Lecture_1

Lecture 1 Electronic Devices 1

ETU 07103Lecture 1


LECTURE 1 Coverage

1. Introduction to electronic devices2. Fundamentals of semiconductors2.1 Semiconductors materials

• Introduction • Atomic structure and energy level• Intrinsic semiconductors• Extrinsic semiconductors• Majority and minority charge carriers

References• A textbook of electronics, 2nd Ed. by S.L. Kakani, pg. 1 to 30• Electronic devices and circuits, 6th Ed. by Theodore F. Bogart,

Jr., page 19 to 31• Electronic devices and circuit theory, 6th Ed. by Robert L.

Boylestad & Louis Nashelsky, page 3 to 10• Basi electronics devices circuit and IT fundamentals by

Santiram Kal, website: http://books.google.com , pg. 1 to 25

http://books.google.com/


1. Introduction to Electronic Devices Introduction

• Electronic devices/components Physical entities capable of controlling the motion of electrons or their associated fields through different media.

• Electron The key particle, flow of which through a medium gives current

• Electronics Branch of science and technology which makes use of the controlled motion of electrons.

Electronic components category• Passive Components

Contribute no power gain (amplification) to a circuit or system Does not require any input other than a signal to perform its function The most common used passive components; resistors, capacitors,

and inductors


Introduction continue…• Active Components

Contribute power gain (amplification) to a circuit or system Require input other than a signal to perform its function The most commonly used active components; diodes and

transistors

Classification of electronics materials• Conductors

Have an abundance of free electrons that act as charge carriers, which means that they have high conductivity.

• Insulators

Have hardly any free electrons, hence offers very low level of conductivity.

• Semiconductors

Have a conductivity level somewhere between the extremes of an insulator and a conductor.


Introduction continue…- The term resistivity is often used when comparing the resistance levels of

materials.

- Typical resistivity values for three broad categories of materials are

shown in table 1.1

Table 1.1 Typical electrical resistivity values of different materials

(at 200 C in Ωcm)


Introduction continue…- Of the three classes of materials, semiconductors are the most

important in electronics engineering. Reasons: Their conductivity can be significantly altered in several different ways They can be manufactured to a very high purity level in the ratio of

1:10,000,000,000. (i.e. 1 part in 10 billions )

- In fact, some of these unique qualities of semiconductors, makes them to be the most prominent materials in the development of electronic devices

- For the other two remaining materials, their conductivity can not be

readily and significantly altered.

After having this little introduction in electronic devices, now let set our

minds toward the semiconductor materials which have received the

broadest range of interest in the development of electronic devices.


2. Fundamentals of Semiconductors

Semiconductor materials- The most commonly used semiconductors

• Silicon (Si)• Germanium (Ge)• The compound semiconductors (i.e. consisting of compound materials)

- Compound semiconductors are general formed from either • Two different elements, and is referred to as binary compound • Three different elements, and is referred to as ternary compound or • Four different elements, and is referred to as quaternary compound

- The III-V compound semiconductors are the most important• Examples of III-V compounds: gallium arsenide (GaAs), indium phosphide

(InP), aluminum arsenide (AlAs), indium arsenide (InAs), etc.


Fundamentals of semiconductors continue…

Atomic structure • Atomic structure

Matter is composed of elements and compounds The elements are the basic materials found in nature Compounds are the combination of more than one element The smallest particle that an element can be reduced to and still

retain its properties is called an atom The atom is composed of three basic particles: the electron, the

proton, and the neutron The protons and neutrons form the nucleus, while the electrons

revolve around the nucleus in a fixed orbit The electrons and protons are the particles that have the electrical

properties (i.e. negative and positive charge respectively) Usually atoms have the same number of electrons and protons, and

so they are electrically neutral Atom with more electrons is called a negative iron while atom with

more protons is called a positive iron


Fundamentals of semiconductors continue… Figure 1.1 shows the representations of the atomic structures of Ge and Si atoms

Figure 1.1 Atomic structure: (a) germanium; (b) silicon


Fundamentals of semiconductors continue… The electrons in the inner shells of an atom do not normally leave

the atom The electrons in the outermost shells may travel from one atom to

another in a crystal lattice These electrons are called as free electrons The tendency of an atom to give up it valence electrons depends on

chemical stability The level of stability is determined by the number of valence

electrons The atoms tends to fill its outermost shell if it is more than half filled Atoms with 5 or more valence electrons make good insulators, since

they tend to accept rather than giving up electrons Atoms with less than 4 valence electrons make the best electrical

conductors, since they tend to give up their electrons The element Ge and Si have 4 valence electrons, and are neither

good conductors and nor good insulators. These are called semiconductors.


Fundamentals of semiconductors continue…• Crystal structure of semiconductors

The atoms of both elements form a very definite pattern The formed pattern is periodic in nature One complete pattern is called a crystal The periodic arrangement of the atoms within a crystal is called a

lattice For Ge and Si the crystal has three-dimensional diamond structure Figure 1.2 shows a two dimensional crystal structure of Si

Figure 1.2 A simplified representation of the Si crystal structure


Fundamentals of semiconductors continue…From figure 1.2

Si atom acquire stability by sharing the valence electrons of four of its neighboring atoms

Every atom duplicates this process and the result is a stable and tightly bound crystal

Strong bound within a crystal between valence electrons and their parent atom can be broken by natural causes

The broken covalent bond produce free electrons

• Energy levels of isolated atom Each isolated atom has only a certain number of orbits available These available orbits represent energy levels for the electrons in

the atom as shown in figure 1.3 According to Bohr’s theory of atomic structure only discrete values

of electron energies are possible An electron can have only certain permissible values No electron can exist at an energy level other than a permissible

one The more the distant the electron from the nucleus, the higher the

energy state


Fundamentals of semiconductors continue… Electron that has left its parent atom has a higher energy state than

any electron in the atomic structure An electron energy is usually expressed in electronvolt (eV) Electronvolt (eV) is the energy acquired by one electron if it is

accelerated through a potential difference of one volt

Figure 1.3 Energy levels of an isolated silicon atom


Fundamentals of semiconductors continue…• Energy band diagrams of a solid crystal

The energy level diagram of Fig. 1.3 is no longer applicable in a solid

A solid crystal is formed when atoms bond together In the solid, the single orbit is influenced not only by the electrons in

its own atom but by electrons of the same orbit with slightly different energy levels from an adjoining atom

The net result is an expansion of the discrete permissible energy levels as shown in Fig. 1.4

From figure 1.4 Beyond the valence band there is a conduction band The gap between these two bands is called forbidden energy gap

(Eg) Thus, Eg is the amount of energy that should be imparted to the

electrons in a valence band to jump to conduction band At absolute zero temperature, the conduction band is empty At room temperature a larger number of valence electrons leave the

valence band, cross the energy gap, and enter the conduction band


Fundamentals of semiconductors continue… Thus an equal number of vacancies are created in the valence band These vacancies in valence band may be treated as positively

charged particles called holes

Figure 1.4 Energy band of a silicon crystal


Fundamentals of semiconductors continue… The holes moves from higher electron energies to lower energies

The electrons in conduction band and holes in valence band carry electric current

The forbidden energy gap helps to classify solids as conductors, insulators, and semiconductors as shown in figure 1.5

Figure 1.5 Energy band diagrams



From Fig. 1.5 For conductors, valence band and conduction band overlap each

other (fig. 1.5a) Eg = 0. This implies that a larger number of valence electrons are

available for conduction at room temperature For insulators (fig. 1.5c), Eg is equal or more than 5 eV, which

severely limits the number of electrons that can enter the conduction band at room temperature

For semiconductors in pure crystalline form, Eg lies in the range o.1-3 eV

Thus, appreciable number of electron-hole pairs is created by thermal process

Increasing temperature causes creation of more electron-hole pairs, hence resistivity falls


Fundamentals of semiconductors continue…- The number of electrons in conduction band or holes in valence

band per unit volume in an ideally pure and perfect semiconductor crystal is called intrinsic carrier concentration (ni or pi)

- Table 1.2 Indicate Eg values and intrinsic carrier concentration of some important semiconductor materials at room temperature

Table 1.2 Properties of some useful semiconductors


Fundamentals of semiconductors continue… Intrinsic Semiconductors

• Introduction A semiconductor in its purest form is called an intrinsic

semiconductor The electron density in electrons/cm3, equals the hole density,

holes/cm3, in an intrinsic semiconductor (i.e. ni = pi) At room temperature, the charge carrier densities for Ge and Si are

approximated to be ni = pi = 2.4 x 1013 carriers/cm3 and ni = pi = 1.5 x 1010 carriers/cm3 respectively

• Conduction in intrinsic semiconductor Both electrons and holes act as charge carriers in semiconductors When an electric field E is applied in a semiconductor, it causes free

electrons to drift in one direction and holes to drift in the other These two components of current add, rather than cancel The total current due to the electric field is called the drift current The drift current depends, among other factors, on the ability of the

charge carriers to move through the semiconductor


Fundamentals of semiconductors continue… The measure of this ability to move is called drift mobility and has the

symbol μ. Drift mobility depends the type of carrier and the kind of materials

Thus the total current density due to holes and electrons is

J = Jn + Jp = nqnμnE + pqpμpE …………………………... (1.1)

= nqnvn + pqpvp

Where J = current density , A/m2

n, p = electron and hole densities, carries/m3

qn = qp = unit electron charge = 1.6 x 10-19 c

μn, μp = electron and hole mobilities, m2/(V.s)

vn, vp = electron and hole velocities, m/s

Note: The conductivity of a semiconductor can be computed using

σ = nqnμn + pqpμp ………………………………………….. (1.2)

Where σ = conductivity, 1/(Ω.m), or siemens/meter (S/m)



Exercise1.1 A potential difference 12V is applied across the ends of the intrinsic silicon bar with length 0.6 cm, width 20 mm, and height 20 mm. Assume that ni = 1.5 x 1010 electrons/cm3, μn = 0.14 m2/(V.s), and μp = 0.05 m2/(V.s), find

1. The electron and hole velocities

2. The electron and hole components of the current density

3. The total current density and

4. The total current in the bar


Fundamentals of semiconductors continue… Extrinsic semiconductors

• Introduction Pure Si and Ge are not suitable for any use except in the

manufacture of heat and light sensitive resistance Their conductivity can, however, be altered significantly by addition

of suitable impurity in a very small proportional The process of adding impurity to a pure semiconductor is called

doping The added impurity is called a dopant A doped semiconductor is called extrinsic semiconductor There are two types of extrinsic semiconductors: n-type and p-type

• n-type semiconductor The n-type is formed by doping pentavalent impurity atoms like P,

As, Sb, etc, at a very low level in Si/Ge This process creates excess unbound electrons in conduction band When group V atom (i.e. As) is added to the Si/Ge crystal, the fifth

valence electrons of As does not enter a covalence bond of Si/Ge and it is loosely bound to its parent atom and it is called free electron as shown in Fig. 1.6 (a)


Fundamentals of semiconductors continue… The impurity atom with five valence electrons is called donor atom

Figure 1.6 Covalent bond structure of (a) arsenic doped n-type silicon and (b) boron p-type silicon


Fundamentals of semiconductors continue…• P-type semiconductor

The p-type is formed by doping a pure Si/Ge crystal with impurity atoms having three valence electrons

The elements which can be used for this purpose are; boron, gallium, indium and aluminium

After doping, there will be insufficient number of electrons to complete the covalent bond of newly formed crystal latice

The resulting vacancy is called a hole The impurity atom with three valence electrons are called acceptor

atom Fig. 1.6 (b) shows unfilled covalent bond, as the fourth bond is

empty since boron has only three electrons to share


Fundamentals of semiconductors continue…• Majority and minority carriers

In an n-type material (Fig. 1.7a) the electron is called the majority carrier and the hole the minority carrier.

In a p-type material (Fig. 1.7b) the hole is the majority carrier and the electron is the minority carrier.

Figure 1.7 (a) n-type semiconductor material; (b) p-type semiconductor material


THE END OF LECTURE 1

Lecture_1

Documents

Transcript of Lecture_1