 electrons Ring No.3 18 electrons Ring No.4 32 electrons

It may be noted that the permissible no. of 1 electrons for each ring or shell is equal to 2N² where N is the ring or shell no. subject to the following conditions.

a) No. of electrons should 'not exceed 8 in the outermost shell of any element and

b) It should not exceed 18 in the last but one, shell.

The atomic structures of silicon or germanium atoms are illustrated in Fig.1.2. Silicon having an atomic number of the 14, has 14 planetary electrons whereas Germanium having an atomic number of 32, has 32 planetary electrons. It may be noted that the outer rings, the 3rd, ring in silicon and 4th ring in germanium, are incomplete, each of these rings having only four electrons. The fact that the outer rings are incomplete is important to the nature of semiconductor devices.

1.4 The atomic weight of an atom designates the mass of the nucleus and is referred to oxygen having an atomic weight of 16 as a standard. The atomic weight may be other than an integral multiple or sub-multiple of the atomic weight of oxygen.

FIG. 1.2 SILICON AND GERMANIUM ATOMIC STRUCTURE

4

1 8

82

+ 3 2

482

+ 1 4

G E R M A N I U M ( 3 2 S I L I C O N )

(1 4 )

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In addition to protons, the nucleus of an atom may also contain neutrons. A neutron is a neutral particle, whose mass is slightly greater than the mass of proton.

The mass of an electron is 9.1083 x 10^-31 Kg.

The mass of a proton is 1.6724 x 10^-27 Kg.

The mass of a Neutron is 1.6747 x 10 ^-27 kg.

An example of an element with neutrons present in the nucleus is carbon shown in Fig.1.3. The net charge of the nucleus is +6, but the additional mass of six neutrons makes the atomic weight of carbon very nearly 12.

A T O M I C W E I G H T 1 2 A T O M I C N o . 6

S E C O N D R I N G C O M P L E T E F I R S T R I N G C O M P L E T E

+ 6

FIG. 1.3 THE CARBON ATOM

1.5 Valency is defined as the chemical combining ability of an element referenced to hydrogen and is a function of the number of planetary electrons in the outer electronic orbit of an atom. Valence is the capacity of the atom to combine with other atoms in order to form a molecule. For example, the element Helium, shown in part A of Fig.1.4 (i) has a valence of zero. This means that the outer orbit is complete i.e. the maximum no of electrons is present. Thus helium does not chemically combine and said to be inert.

On the other hand, hydrogen, shown in part B of the figure, has a valence of 1 and will go into chemical combination readily. The reason for this is that hydrogen, has an incomplete outer ring which requires one more electron in order to be complete. One atom of oxygen (Part C) has a combining power of valence of 2 since there are two vacancies in the outer orbit of the atom. Where oxygen and hydrogen combine. to form water, two hydrogen atoms contribute their electrons to the outer ring, of the oxygen atom in such a way as to complete the outer rings of both atoms as shown in part A of Fig.1.4(ii). All atoms continually strike to complete their outer ring of electrons and when this is accomplished a stable state exists. The type of bond produced, when hydrogen and oxygen combine to form water, is called an ionic valence bond or an electro valence

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bond. The tendency of an atom to complete its outer ring is illustrated by the atomic form of hydrogen. In this form hydrogen has only one electron in the outer ring. The atom, is therefore unstable since its outer ring required two electrons to be complete. As a result one hydrogen atom will combine with another hydrogen atom to produce the molecular form of hydrogen, consisting of two atoms bonded together each sharing the others electron to form a stable molecule. This type of bond is called covalent bond. An example of covalent bond is shown in part 6 of fig.1.4 (ii).

1.6 Referring again to the silicon and Germanium, atoms it appears that in order for the silicon atom to chemically combine with another atom 14 additional electrons are required to complete its outer. Ring similarly it appears that 28 additional electrons are required to complete the outer ring of the Germanium atom. However, for atoms having 3 or more rings, it is found that if the outermost ring contains eight electrons it can be considered to be complete and the atom can be considered to the stable. Therefore, the outer ring of silicon or germanium require only four electrons to become stable hence the valence of silicon or germanium is 4.

1.7 In some elements, the electrons are bound closer to the nucleus than in others and the effect of external forces such as due to gravity and magnetism on the highly bound electrons is much smaller than the effect of forces within. the atom where the electrons are tightly bound, they are difficult to remove. It is the case of difficulty encountered in dislodging the outer electrons that determine whether the element is a conductor, insulator or semiconductor.

F I R S T R I N G I N C O M P L E T E F I R S T R I N G

C O M P L E T E

+ 8

F I R S T R I N G C O M P L E T E

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)O X Y G E N V A L A N C E - 1

(2

)H Y D R O G E N F I R S T R I N G

+ 1 I N C O M P L E T E

+ 2

F I G . 1 . 4 (i

)C O M P A R I S I O N O F H E L I U M , H Y D R O G E N A N D O X Y G E N (1

)H E L I U M

F I G . 1 . 4 ( i i

)V A L A N C E B O N D S⁽

B) C O V A L E N T B O N D (A

) I O N I C V A L A N C E B O N D F O R M S W A T E R

S H A R E D E L E C T R O N S

+ 2 + 2

+ 1 + 1

O X Y G E N A T O M H Y D R O G E N A T O M

+ 8

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1.8 It is established that a definite amount of energy must be supplied in order to affect an electron held at an atom. That is in order to move an electron from one energy level to higher energy level, a given amount of energy is required. If less than the required amount of energy is supplied to the electron, it will remain at its original level. If more than the required amount of energy is supplied to the electron, it will leave its orbit and move to the next higher level. The excess energy will be of no use, unless it is sufficient to cause the electron to move to a higher energy level. These definite amounts of energy are called "quanta" and they can be supplied to the electrons only in whole numbers such as .1, 2, 3, 4 etc.

It is possible for electrons to lose energy as well as receive it. When an electron in an atom loses energy, it moves to, a lower energy level or closer to the nucleus. The energy that is lost in this process may appear in the form of heat as in a conductor passing current or visible light as in gaseous tubes. The different elements have different energy levels; hence the amount of energy released or absorbed by the electrons of different atoms varies.

1.9 A fact which is brought out by quantum theory explains more precisely the difference between conductors and insulators. The reasoning that the difference between conductors and insulators is due to the no. of electrons in the outer ring of an atom still holds true. However in a solid crystal the energy levels are considered as bands instead of rings or orbits.

An example of the band structure of an insulating material is shown in part A of Fig.1.5.

Since such a diagram illustrates the electron energy bands of a material it is often reported to as energy band diagram or simply energy diagram.

CONDUCTION BAND

VALANCE BAND

(B) CONDUCTORS

ENERGY IN ELECTRON VOLT

Fig. 1.5 ENERGY DIAGRAM FOR INSULATORS & CONDUCTORS

ENERGY IN ELECTRON VOLT

(A) INSULATORS FORBIDDEN REGION OR ENERGY GAP

VALANCE BAND CONDUCTION BAND

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The lower portion of the diagram, called the valence band, represents the energy levels closest to the nucleus of the atom. In the normal atom the energy levels in the valance band contain the correct number of electron necessary to balance the positive charge of the nucleus. Thus band is called the filled band. The electrons in this band are tightly bound to the nucleus with the electrons being more lightly bound in each succeeding energy level, toward the nucleus. It is more difficult to disturb electrons in the energy levels, closer to the nucleus since their movement involves greater energies. The top or outermost band in the diagram is called the conduction band. An electron in an energy level which lies within this band is relatively free to move about the, crystal and hence conduct ails electric current.

Between the bands is a range of energy values across which electrons may pass but the values of which they actually may not have. That is although electrons can jump across this region from the valence band to the conduction band, they never have energy values till this range. Hence this region is appropriately called the forbidden region or energy gap. Note that the forbidden region of an insulator is relatively wide. When compared to the valence band and the conduction band. The wider the energy gap in a material the greater the amount of energy required to cause an electron from the valence band to jump the gap ad appear in the conduction band where it can be used as a current carrier. It is apparent that due to the wide energy gap, a large amount of energy is required to produce a small amount of current through an insulating material.

Part B of figure shows the band structure of a conducting material. Notice that the valence band and conduction band touch each other and that there is no forbidden region. Whenever these two band touch, only an extremely small amount of energy is required to move electrons into the conduction band and an electrical current is readily passed by the material.

1.10 The measure used in the diagram is the electron volt, one electron volt being equal to the energy acquired by an electron in passing through a difference of potential of one volt. By applying this method of measuring energy to an insulator the width of energy gap is generally one electron volt or more. For conductors, the energy gap is less than 0.05 electron volt from the valence band to the conduction band.

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Subjective:

1. Draw and explain the structure of an atom

2. What is the difference between Atomic Weight and Atomic Number?

3. What is meant by valency of an atom?

4. Explain with example

5. What are valence electrons?

6. Draw and explain the energy band diagram for conductors and insulators 7. Give two examples of semiconductor materials.

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TYPES OF SEMICONDUCTOR MATERIALS

CHAPTER 2 TYPES OF SEMICONDUCTOR MATERIALS

2.1 The conductivity of a semiconductor is midway between that of a conductor and that of an insulator. Germanium or silicon which is considered to be semiconductor, in pure form actually is insulators. However in the manufacture of these elements for electronic use, impurities are added to them so that they become semiconductors.

2.2 The fact that the movement of electrons through a conducting material produce a current has been used as the basis for explaining both alternating and direct current. This is called the electron theory.

Although the movement of electrons in a semiconductor material causes a current to pass, a current also results in such materials from the movement of positive charges or holes, through the material. A hole is nothing more than the space left by the electron.

Since this space has an attraction for any negatively charged electron, the hole is considered to have a positive charge.

2.3 Holes are considered to be capable of motion around in the covalent bonds. Hole movement is somewhat different from electron movement. Electrons in motion out of an orbit of an atom are considered to be free. Holes however make only when electrons leave their positions or orbits.

An analogy to hole motion can be drawn from the arrangements of bearing in a tube or cylinder as shown in Fig.2.1. By removing No.1 bearing a hole or space is left which is then filled by the No.2 bearing. The No.3 bearing then moves into the No.2 space. This action continues until all the bearings have moved one space to the left at which time there is a space, left by the No. 7 bearing, at the right end of the tube. Therefore whether this process is looked upon as a motion of the bearings to the left or a motion of the space (absence of a bearing) to the right the end result is the same. This motion is similar to that of a hole in the covalent bond structure of a semiconductor material with the hole movement being governed by the shifting of electrons. In the covalent bonds, the same electrical effect is obtained whether electrons move in one direction (electron current) or holes move in the opposite direction (hole current). This is an important concept and is fundamental to the study of transistors, since both types of current occur in transistors. Usually electrons move through the conduction band and holes move through the valence band.

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2.4(a) Germanium has four electrons in its outermost shell, in bonds atoms are shown with their outer electrons only since these electrons which a maximum of 32 electrons is permitted. The germanium atoms will share valence electrons in a covalent bond. This is shown in Fig.2.2. The, germanium are the ones associated with the covalent bond. The crystalline form of germanium called the crystal lattices structure has the atoms arranged in this manner. The electrons in such an arrangement are in very stable condition and thus are less apt to be associated with conductors. Germanium in a pure form is an insulating material and is called an intrinsic semiconductor.

2.4(b) Silicon is also used in the manufacture of semiconductor devices. Silicon also has four electrons in its outermost shell. The atomic structure of silicon and germanium are shown in the sketch. The crystal lattice structure of silicon is that similar to the Germanium.

2.5 Pure form of germanium is of no use as a semiconductor device. By the addition of impurities however a desired amount of conductivity can be obtained. In order to do this, the quality of added impurity must be carefully controlled. The added impurities will create either an excess or a deficiency of electrons depending on the type of impurity added.

1 2 3 4 5 6 7

5 6 7

3 4

2

2 3 4 5 6 7

1

BEARING IN A TUBE

SPACE LEFT BY No. 1 BEARING

No. 1 BEARING MOVED

SPACE LEFT BY No. 7 BEARING

FIG. 2.1 ANALOGY TO HOLE MOTION

GE

GE GE

GE

GE GE

GE GE GE

GE

GE GE

GE

4

1 8

82

+ 3 2

482

+ 1 4

G E R M A N I U M(3 2) S I L I C O N(1 4)

Fig. 2.2 COVALENT LATTICE STRUCTURE OF PURE GERMANIUM

Fig. 2.2(b) SILICON AND GERMANIUM ATOMIC STRUCTURE

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GE

GE GE

GE

GE GE

GE GE GE

GE

GE GE

GE

(N-TYPE GERMANIUM)

FIG. 2.3. GERMANIUM LATTICE WITH A DONAR IMPURITY ADDED

EXCESS ELECTRON DUE TO IMPURITY ELEMENT

DONAR IMPURITY

GE GE

2.6 Of primary importance in semiconductors are these impurities that align themselves in the regular germanium lattice structure despite the fact that they have one valance electron too many or one too few. The first type easily loses its extra electron, and in so doing it increases the conductivity of the material by contributing a free electron. This type of impurity has five valence electrons and is called as pentavalent impurity. Arsenic, Antimony, Bismuth and Phosphorous are pentavalent impurities. Since these impurities give up or donate, one electron to the material, they are referred to as donor impurities.

The second type of impurity tends to make up its deficiency of one electron by acquiring an. electron from its neighbour. Impurities of this type in the lattice structure have three valence electrons and are, therefore, called trivalent impurities. Examples of trivalent impurities are Aluminium, Gallium, Indium and Boron. Since these impurities accept one electron from the material, these are referred to as acceptor, impurities. Semiconductors produced by adding either acceptor or donor impurities are called extrinsic semiconductors.

2.7 When a pentavalent impurity such as Arsenic is added to germanium, it will form covalent bonds with the germanium atoms. Fig.2.3 illustrates the presence of an Arsenic atom (As) within the germanium lattice structure. Only four of the five electrons of arsenic in the outer ring is used to form covalent bonds leaving one electron relatively free in the crystal structure. Since this semiconductor material conducts by electron movement it is termed a negative carrier type of N- type semiconductor. Pure Germanium may be turned into an N-type semiconductor by doping it with an element containing five electrons in its outer ring. The amount of impurity added is ordinarily in the neighbourhood of one atom of impurity material per ten million atoms of germanium.

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2.8 Application of an electric field to an N-type semiconductor causes a current conducted by negative (electron) carriers. Fig. 2.4 illustrates one N-type semiconductor with an electric field applied. Electric field causes the loosely bound electron to be released from the impurity atom and move toward the positive potential point. The conduction is similar to that in a copper conductor. But, certain difference exists between a semiconductor and the familiar copper conductor. For example, the semiconductor resistance decreases with increasing temperature because more carriers are made available at higher temperatures, while the resistance of copper increases with temperature.

2.9 A Trivalent impurity element can be added to pure germanium to dope the material. In this case the valence electrons of the trivalent element will also enter into covalent bonds with the germanium atoms. However, the trivalent impurity will borrow a fourth electron from a Germanium atom to complete the covalent bond structure. This removal of an electron from the covalent bonds of the Germanium by the trivalent impurity creates a hole or space.

2.10 In Fig.2.5, the germanium lattice structure is shown with the addition of an Indium atom (In). The Indium atom takes a hole in the structure. Other, elements which display the same characteristic are Gallium and Boron. The holes are present only if a trivalent impurity is used. Since such a semiconductor material conducts by the movement of holes which are positively charged, it is termed positive carrier type or P-type semiconductor.

Fig. 2.4 ELECTRON MOVEMENT IN A N-TYPE SEMICONDUCTOR

DIRECTION OF ELECTRON MOVEMENT

DIRECTION OF ELECTRIC FIELD

+ -

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GE GE GE GE

HOLE CAUSED BY IMPURITY ELEMENT ACCEPTOR

IMPURITY

FIG. 2.5. GERMANIUM LATTICE WITH AN ACCEPTOR IMPURITY ADDED (N-TYPE GERMANIUM)

GE

GE GE

GE

GE GE

GE

GE GE

GE GE GE

2.11 Application of an electric field to a P-type semiconductor causes a current conducted by positive carriers (holes). In order for the hole to move, an electron from a near by site must shift to the position where the hole originally existed. Hence, the holes illustrated in Fig.2.6 move from the positive terminal to negative terminal. Electrons from the negative terminal cancel holes at the vicinity of the terminal while at the positive terminal;

electrons are being removed from the covalent bonds, thus creating new holes. The new holes then move towards the negative terminal and are cancelled by more electrons emitted from the negative terminal.

2.12 It should be realised that in either N-type of P-type germanium both electrons and holes are present and can act as current carriers. In N-type germanium, electrons greatly out number the holes and thus are said to be the major current carriers while the holes are referred to as minor current carriers. On the other hand, P-type germanium contain a

- +

HOLE MOVEMENT ELECTRON MOVEMENT

Fig. 2.6 HOLE MOVEMENT IN P-TYPE SEMICONDUCTOR

+ -

+ +

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greater number of holes than electrons and thus in this material holes are the major current carriers while electrons are considered to be minor current carriers. In an intrinsic semiconductor there is thermal break up of covalent bond producing an electron hole pair i.e. the no of electrons = no. of holes. When pentavalent impurity is added the no. of electrons increases without corresponding increases in holes and when a trivalent impurity is added, the no. of holes increases without a corresponding increases the no.

of electrons. However, the product of electron and hole concentration remain the same whether impurity is added or not.

Thus, if the hole concentration is TIP and electron is ηe

Then ηp X ηe = n² where η is the no. of electrons = no. of holes in the intrinsic semiconductors.

2.13 Mobility µ of the charge carriers is defined as the speed at which they drift in, unit electric field.

The intrinsic Conductivity σi is given by the formula σi = eη(µ1+µ2)

where,

σi is conductivity in semiconductor e is charge of electron (or hole)

η is intrinsic concentration of carriers per cc.

ηi = nxp where n & p are the concentration of electrons and holes µη is mobility of free electron, em/sec. per volt/cm.

µp is mobility of free hole cm/sec per volt/cm.

The mobility of electron and holes in silicon is 1250cm²/V.sec and 480 cm²/V. whereas in germanium it is 3900 cm² /V.sec and 1900 cm²/V.sec.

This applies to lightly doped silicon and germanium. These values will decrease for higher doping levels. The ratio of µη/µp however remains relatively constant. This ratio is about 2 to 6 in silicon and 2 in germanium.

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IRISET 12(i) T3 - FUNDAMENTAL OF ELECTRONICS

Subjective:

1. How do you differentiate between electrons and holes?

2. What is an impurity? What are the different types of impurities available?

3. Differentiate between trivalent and pentavalent imlpurities

4. What is meant by Mobility? Explain with the equation for Mobility.

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PN JUNCTION

CHAPTER - 3 PN JUNCTION

3.1 Representative diagrams of both P-type and N-type semiconductor materials are illustrated in Fig.3.1. A P-type semiconductor is shown in part A with the symbol representing the accepter atoms of the added impurity and the plus sign without the circle representing hole carriers. A N-type semiconductor is represented in part B, the plus sign within the circle + representing the donor atoms and the minus sign without the circle representing free electrons.

3.2 If a piece of P-type semiconductor material and a piece of N-type semiconductor material are joined together, the result is known as PN junction (Fig.3.2). A PN junction is formed during the process of manufacturing the semiconductor crystal. Several methods are employed, one of which is to add the desired impurities as the crystal is being made so that one section of the crystal is N-type and the other P-type. This impurity in the process of taking electrons from covalent bonds of the N-type crystal, creates an area of P-type crystal. It should be noted that the addition of impurity atoms to a semiconductor does not create a change or potential difference in the semiconductor. Such impurity atom added is electrically neutral. When it enters into a covalent bond it allows the carrier to be free through the semiconductor under the influence of an electric field.

FIG. 3.2. THE P-N JUNCTION

P N

ACCEPTOR ATOMS DONOR ATOMS

+ + +

+ +

+ + +

+ +

+ + +

- - -

- -

- - -

- -

- - -

- +

+

+ +

+

+ +

+

+ +

+

-

- -

-

- - -

-

(A) P - TYPE MATERIAL (A) N - TYPE MATERIAL +

- - - -

+ + +

+ +

+ + +

- - - -

+

+ + + +

HOLES

ELECTRONS ACCEPTOR

ATOMS

DONOR ATOMS

Fig. 3.1 P-TYPE & N-TYPE SEMICONDUCTOR MATERIAL

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PN JUNCTION

3.3 The addition of arsenic to a germanium crystal supplies electron carriers that are not bound by the covalent forces. Similarly the addition of Indium supplies hole carriers that are not bound by the covalent forces. The atomic state remains neutral in charge as long as the carrier is present. When the electron or hole carriers move off under the influence of an electric field, the atomic state may temporarily be positive or negative, respectively but the net charge of the material is still zero.

3.4 In the absence of external forces, there is a process of carrier movement called diffusion occurring across the PN junction. This is caused by the holes attempting to move to the N-type material and the electrons attempting to move to the P-type material. However, only a few electrons and holes actually cross the junction. As soon as a crossing takes place, a few atomic states near the junction lose their compensating carrier and become uncompensated and are no long neutral as shown in Fig.3.3. The donor atomic sites become positive, having lost a neutralising electron, the acceptor sites becomes negative having gained an electron. The carrier movement reaches an equilibrium condition at which the net current between P-type and N-type materials is zero and a potential difference exists between the materials.

FIG.3.3 CONDITION OF EQUILIBRIUM ACROSS A PN - JUNCTION

3.5 The potential difference existing between the P-type and N-type materials is called the

"barrier region" or "potential hill" and can be represented by a battery. This does not mean that a potential may be measured from one end of the material to the other. The overall piece of material is neutral even though a charge is displaced within the semiconductor to create the barrier.

3.6 In order to produce a current across a PN junction, the potential hill existing at the junction must be neutralized. The potential hill can reduce or neutralized by the addition of a bias battery across the two crystal section. For the reduction of the potential hill, the polarity of the bias battery must be opposite to the polarity of the potential hill battery. In

- - - -

- -

- - -

+ +

+ + +

- - -

- -

- - -

- -

- - -

+ + +

+ +

+ + +

+ +

+ + +

UNCOMPENSATED DONOR ATOMS UNCOMPENSATED

ACCEPTOR ATOMS

P N

DIFFUSSION OF CARRIER ACROSS JUNCTION

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PN JUNCTION

this case the junction is said to be in the forward bias direction. When the polarity of the bias battery has the same polarity as the potential hill battery, little or no current will cross the junction. In this case, the junction is said to be reverse biased. Since the PN junction has the same directional characteristic, as a vacuum tube diode. It is called a PN junction diode or simply a junction diode.

3.7 Fig.3.4 illustrates the junction diode in the forward bias connection. Where only the electron carriers and hole carriers are shown. The negative terminal of the battery is connected to the N-section and the positive terminal to the P-section which is just opposite of the potential hill battery. The N-section electrons are driven toward the junction by the negative battery terminal while holes in the P-section are forced towards the junction by the positive terminal. A number of electrons and holes depending on the battery potential cross the barrier region or junction and combine.

Along with the combination of holes and electrons in the barrier region, there are two simultaneous actions that take place. Near the positive terminal in the P-section the covalent bonds of the atoms are broken and electrons are freed. The free electrons enter the positive terminal. This creates a new hole which is attracted toward the N-section. At the same time an electron enters the negative terminal of the N-section and move toward the positive terminal of P-section.

This action in effect reduces the potential hill to a value which no longer prohibits current across the junction. The reduction of potential hill or barrier is illustrated in the upper para of the Fig. 3.7. Current in the P – region is be holes and the N – region is by electrons. So far as overall circuit is concerned, the current is dependent on bias battery potential.

BARRIER LIMITS WITHOUT BIAS BATTERY

P-REGION N-REGION

BARRIER AREA HOLES & ELECTRONS COMBINE

+ - POTENTIAL HILL

BIAS BATTERY

FIG. 3.4. FORWARD BIAS CONNECTION OF A JUNCTION DIODE BARRIER AREA HOLES & ELECTRONS COMBINE

NEW HOLES DUE TO ELECTRON REMOVAL

NEW ELECTRONS ENTERING TO REPLACE THOSE REMOVED

P N

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PN JUNCTION

3.8 Although the voltage of the potential hill is in the order of tenths of a volt, the applied voltage must overcome the, internal resistance of the diode. Since the germanium used is a semiconductor, its internal resistance is many times that of a conductor. Hence the voltage across P-N type circuit sections is large leaving small voltage a to reduce the potential hill. If the actual size of junction is considered, the external voltage source need only be of the order of 1 or 2 volts to neutralize the potential hill. As the battery potential' increases, causes a rise in current, the resistance of the barrier decreases if the battery potential is allowed to reach a level at which, the potential hill is completely neutralized, heavy current will pass and junction may be damaged by the resultant heat. For this, reason the potential of the bias battery should not be too large.

3.9 Fig.3.5 illustrates the reverse bias connection of an external battery to a junction diode.

Positive terminal of the battery is connected to the N region and Negative terminal to the P region. These connections are the same as these of the potential hill battery.

Consequently the bias battery potential now aids the potential hill battery and very little or no current passes across the junction. In the P region the negative battery terminals draws the holes away from the barrier area. The electrons in the N region are similarly affected by the positive battery terminal. The potential hill is reinforced by the bias battery. This condition is shown in the upper portion of the fig.3.5.

JUNCTION INCREASES

WITH REVERSE BIAS

BARRIER LIMITS WITHOUT BIAS BATTERY

P-REGION N-REGION

HOLES DRAWN AWAY FROM BARRIER AREA BY NEGATIVE OF BATTERY

ELECTRONS DRAWN AWAY FROM BARRIER AREA BY POSITIVE OF BATTERY BARRIER AREA

P N

BIAS BATTERY

POTENTIAL HILL BATTERY

FIG. 3.5 REVERSE BIAS CONNECTION OF A JUNCTION DIODE +

- + +

+ + + +

+

- - - - - - - -

+ -

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PN JUNCTION

3.10 Actually, there is a very small current due to the minority carriers present in each side of the crystal with their action being similar to the action of the majority carriers during the forward bias condition. The reverse current remains small with the increase in battery potential until certain voltage is reached. At this voltage the covalent bond structure begins to break down and a sharp rise in reverse current occurs. This action is called

"avalanche breakdown" and is due to the acceleration of the few electrons and holes that comprise the reverse current to such a point that they have violent collisions with the germanium crystal atoms. The maximum reverse voltage of the semiconductor diode corresponds to the peak inverse voltage of the vacuum tube. When a PN Junction is reverse biased all the applied voltage appears across the depletion region, since it has no free charge carriers hence an infinite resistance. If the impurity content is about one part in ten the relatively small dimensions of the depletion layer result in very high field strength in this region. This causes a rapid increase of current at low voltages due to the breaking of the covalent bonds and is known as the Zener effect. Zener breakdown is a field emission phenomenon, the strong electric field in the junction region pulling carriers from their atoms.

Zener diodes using this mode of operation are usually made of silicon with a zener breakdown voltage lying between 3V and 6V and a negative temperature coefficient.

Higher voltage stabilising diodes utilise the avalanche effect and have much lower impurity content. They have a positive temperature co-efficient. It is difficult to separate the two effects in a practical zener diode.

3.11 The fig.3.6 illustrates the static characteristic of a junction diode. There are different current scales for forward bias and reverse bias operations. The forward portion of the curve indicates that the diode conducts easily when the P region is made positive and the N region negative. The diode conducts poorly in the high resistance direction i.e.

when the P region is made negative and the N region is made positive. Now the holes and electrons are drawn away from the junction, causing the barrier hill to increase. This condition is indicated by the reverse current portion of the curve. The dotted section of the curve indicates the ideal curve which would result if it were not for avalanche breakdown.

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PN JUNCTION

It is therefore seen that the forward resistance is low, the reverse resistance is very high.

The current voltage relationship of a diode is given by

l=l_R(e QV/KT^-1)

Where Q is the' charge on an electron (1.602 X 10 - 19 coulomb) V is the potential difference in volts.

K is the Boltzmann constant (I.38 X10^-23 Joules/Kelvin) T is the absolute temperature in degree Kelvin.

IR is the reverse saturation current.

FORWARD CURRENT IN mA

REVERSE BIAS IN VOLTS

FORWARD BIAS IN VOLTS

REVERSE CURRENT IN microA

FIG. 3.6. TYPICAL STATIC CHARACTERISTIC CURVE FOR A SEMICONDUCTOR DIODE IDEAL

CURVE

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PN JUNCTION

Subjective:

1. What is a P-N Junction explain with a suitable diagram.

2. What is a Forward biased PN Junction? Explain the Forward biased connection of a junction diode

3. What is a Reverse biased PN Junction? Explain the Reverse biased connection of a junction diode

4. Draw and explain the static characteristic of a semiconductor diode

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TRANSISTOR OPERATION

N P N

P O T E N T I A L H I L L B A T T E R Y

E M I T T E R C O L L E C T O R

B A S E

_ + _ +

F I G . 4 . 1 T H E N P N J U N C T I O N T R A N S I S T O R

CHAPTER 4 TRANSISTOR OPERATION

4.1 A junction of two junction diodes with either P-type or section being, common to, both, the resultant transistor is either an NPN or PNP type Junction transistor. In either case, 'the middle or section is very narrow compared to other sections. The junction transistor is produced in several different ways but the end result is the formation of PN, junctions.

The junction may be formed in the process of growing the crystal. The thickness of the germanium or silicon crystal is important because of the possibility of its shorting out if it is too thin. On the hand the crystal is too thick; the operation of the transistor will be poor.

4.2 The description given below for a germanium transistor applies equally to a silicon transistor. The NPN transistor consists of a, very thin layer of P-type germanium between two sections of N-type germanium as shown, in, Fig. 4.1. The potential hills of the two junctions are positive for the N-section and negative for the P-section. The emitter (or input NP section) is biased in the forward direction. The collector (or output PN section) is biased in the reverse direction that is the collector is positive with respect to base.

4.3 With the aid of negative potential applied to it, the free electrons in the emitter N-section will be pushed towards the first junction. The potential hill of this junction is essentially reduced by the polarity of the emitter bias battery. A number of electrons will pass through the junction and enter the middle or P-section where some of them combine with holes while others pass through. The electrons that pass through the P-section do so because of the thickness of the section and the effect of the potential hill of the second or PN junction. Actually, the potential hill, at the second junction accelerates the electrons into the collector N-section. In the collector area, the free electrons are attracted by the applied positive collector base voltage. It is important to note that the movement of electrons and holes is not in one for one process. A small percentage (about to 5%) of the electrons entering the P-section (base region) form the emitter N-section combine with the P-section holes. However the majority of the

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electrons from the emitter do pass through the P region. Thus most of the electron flow is between emitter and collector. The electrons leaving the emitter are controlled by the bias potential between the emitter and base. (The similarity to a vacuum tube triode, where the bias is between the control grid and the cathode controls the electron flow to the plate which receives most of the, electrons should now be obvious).

4.4 The PNP transistor consists of a very thin layer of N-type germanium between two sections of P-type germanium as shown in Fig.4.2.In this type, the potential hills of the two junctions are positive for the PN-section and negative for the NP-section. In the PNP transistor the connection of emitter bias battery must be positive to the emitter and negative to the base in order to forward bias the emitter. The collector bias battery must have its positive terminal connected to the base to reverse bias the collector. The potential hill of the emitter junction is reduced by the forward bias.

FIG.4.2 THE PNP JUNCTION TRANSISTOR

4.5 In the operation of the PNP junction transistor holes are forced from the emitter P-section into the base N-region by the positive potential of the emitter which is also creating more holes by electron removal. In the base region a small number of holes (about 1% to 5%) combine with electron from the base. Because the base region is very narrow most of the holes move on into the collector P region before they can combine with base electrons. In the collector P region the holes are attracted to the collector negative terminal and combine with electrons from the collector. Thus the major hole current is from emitter to collector, while emitter base current is very small. It is important to note that the major current carriers in the PNP transistor are holes while in the NPN transistor electrons are the major current carriers.

4.6 The NPN and PNP transistors are identified on schematics by the symbols shown in fig.4.3. The three regions comprising the transistor are called the collector, base and emitter. Emitter-base junction is always forward biased while collector-base junction is reverse biased. The emitter region is so called because it emits majority carriers into the base region. The collector gets its name because it collects the majority carriers from the base region. The base region is so called, because it is a support or base for emitter and collector materials.

B A S E

C O L L E C T O R E M I T T E R

P O T E N T I A L H I L L B A T T E R Y

P

N

P + _+ _

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Fig. 4.3 SCHEMATIC SYMBOLS OF NPN AND PNP TRANSISTORS

E E

B CNPN

B CPNP

4.7 The direction of electron flow in the wires connected to the transistor is shown in fig. For the NPN transistor where, electrons are the majority carriers, the electron flow shown is continuation of the internal flow. For the PNP, the majority current carriers are holes and the internal conduction is due to hole current. However, hole conduction takes place only within the semiconductor crystal itself. This internal hole conduction leads to electron flow in the external wires connected to the semiconductor material. The direction of the electron flow is opposite to the internal hole conduction and it is electron direction that is indicated for the PNP transistor.

4.8 Fig. 4.4 shows the basic current paths for the NPN and PNP transistors. The battery labelled V_BB provides the forward bias for the base-emitter junction. Forward biasing causes current from one terminal of the battery through the junction and resistor and back to other battery terminal. This is called base current. The resistor is included in this path to indicate that some means of controlling this current is necessary. Recall that most of the majority carriers that are injected, into the base region from the emitter do not continue in the base emitter path. They are attracted toward the larger potential applied to the collector region. This potential is supplied by the battery marked Vcc. This current that is attracted to Vcc battery is called collector current. Since both the base current and collector current come from the emitter region, a simple relationship exists between the currents:

I_b + I_C = le

In words the emitter current separates in the transistor into the base current and also the collector current.

V_BB

V_CC

Ib E

Ie B CNPN

b E I

e V_BB I

B CPNP c

I Ic

Fig. 4.4 BASIC CURRENT IN A TRANSISTOR

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4.9 The amount of collector current depends on the amount of base current. More the base current, the more the majority carriers that are injected into the base region and the collector current is, therefore, larger. The base current converts the current supplied by Vcc into a controlled current, namely the collector current. The amount of collector current is related to the base current by the following simple but important relationship:

lc = βIb

The Greek letter β (Beta) represents the current gain of the transistor. It is important to understand the twin loop concept depicted in the preceding illustration. One loop is the base current path (the input circuit) and the other loop is the collector current path (the output circuit). As will be seen later, the signal to be amplified is added to the base bias current and the output signal is derived from the collector current. The idea of base current regulating or controlling collector current is the basic operation of the transistor amplifier.

4.10 The most important thermal consideration is the increase in base to collector reverse current that occurs as temperature increases. The reverse biased base to collector junction has very small current through it due to minority carriers. The situation is shown for the NPN transistor in figure. This current is referred to as I_CE0 (an abbreviation for collector cut off current) This is the collector current that would flow if the base lead were left disconnected. I_CE0 has a particular value at room temperature, but it increases as the temperature increases. This results in a situation where there is a certain amount of collector current which is not controlled by the base current, leading to unpredictable results. Precautions have, therefore, to be taken to minimize ICEO, and related effects due to change in ICEO with temperature.

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IRISET 22(i) T3 - FUNDAMENTAL OF ELECTRONICS

Subjective:

1. Draw and explain the working of a PNP Transistor

2. Draw and explain the working of a NPN Transistor

3. Draw and explain the basic current path for a PNP Transistor

4. Draw and explain the basic current path for an NPN Transistor

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TRANSISTOR CURRENT CONFIGURATIONS

CHAPTER 5 TRANSISTOR CURRENT CONFIGURATIONS

There are three basic configurations for transistor circuits. The three configurations are called the common emitter, the common base, and the common collector circuit.

The input signal to a transistor is applied between two elements. The output signal is taken between two elements. Since there are only three elements in a transistor, one of the elements has to be part of both the input and output circuits. The type of configuration derives its name from the element that is common to both input and output.

The most widely used transistor circuit is the common emitter. It is called thus because the emitter is common to both the input and output circuits. This is shown in figure. Figure A shows the input circuit and figure B shows the output circuit. An important point should be mentioned concerning the illustration is Fig. 5.1. The emitter is shown as grounded. Ground is the reference point in the circuit from which voltages are measured.

Fig.5.1 COMMON EMITTER INPUT AND OUTPUT CIRCUIT

It will be noticed in Fig. 5.4 that both the input and output signals are measured with reference to ground. This reference point is called ground because quite often it is connected to the actual earth or ground. Because the common element, the emitter in this case, is grounded this circuit is sometimes referred to as a grounded emitter circuit. Common emitter or grounded emitter refers to the same type of circuit. The drawing shows a common emitter stage. Figure does not show all the components usually needed for a working circuit, but is intended to show that emitter is common to both the input and output.

SIGNAL

INPUT SIGNAL

V_CC OUTPUT Rc

Fig.5.2 COMMON EMITTER STAGE

0 CC

GROUND

(A) INPUT CIRCUIT (B) OUTPUT CIRCUIT

EMITTER BASE

E V

CC Ie

PNP

C Rc

COLLECTOR +V

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The example shown above is for the NPN type transistor. Every thing would still be valid for the PNP type, except that the power supply polarity would be reversed.

Figure shows the common base configuration. The input signal is applied to the emitter base circuit. Thus the base of the transistor is the common element. As was the case for the common emitter, figure is only intended to show' why this circuit is called the common base and does not represent a complete working circuit. In PNP transistor except that the polarity of the power supply would be reversed and naturally, the arrow on the emitter lead would point in the opposite direction.

Fig.5.3 COMMON BASE STAGE

The third and final type of configuration is called is common collector and is illustrated in figure A and B. The input signal is applied between the base and collector, and the output signal is taken between the emitter and collector. Figure A shows the circuit as is normally drawn, but it does not clearly illustrate why it is called a common collector. The identical circuit is redrawn in figure B. The transistor has been turned around and this shows clearly that the collector is common to both the input and output signals.

V_CC

(A)

INPUT

SIGNAL SIGNALINPUT

OUTPUT SIGNAL

(B)

V_CC

SIGNAL OUTPUT

Fig.5.4 COMMON COLLECTOR STAGE

As in the case of the other two configurations, another name for the common collector is the grounded collector. The most popular name for this circuit is the emitter follower.

SIGNAL OUTPUT INPUT

SIGNAL

V_CC c R

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Subjective:

1. Draw and explain the CE input and output configuration of a NPN Transistor

2. Draw and explain the CB input and output configuration of a NPN Transistor

3. Draw and explain the CC input and output configuration of a NPN Transistor

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THE COMMON EMITTER

CHAPTER 6 THE COMMON EMITTER

6.1 The common emitter circuit is the most popular and versatile of the three types. The best way to arrive at a clear understanding of the performance of the common emitter is to start with the basic concepts and add ideas bit by bit until a working circuit is attained.

6.2 For a transistor to conduct, the base to emitter junction is forward biased and the base to collector junction is reverse biased. This will cause a base current, which in turn results in a collector current (see. Fig.6.1). They are related by the expression lc = βI_b, where β is the current gain.

N

E ^Ie N

B Ib

P Ib

B

E ^Ie C

Ic C Ic

Fig.6.1 BASE COLLECTOR CURRENT

6.3 The first step then is to establish a forward biased base to emitter junction and produce some base current. This is done by connecting the base to a power supply through a resistor to establish the desired current. This is shown in Fig. 6.2. A. To provide a reverse bias to collector junction and a path for collector current, the collector is connected to a power supply through a resistor (Fig. 6.2). The two currents together are shown in Fig. C. The same power supply is used for both currents. The process of establishing a base current and a collector voltage is called biasing the transistor. So far only biasing current has been established and no mention has been made of the signal to be amplified.

(C) BASE & COLLECTOR CURRENTS (A) BASE CURRENT (B) COLLECTOR CURRENT

b ^Ib

R

Ic R

20 V ^b

20V

Ic b

R_b _20V

I

Fig.6.2 COMMON EMITTER BIASING

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THE COMMON EMITTER

6.4 The signal to be amplified is superimposed or added to the base bias current. It is introduced into the circuit by means of a capacitor, as shown in figure. This capacitor is called a coupling capacitor because it couples or joins the input signal to the circuit. A capacitor is used because it blocks the base biased direct current from flowing into the source of the signal and yet, lets the signal flow into the circuit.

COUPLING 20V CAPACITOR

SIGNAL OUTPUT SIGNAL

INPUT

CURRENT CURRENT

CURRENT

BASE BIAS RESISTANCE LOAD

FIG.6.3 INTRODUCTION OF SIGNAL

6.5 The collector current is P times the base current. P can be looked up in the specifications for the particular type transistor used. Whatever the base current is, P times that current will flow in the collector circuit (assuming linear operation to be explained later). A simple numerical example will help to illustrate these ideas.

Assume we are given the following data:

β = 50 from transistor specifications.

R_L= 5000 ohms, determined by load requirements.

It is desirable that when no signal current is present, the collector is midway between its minimum and maximum possible operating excursion. In this case, that would be +10 volts since the power supply voltage is 20 volts. This means that with no signal current we want a 10 volt drop across the 5000 ohm resistor. The desired collector current is then:

I_C = 10 volts = .002 ampere = 2 mA 5000 ohms.