atoms - city university londondanny/6_introsc.pdf · • such atoms have three valence electrons,...

6. Introduction to Semiconductor Devices

1

ATOMS

• All matter is made of atoms, which consist of electrons,

protons and neutrons

• An atom is the smallest particle of an element that

retains characteristics of that element

• There are 109 elements, each of which has a different

atomic structure

• In the classic Bohr model, an atom is visualised as

having a planetary type structure that consists of a

central nucleus, surrounded by orbiting electrons

• Nucleus consists of positively charged particles called

protons, and equal number of uncharged particles

called neutrons

• Electrons have a negative charge

• Each type of atom has a certain number of electrons

and protons that distinguishes it from other atoms of

other elements

• The simplest atom is that

of hydrogen, with one proton

and one orbiting electron

• Helium has two protons,

two neutrons and two electrons

Electron Proton Neutron


2

ATOMIC STRUCTURE

• Elements are arranged in the periodic table according

to their atomic number, which is the number of protons

in the nucleus

• Hydrogen has an atomic number of 1, and helium 2

• In their neutral state, all atoms of an element have an

equal number of electrons and protons

• Thus the positive charges cancel out the negative

charges, so an atom is electrically balanced

• Electrons orbit the nucleus of an atom at certain

distances from the centre

• Electrons near the nucleus have less energy than those

in more distant orbits

• Only separate and distinct (discrete) values of electron

energies exist within atomic structures

• Thus electrons orbit at discrete distances from nucleus

• Each discrete orbit corresponds to an energy level

called a shell

• Each shell has a fixed maximum number of electrons at

permissible energy levels (orbits)

• Shells are designated 1, 2, 3 and so on, with 1 nearest

the nucleus

• Maximum number of electrons permitted in each shell

follows 2N2, where N is number of shell

• So first shell has 2 electrons, 2nd has 8, 3rd 18, etc..


3

VALENCE ELECTRONS

• Electrons in orbits farthest away from the nucleus have

higher energy, and are less tightly bound to the nucleus

than those closer to it

• This is due to force of attraction between positively

charged nucleus and negatively charged electrons,

which decreases with distance

• Electrons with the highest energy exist in the outermost

shell of an atom called the valence shell

• Electrons in valence shell are called valence electrons

and determine a material’s electrical properties

• If an electron absorbs a photon (particle of

electromagnetic radiation) with sufficient energy, it can

escape the atom and becomes a free electron

• When an atom is left with a net charge (i.e. when there

are an unequal number of electrons and protons), it is

called an ion

• When an electron escapes from a parent atom, the

atom gains a net positive charge as there are now more

protons than electrons than protons – the atoms

becomes a positive ion

• When an atom acquires an electron, it becomes a

negative ion – more electrons than protons


4

THE COPPER ATOM

• Copper is the most commonly used metal in electrical

applications

• It has 29 electrons in orbit around nucleus in 4 shells

• In the valence shell, there is only 1 valence electron

• When this valence electron gains sufficient thermal

energy, it can break away from the parent atom and

thus becomes a free electron

• In a piece of copper at room temperature, several of

these free electrons are present, and are free to move

in the copper material

• These free electrons make copper an excellent

conductor and make electrical current possible


5

CATEGORIES OF MATERIALS

• Conductors are materials that readily allow current

• They have a large number of free electrons, and are

characterised by one to three valence electrons in their

atomic structure

• Most metals are good conductors

• Silver is the best conductor, followed by copper

• Copper is more widely used as it is less expensive than

silver

• Semiconductors are classed below conductors in their

ability to carry current as they have fewer free electrons

• Semiconductors have four valence electrons, yet

because of their unique characteristics, semiconductor

materials are the basis of electronic devices such as

diodes and transistors

• Silicon and germanium are common semiconductors

• Insulators are poor conductors of electrical current

• They are used to prevent current flow where it is not

wanted

• Insulators have very few free electrons and are

characterised by more than four valence electrons


6

ELECTRICAL CHARGE

• The charge on an electron and proton are equal in

magnitude

• Electrical charge exists because of an excess or

deficiency of electrons (Q)

• Static electricity is the presence of a net positive or

negative charge in a material

• Materials with charges of opposite polarity are attracted

to each other, those of the same polarity are repelled

• A force acts between the charges (attraction or

repulsion), and is called an electric field

• Unit of charge is the coulomb, where 1 coulomb is the

total charge possessed by 6.25×10-19 electrons


7

POSITIVE AND NEGATIVE CHARGE

• A neutral atom has the same number of electrons and

protons – it has no net charge

• When a valence electron gains enough energy to pull

away from the atom, the atom is left with a net positive

charge (more protons than electrons)

• It thus becomes a positive ion

• If the atom acquires an extra electron, it becomes a

negative ion as there are now more electrons than

protons, and so it has a net negative charge

• The amount of energy required to free a valence

electron is related to the number of electrons in the

outer shell

• The more complete an outer shell, the more stable the

atom and thus the more energy is required to release

an electron


8

SILICON AND GERMANIUM ATOMS

• Both silicon and germanium have 4 valence electrons

• Silicon has 14 protons, whereas germanium has 32

• Valence electrons in germanium are in the 4th shell, and

those of silicon are in the 3rd shell

• Germanium valence electrons are at higher energy

levels than those of silicon and thus require a smaller

amount of additional energy to escape from the atom

• This makes germanium more unstable than silicon at

high temperatures, and is the main reason silicon is the

most widely used semiconductive material


9

ATOMIC BONDING

• When certain atoms combine into molecules to form a

solid material, they arrange themselves into a fixed

pattern called a crystal

• Atoms within the crystal structure are held together by

covalent bonds

• These are created by the interaction of valence

electrons of each atom

• In a silicon crystal, each atom positions itself with four

adjacent atoms

• A silicon atom with its four valence electrons shares an

electron with each of its four neighbours

• This creates eight valence electrons for each silicon

atom, and hence improves chemical stability

• This sharing produces covalent bonds that hold the

atoms together

• Each shared electron is attracted equally by two

adjacent atoms

• An intrinsic crystal is one without impurities


10

CONDUCTION ELECTRONS AND

HOLES

• Here is an energy band diagram for a silicon crystal with only unexcited atoms (no external energy)

• An intrinsic (pure) silicon crystal at room temperature has enough heat energy for some valence electrons to jump the gap from the valence band into the conduction band, becoming free electrons

• When an electron jumps to the conduction band, a vacancy (a hole) is left in the valence band

• For every electron raised to the conduction band by external energy, there is one hole left in the valence band, creating an electron-hole pair

• Recombination occurs when a conduction band electron loses energy and falls back into a hole in the valence band

• Thus a piece of intrinsic silicon at room temperature has a number of free, drifting conduction band electrons, and an equal number of holes in the valence band


11

ELECTRON AND HOLE CURRENT

• When a voltage is applied across a piece of silicon, the

free electrons in the conduction band are attracted to

the positive terminal of the voltage source

• The corresponding movement of free electrons is one

type of current in semiconductive material called

electron current

• Another type of current occurs at valence level, where

the holes created by the free electrons exist

• Electrons that remain in the valence band are still

attached to their parent atoms and are not free to move

randomly in the crystal

• Yet a valence electron can move into a nearby hole,

thus leaving another hole where it came from

• The hole has effectively (not physically) moved from

one place to another

• This is called hole current


12

N TYPE AND P TYPE

SEMICONDUCTORS

• Semiconductors do not conduct current well, and are of

little use in their intrinsic state

• So intrinsic silicon (or germanium) must be modified by

increasing the free electrons and holes to increase its

conductivity

• This is done by adding impurities to form an extrinsic

semiconductive material

• There are two types of extrinsic semiconductors

• N-type and P-type

• Doping is the process where impurities are added to a

semiconductor to increase its conductivity


13

N-TYPE SEMICONDUCTOR

• Pentavalent impurity atoms

are added to intrinsic silicon

to increase the number of

conduction band electrons

• Such atoms have 5 valence

electrons, and are known as

donor atoms – arsenic (As),

Phosphorus (P), antimony (Sb)

• Donor atoms provide an extra electron to the

semiconductor’s crystal structure

• Each pentavalent atom forms covalent bonds with 4

adjacent silicon atoms

• Four of the pentavalent atom’s valence electrons are

used to form the covalent bonds with silicon atoms,

thus leaving one extra electron

• This extra electron becomes a conduction electron as it

is not attached to any atom

• In an n-type (n - negative electron charge) semi -

conductor, most of the current carriers are electrons

• Hence in this case the majority carriers are electrons

• There are a few holes, but in n-type material they are

minority carriers


14

P-TYPE SEMICONDUCTOR

• To increase the number of

holes in intrinsic silicon,

trivalent impurity atoms

are added

• Such atoms have three

valence electrons, such

as aluminium (Al), boron

(B), Gallium (Ga), and are

known as acceptor atoms

• Acceptor atoms leave a hole in the semiconductor’s

crystal structure

• Each trivalent atom forms covalent bonds with four

adjacent silicon atoms

• All three of the trivalent valence electrons are used in

the covalent bonds

• Since four electrons are required, a hole is thus formed

with each trivalent atom

• Here most of the current carriers are holes, which can

be thought of as positive charges

• Thus holes are the majority carriers in p-type material,

and electrons are the minority carriers

• Silicon doped with trivalent atoms is a p-type

semiconductor


15

DIODES

• If you take a block of silicon and dope one half of it with

a trivalent impurity and the other half with a pentavalent

impurity, the boundary between the two regions is

formed called the pn junction

• A diode consists of an n region and a p region

separated by a pn junction

• The n region has many conduction electrons, and the p

region has many holes

• There is no movement of electrons (current) through a

diode at equilibrium

• A diode has the ability to allow current flow in only one

direction, which is determined by the bias

• Bias refers to the use of a DC voltage to establish

certain operating conditions for a device

• For a diode, there are two bias conditions: forward and

reverse

• These conditions are created by application of a

sufficient external voltage of the proper polarity across

the pn junction


16

THE DEPLETION REGION

• With no external voltage, conduction electrons in the n region randomly drift in all directions

• At the instant of junction formation, some of the electrons nearthe junction drift into the p region and recombine with holes close to the junction

• For each electron that crosses the junction and recombines with a hole, a pentavalent atom is left with a net positive charge in the n region near the junction, making it a positive ion

• When an electron recombines with a hole in the p region, a trivalent atom acquires a net negative charge, making it a negative ion

• Due to this recombination process a large number of positive and negative ions build up at the pn junction

• Electrons in the n region must overcome attraction of the positive ions and repulsion of negative ions in order to migrateinto p region

• As the ion layers build up, both sides of the junction become depleted of any conduction electrons or holes, and forms the depletion region

• At equilibrium, the depletion region has widened to a point where no more electrons can cross the pn junction

• The barrier potential is the amount of voltage needed to move electrons through the depletion region (Silicon = 0.7V, germanium = 0.3V


17

DIODE: FORWARD BIAS

• Forward bias permits diode current flow

• Negative terminal of DC bias voltage connected to n region, positive terminal to p

• Negative terminal of bias voltage pushes conduction band electrons in the n region toward the pn junction

• Positive terminal pushes holes in the p region also toward the pn junction

• When the bias voltage is greater than the barrier potential, there is enough energy for the n region electrons to penetrate the depletion region, and move through the junction and recombine with p region holes

• As electrons leave the n region, more flow from the negative terminal of the dc voltage source

• Current is thus formed through the n region by the movement of majority electrons to the pn junction

• Once the conduction electrons enter the p region and combine with holes, they become valence electrons

• They move as valence electrons from hole to hole to the positive terminal of the voltage source

• Thus the current in the p region is formed by the movement of holes toward the pn junction


18

DIODE: REVERSE BIAS (1)

• Prevents current flow

• Negative terminal of

DC voltage source

connected to p region,

positive to n region

• Negative terminal attracts

holes in p region away from pn junction

• The positive terminal attracts electrons away from the

pn junction

• This causes the depletion region to widen

• More positive ions created in the n region, and more

negative ions created in the p region

• The depletion region widens until the voltage across it

equals the source bias, and at this point the holes and

electrons stop moving away from the pn junction

• When reversed bias, depletion region acts as an

insulator between layers of oppositely charged ions

• The depletion region widens with increased reverse

bias voltage

• Majority current becomes zero with reverse bias

• Small amount of minority current is leaked (nA)

• Some electrons manage to diffuse across the pn

junction


19

DIODE: REVERSE BIAS (2)

• As electrons and holes move away from the pn

junction, the depletion region widens

• More positive ions are created in the n region, and

more negative ions in the p region

• Initial flow of majority carriers away from the pn junction

is called transient current and lasts only for a very

short time on application if reverse bias


20

REVERSE BREAKDOWN

• If the external reverse bias voltage is increased to a

large enough value, reverse breakdown occurs

• Most diodes normally are not operated in reverse

breakdown

• Diodes can be damaged when reverse breakdown

occurs

• Zener diodes are specifically designed for reverse

breakdown operation


21

DIODE SYMBOL

• The arrowhead in the symbol points not in the direction

of electron flow, but in the direction of conventional

current flow

• Metal contacts are connected to each region, anode to

the p region, cathode to the n region

• When the anode is positive with respect to the cathode,

diode is forward biased and current IF

is from cathode

to anode

• When the diode is forward biased, the barrier potential,

VB, always appears between the anode and cathode

• When the anode is negative with respect to the

cathode, diode is reverse biased, and there is no

current flow

VBIAS

+ VB -

R

+ VB -

RVBIAS

IF

I = 0


22

IDEAL DIODE MODEL

IF

VR VF

IR


23

PRACTICAL DIODE MODEL (1)

• The ideal model on the previous slide neglected the

effect of the barrier potential, the internal resistances

and other parameters

• The practical diode model offers more accuracy

• The forward bias diode is represented as a closed

switch in series with a small battery equal to the barrier

potential, VB

(0.7V for silicon)

• The positive end of the battery is towards the anode

• Remember that the barrier potential is not a voltage

source, and can’t be measured with a voltmeter

• It only has the effect of a battery when forward bias is

applied because the forward bias voltage must

overcome the barrier potential for the diode to conduct

• The reverse biased diode is represented by an open

switch (as ideal case) because the barrier potential

does not affect reverse bias


24

PRACTICAL DIODE MODEL (2)


25

COMPLETE DIODE MODEL

IF

VR VF

IR

0.7V

Slope due to low forward

resistance

Small reverse current due

to high reverse resistance

atoms - city university londondanny/6_introsc.pdf · • such atoms have three valence electrons,...

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