1

H63SSD

Solid State Devices

Dr. Mumtaj Begam

Department of Electrical and Electronic Engineering

Room: DB24

Extn. 3488

Email: Mumtaj.Begam@nottingham.edu.my

An understanding of the physics of semiconductor materials

2

Prerequisites

Co-requisites: None

Basic understanding of calculus and differential equations

A good understanding of p-n junctions

Understanding of the nature of signals in information transfer

3

Teaching

- Lecture 2hrs

Thursday: 11:00 -13:00 @ F1A09

- Tutorial 1hr (Example sheet discussion)

Monday: 15:00 -16:00 @ F1A10

4

Method of Assessment

Assessments:

1% Written Exam Paper (2 hours - 3 compulsory

questions)

5

Student Evaluation (Feedback)

Student evaluation of teaching (SET)

Evidence from student evaluation of teaching is required for all

professors, readers, senior lecturers, lecturers, teaching fellows and

other University of Nottingham staff with responsibilities for teaching

who have either a full-time or part-time (50% or more) contract with the

University.

All teaching staff are to be evaluated by students for the purposes of

appraisal.

Student evaluation of module (SEM)

SEM is required to gather feedback from students on modules for

curriculum development.

SEM should be implemented as part of a school's 'Course Review'

strategy.

6

Aims and Objectives of the module

To introduce students to the internal operation of

commonly used electronic and optoelectronic

semiconductor devices

To illustrate how the device design and performance

relates to the underlying materials and physical

processes

To illustrate how the demands of the targeted application

affect the device design

7

Module Contents

Develop a detailed understanding of the internal operating

mechanisms of semiconductor electronic and opto-electronic

devices

Focus on devices based on pn junctions (e.g. diodes, bipolar

junction transistors) and devices based on MOS capacitors (e.g.

memory cells, CCD detectors, MOSFETs)

Consider how the targeted application for a device impacts upon its

design. (For example, signal-mixing diodes, power diodes, light-

emitting diodes and solar cells are all based upon the pn diode, but

provide very different functionality)

Discuss the characteristics required of these devices in relation to

their incorporation into appropriate electronic systems

8

Learning Outcomes

By the end of the module, students should be able to:

demonstrate a good understanding of the operating principles and

underlying physical processes used in electronic and optoelectronic

semiconductor devices

relate the performance limitations of semiconductor devices to the

underlying physical processes and materials properties

demonstrate an appreciation of how the performance requirements

of different applications influences the device design

9

Modern semiconductor Devices and

Applications

Diodes - Power rectification, signal mixers

Photodetectors - Telecommunications, Optical sensors/switches

Solar cells - Power generation (space/remote applications)

LEDs - Indicators, Displays

Laser diodes - Telecommunications, CD players, Surgery,

printers/displays

MOS capacitor - Dynamic Random Access memories (DRAMS)

CCDs - Imaging, Detectors

FETs - VLSI, CCD arrays

Bipolar Transistors - Power electronics, Electronics

Thyristors - Power electronics

Approach

10

Explore two of the most important structures pn junction diodes and

MOS capacitors

Develop an understanding of the devices based on pn junctions (eg.

LED/lasers and photodetector/solar cells)

Develop an understanding of devices based on MOS capacitors (e.g.

CCD arrays and MOSFETs)

Solid-state gets its name from the path

that electrical signals take through solid

pieces of semi-conductor material

Prior to the use of solid-state devices,

such as the common transistor, electricity

passed through the various elements

inside of a heated vacuum tube

Solid-state devices, such as a transistor,

use conductors to control the flow of

signals through a circuit

Solid-state electronic devices are part of

our everyday lives

The transistor, invented in 1947 by Bell

Labs, was the first solid-state device to

come into commercial use in the 1960s

Solid-state electronic devices have

replaced vacuum tubes in just about all

electronics devices.

11

What does solid-state mean in relation

to electronics?

Basic information

To understand why solid-state devices function as they do, we will have to

examine closely the composition and nature of semiconductors. This entails

theory that is fundamental to the study of solid-state devices.

Rather than beginning with theory, let's first become reacquainted with some

of the basic information you studied earlier in the previous module.

The UNIVERSE consists of two main parts-matter and energy.

MATTER is anything that occupies space and has weight. Rocks, water,

and air are examples of matter.

Matter may be found in any one of three states: solid, liquid and

gaseous.

It can also be composed of either an element or a combination of

elements.

An ELEMENT is a substance that cannot be reduced to a simpler form by

chemical means. Iron, gold, silver, copper, and oxygen are all good

examples of elements

12

Basic information

A COMPOUND is a chemical combination of two or more elements.

Water, table salt, ethyl alcohol, and ammonia are all examples of

compounds.

A MOLECULE is the smallest part of a compound that has all the

characteristics of the compound. Each molecule contains some of

the atoms of each of the elements forming the compound.

The ATOM is the smallest particle into which an element can be

broken down and still retain all its original properties. An atom is

made up of electrons, protons, and neutrons. The number and

arrangement of these particles determine the kind of element.

13

An ELECTRON carries a small negative charge of electricity.

The PROTON carries a positive charge of electricity that is equal

and opposite to the charge of the electron. However, the mass of

the proton is approximately 1,837 times that of the electron.

The NEUTRON is a neutral particle in that it has no electrical

charge. The mass of the neutron is approximately equal to that of

the proton.

14

Basic information

An ELECTRON'S ENERGY LEVEL is the amount of energy

required by an electron to stay in orbit.

Just by the electron's motion alone, it has kinetic energy.

The electron's position in reference to the nucleus gives it potential

energy.

An energy balance keeps the electron in orbit and as it gains or

loses energy, it assumes an orbit further from or closer to the

center of the atom.

15

Basic information

CHEMICAL REACTIVITY OF AN ATOM:

The chemical activity of an atom is determined by the number of

electrons in its valence shell.

When the valence shell is complete, the atom is stable and shows

little tendency to combine with other atoms to form solids.

Only atoms that possess eight valence electrons have a complete

outer shell.

These atoms are referred to as inert or inactive atoms.

However, if the valence shell of an atom lacks the required number

of electrons to complete the shell, then the activity of the atom

increases.

16

Basic information

SHELLS and SUBSHELLS

Shells and Subshells are the orbits of the electrons in an atom.

Each shell can contain a maximum number of electrons, which can

be determined by the formula 2n2.

Shells are lettered K through Q, starting with K, which is the

closest to the nucleus.

The shell can also be split into four subshells labeled s, p, d, and f,

which can contain 2, 6, 10, and 14 electrons, respectively. (see

Figure)

17

Basic information

VALENCE is the ability of an atom to combine with other atoms.

This shell is referred to as the VALENCE SHELL.

The valence of an atom is determined by the number of electrons

in the atom's outermost shell. The electrons in the outermost shell

are called VALENCE ELECTRONS.

IONIZATION is the process by which an atom loses or gains

electrons.

An atom that loses some of its electrons in the process becomes

positively charged and is called a POSITIVE ION.

An atom that has an excess number of electrons is negatively

charged and is called a NEGATIVE ION.

18

Basic information

The collection of densely packed atoms is called a SOLID. In other

words, isolated atoms are brought together to form a solid.

The volume density of atoms in a solid matrix contains ~ 1022 atoms/cm3

as per the following simple relation:

Density = No. of atoms in the unit cell/ volume of crystal

The volume density of atoms represents the order of magnitude of

density for most materials.

However, the actual density is a function of the crystal (solid) type and

crystal structure since the packing density number of atoms per unit

cell depends on crystal structure.

According to band theory of solids, the solids can be classified on the

basis of their resistivity () as metals (conductors), semiconductors and

insulators.

19

What is a SOLID?

Crystalline and amorphous

Broadly speaking, these solids (conductor, semiconductor and

insulators) can fall into one of two categories: those which possess

long-range-order in the disposition of their atoms, and those which

do not.

The first type of material is known as a crystal (single crystal or

polycrystalline), while the second is termed an amorphous

material.

20

Single crystal structure Polycrystalline structure Amorphous structure

Atomic structure of Silicon

21

Energy band theory

Metals are good conductors of electricity - have "free electrons" that can move

easily between atoms, and electricity involves the flow of electrons.

In silicon crystals all of the outer electrons are involved in perfect covalent bonds,

so they can't move around. A pure silicon crystal is nearly an insulator -- very little

electricity will flow through it.

The interaction between the electrostatic field of the atoms splits each energy level

into two (Pauli Exclusion Principle).

Many atoms are brought together, the split energy levels form essentially

continuous bands of energies.

22

Energy band theory

23

Note: With such a small gap, the

presence of a small percentage of a

doping material can increase

conductivity dramatically.

atom atom in a solid

Energy arrangement

At the distance between atoms

equilibrium interatomic spacing (Si:

5.43 )

The Band splits into two bands,

separated by forbidden gap (band

gap),

upper band : conduction band

(completely empty of electrons

at 0K)

lower band: valence band (fully

occupy by electrons at 0K)

Energy band theory

24

Metals Semiconductors Insulators ( -cm)

10-8 10-4 100 104 108 1012 1016 1020

25

Insulator, conductor & semiconductor

Insulator, Eg = 7 eV

No conduction is possible even under the

influence of an electric field.

Ex: Diamond, Glass, SiO2, Al2O3, etc.

Conductor, no energy gap

Conduction band & valence band very close

to one another & may even overlap; high

conductivities

Ex: Copper, aluminum, silver, etc.

Semiconductor, Eg ~ 1.1 eV (narrow gap)

Conductivities lie in between insulators and

conductors.

At zero degree temp. - behaves as an

insulator.

Sensitive to temperature, illumination,

magnetic field and impurity atoms.

Ex: Si, Ge and GaAs

Compound semiconductor:

III-V compounds: GaAs and InP

II-VI compounds: CdS and ZnSe

Empty c.b

v.b

Energy gap

Partly filled c.b

v.b. filled up with electrons

Eg= Ec-Ev

Ev

Ec

Bottom of conduction band

Top of valence band

26

Semiconductors partially conduct electricity

Their conductivity can be controlled by introducing dopant impurities

Their conductivity can be changed by more than 8 orders of magnitude from

semi-insulating to semi-metallic

Semiconductors Electrical properties

How does conduction occur in a semiconductor?

How do dopant impurities change the conductivity?

The valence and conduction bands are responsible for most of the

interesting electrical and optical properties of a semiconductor

material.

Intrinsic semiconductors

27

Silicon and germanium, are the most frequently used semiconductors

Both are quite similar in their structure and chemical behavior

Silicon has 14 electrons. The four electrons that orbit the nucleus in the

outermost, or "valence," energy level are given to, accepted from, or

shared with other atoms

The sharing of valence electrons between two or more atoms produces a

COVALENT BOND between the atoms

Intrinsic semiconductors

28

There are no energy levels in the band gap in a pure (intrinsic) semiconductor.

Physically, Eg corresponds to the energy needed to remove a valence electron

from its orbit to turn it into a conduction electron (leaving a hole behind, a hole

is the vacancy created by moving an electron from VB to CB).

At T = 0 K, the valence band (VB) is completely full and the conduction band

(CB) is completely empty.

No conduction in conduction band (i.e., no electron)

No conduction in valence band.

Hence no net current

At T > 0 K, increases, electrons jump from VB to CB to form electron-hole

pairs.

Thermal generation of an electron-hole pair

A hole is a fictitious, positively charged particle created in the

valence band by the removal of an electron.

29

Where Nc and Nv are the effective density of states in the conduction

and valence bands.

Conductivity for an intrinsic semiconductor is given by

where represent the mobility of electrons and holes

respectively.

n varies with temperature and different for different semiconductors.

Then the number of holes in the valence band or the number of electrons

in the conduction band is given by

Extrinsic semiconductor

Injecting a foreign atom into an intrinsic semiconductor.

Foreign atom injected is called dopant or impurity.

The process is called doping and the dopants are elements from gp.

III (B, Al, In, Ga) and V(As, P, Sb).

The impurity-added semiconductors are known as doped

semiconductors.

These semiconductors have unequal concentration of

electrons and holes - called extrinsic semiconductors.

Almost all semiconductor devices employ extrinsic

semiconductors.Why???

30

N-type semiconductors

If a group IV (see Figure below) semiconductor (eg. Si), is

doped with a group V element such as P ( which has five

electrons), the spare electron occupies an energy level just

below the CB.

31

Si+

4

As

+5

Si+

4

Si+

4

Si+

4

Si+

4

Si+

4

Si+

4

Si+

4

Conduction

electron

P has 5 valence electrons

Four electrons make covalent bonds with Si

atoms.

Fifth electron bound to P atom by weak

electrostatic forces

N-type semiconductors

We normally assume that at room temperature all of these spare

electrons pick up enough energy (Only a small thermal energy (kT/q at

290K = 0.025eV) to be excited to the CB, producing Nd conduction

electrons/m3, i.e., to a good approximation, . (In an n-type

semiconductor, ). This process leaves behind positively charged

donor atoms. That is,

Since impurity atoms donates one free electron - called donor atom or

donor.

The electron concentration will be more than the intrinsic carrier

concentration (ni) and becomes higher than the hole concentration.

The semiconductor is called n-type semiconductor.

Here, electrons are the majority, termed majority carriers whereas the

holes are the minority - termed minority carriers.

32

P-type semiconductors

If Si is doped with a group III dopant such as B, an acceptor level is

created just above the VB.

This level can accept electrons from the VB, leaving behind holes.

That is, for p-type SCs, so

33

P-type semiconductors

At 0K the hole remains bound to the impurity atom but breaks away

from it at higher temperature and wanders freely in the crystal. An

impurity atom that contributes a hole is called an acceptor because it

accepts a bound electron from the covalent bond.

These atoms ionize at room temperature, contributing one hole per

impurity atom.

The crystal has an excess of holes - called p-type semiconductor.

If the crystal is doped with both donor and acceptor, neither electrons

nor holes will be produced - process known as compensation.

If donors exceed acceptor - n-type vice versa for p-type.

If electron and hole concentrations are the same - fully compensated.

Properties different from intrinsic.

34

E-k diagram

35

Electron-Hole product:

At equilibrium, n x p = ni2

where ni depends on temperature.

Probability Function:

The probability of a level at energy E being occupied by

an electron is given by the Fermi function:

where EF is the Fermi Energy (or Fermi Level) and is the

energy at which the probability of occupation is 0.5.

Similarly, the probability of a state at E being occupied by a

hole is [1- f(E)].

E-k diagram

Each electron state in the valence and conduction bands has an energy

and a momentum associated with it. Sometimes, it is useful to represent

these bands as an energy-vs-momentum plot (E vs k) plot of the

energy states which make up the bands.

36

Direct band gap semiconductors

Direct transition:

A photon of energy h =Eg can excite an electron from

the top of valence band

directly into a state at the

bottom of the conduction

band.

If electron from c.b. decays

to v.b., radiation (photons)

will be emitted*.

37

*Application as

lasers & LED

photons

=

In a direct band gap semiconductor (e.g:

GaAs, InP, CdS etc), the top of the valence

band and the bottom of the conduction band

occur at the same value of momentum.

Indirect band gap semiconductors

Indirect transition:

The valence band maximum and the lowest conduction band minimum are at different

values of k. Direct transition of the electron from VB to CB by a photon of energy Eg is

not possible.

Because a photon has a very small momentum whereas the transition involves a

change in momentum. The transition can occur indirectly by cooperation of a lattice

phonon (vibrational modes) which can supply the required momentum.

38

This is called an indirect semiconductor. In the above case, most of the CB

electrons are near a and most of the holes are near a, so a a transitions do

happen. But they are phonon transitions, not PHOTON transitions. So silicon is

not suitable for LEDs, for instance.

VB

CB

a

a

39

The difference between the two is most important in optical devices

Some semiconducting elements and compounds at 300 K

DBGS and IBGS

40

Indirect

band

gap

semico

nductor

semiconductor in which bottom of the conduction

band does not occur at effective momentum k=0,

i.e. is shifted with respect to the top of the valence

band which occurs at k=0; energy released during

electron recombination with a hole is converted

primarily into phonon; e.g. Si, Ge, GaP.

direct

band

gap

semico

nductor

semiconductor in which the bottom of the conduction

band and the top of the valence band occur at the

momentum k=0;in the case of d.b.s. energy released

during band-to-band electron recombination with a

hole is converted primarily into radiation (radiant

recombination); wavelength of emitted radiation is

determined by the energy gap of semiconductor;

examples of d.b.s. GaAs, InP, etc.

41

Text Books and Reference Books

Text Books:

1. Solid State Electronic Devices (6th edition), Ben G.

Streetman & Sanjay Kumar Banerjee, Pearson

Prentice Hall, 2006. ISBN: 0132017202

2. Semiconductor Devices, S.M. Sze, Wiley Inter-

Science, ISBN: 0471056618.

Reference Books:

1. Semiconductor Physics and devices (2nd edition),

Donald A. Neamen, McGraw-Hill.

2. Semiconductor device fundamentals, Robert F.

Pierret, Addison Wesley