Fundamentals of Semiconductors

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Fundamentals of
Semiconductors

Definition


A

semiconductor

is

a

material

that

is

an

insulator

at

very

low

temperature,

but

which

has

a

sizable

electrical

conductivity

at

room

temperature
.



A

semiconductor

is

a

substance,

usually

a

solid

chemical

element

or

compound,

that

can

conduct

electricity

under

some

conditions

but

not

others,

making

it

a

good

medium

for

the

control

of

electrical

current
.



The

semiconductor

name

derives

from

the

materials

ability

to

sometime

be

a

conductor

to

electricity

and

other

times

act

as

nonconductor

to

electricity
.



The

earliest

semiconductor

material

was

germanium

built

into

a

single

with

one

function

(referred

as

discrete

component)
.


Nowadays,

greater

than

85
%

of

all

microchips

are

made

from

silicon

semiconductor

material
.

Introduction


The

distinction

between

a

semiconductor

and

an

insulator

is

not

very

well
-
defined,

but

roughly,

a

semiconductor

is

an

insulator

with

a

band

gap

small

enough

that

its

conduction

band

is

appreciably

thermally

populated

at

room

temperature
.


It

has

a

resistivity

between

that

of

conductor

and

insulator
.
The

resistivity

of

a

semiconductor

decreases

as

temperature

increases
.

So

at

higher

temperatures

semiconductor

act

like

conductors

and

at

lower

temperatures

they

act

like

insulator
.


Its

conductance

varies

depending

on

the

current

or

voltage

applied

to

a

control

electrode,

or

on

the

intensity

of

irradiation

by

infrared

(
IR
),

visible

light,

ultraviolet

(UV),

or

X

rays


Silicon

dioxide

is

an

example

of

a

nearly
-
perfect

insulator,

while

silicon

is

the

archetypical

semiconductor
.



Many

materials

that

in

the

past

would

have

been

considered

insulators

are

now

called

wide

bandgap

semiconductors
.


Semiconductor

material

do

not

have

free

electrons

to

support

the

flow

of

electrical

current

through

them

at

room

temperature
.


However,

valence

electron

may

become

free

electrons

if

sufficient

energy

is

induced

into

the

material,

for

example

by

heating

the

material
.


The

specific

properties

of

a

semiconductor

depend

on

the

impurities,

or

dopants
,

added

to

it
.


An

N
-
type

semiconductor

carries

current

mainly

in

the

form

of

negatively
-
charged

electron
s,

in

a

manner

similar

to

the

conduction

of

current

in

a

wire
.



A

P
-
type

semiconductor

carries

current

predominantly

as

electron

deficiencies

called

hole
s
.

A

hole

has

a

positive

electric

charge,

equal

and

opposite

to

the

charge

on

an

electron
.



Elemental

semiconductor

include
:


Antimony



-

germanium,



Arsenic



-

selenium



Boron



-

silicon


Carbon



-

sulfur



Silicon

is

the

best
-
known

of

these,

forming

the

basis

of

most

Integrated

Circuits

(IC)



Common

semiconductor

compounds

include
:
-


gallium

arsenide



indium

antimonide


the

oxides

of

most

metals



gallium

arsenide

(GaAs)

is

widely

used

Intrinsic semiconductors (Pure Silicon)


An

intrinsic

semiconductor(intrinsic

silicon)

is

one

which

is

pure

enough

that

impurities

do

not

appreciably

affect

its

electrical

behavior
.



In

this

case,

all

carriers

are

created

by

thermally

or

optically

exciting

electrons

from

the

full

valence

band

into

the

empty

conduction

band
.



The

band

gap
,

or

energy

spacing

between

the

valence

band

and

the

conduction

band,

corresponds

to

the

energy

necessary

to

free

charge

carriers

in

this

way
.

Intrinsic semiconductors (Pure Silicon)

Intrinsic semiconductors (Pure Silicon)

Si

Si

Si

Si

Si

Si

Si

Si

Si

Si

Si

Si

Silicon crystal with
covalent bonds between
silicon (Si) atoms, for a
pure semiconductor
without doping

Covalent
Bond

Doped


The

structure

of

silicon

can

be

altered

to

greatly

enhanced

its

conductivity

by

adding

small

amounts

of

other

elements

to

the

material

through

a

process

known

as

doping
.



Doping

is

the

process

of

adding

certain

elements

to

pure

silicon

to

improve

the

conductivity

of

the

semiconductor
.



The

elements

added

during

doping

are

referred

to

as

dopants

or

impurities

because

the

silicon

is

no

longer

pure
.


Doped (Cont.)


The

more

impurity

added

then

the

higher

the

conductivity

(

or

the

lowest

resistivity)
.



The

term

impurity

used

to

indicate

that

another

element

have

been

added

to

the

silicon
.



Doped

silicon

is

also

known

as

extrinsic

silicon
.


Doped (Cont.)


Two

kinds

of

impurities

are

used
:


The

impurities

with

five

valence

electrons

in

the

outer

ring

are

called

“donors
.


The

semiconductor

material

doped

with

a

donor

impurity

is

known

as

n
-
type

semiconductor

material
.

Phosphorous,

Arsenic,

and

Antimony

are

examples

of

donor

impurities
.


The

impurities

with

three

valence

electrons

in

the

outer

ring

are

called

“accepters
.


The

semiconductor

material

doped

with

an

accepter

impurity

is

known

as

p
-
type

semiconductor

material
.


Doped (Cont.)

Atomic structure of the Silicon Atom

N
-
type Semiconductors

Si

Si

Si

P

Si

Si

Si

Si

Si

Si

Si

Si

Figure 1.0

Crystal lattice structure
of Si atoms doped with
phosphorus (P). The
covalent bonds have one
free electron for each
phosphorus atom

Negative
aluminium
ion

Covalent
Bond

-

Free positive
hole charge

N
-
type Semiconductors (cont.)


The

doping

element

can

be

arsenic,

antimony,

or

phosphorus

which

have

an

electron

valence

of

5
.

This

value

means

one

extra

electron

for

each

group

of

four

in

the

outermost

shell

of

the

atom
.



As

a

result,

each

impurity

atom

provides

an

extra

electron

in

the

covalent

bonds
.

Figure

1
.
0

show

silicon

with

atomic

number

14

doped

with

phosphorus

(P),

which

has

atomic

number

15

and

5

valence

electron
.


Four

of

these

become

part

of

covalent

bond

structure
.

The

extra

electron

can

be

considered

as

a

free

negative

charge
.

The

result

is

N
-
type

doped

silicon
.

P
-
type Semiconductors

Si

Si

Si

Al

Si

Si

Si

Si

Si

Si

Si

Si

Figure 2.0

Crystal lattice structure
of Si atoms doped with
Aluminium (Al). The
covalent bonds have one
free positive hole charge
for each aluminium atom

Positive
phosphorus
ion

Covalent
Bond

+

Free negative
electron charge

P
-
type Semiconductors (cont.)


The

doping

element

can

be

aluminum,

boron,

gallium

or

indium

which

have

a

electron

valence

of

3
.

Each

atom

has

three

electrons

in

the

outermost

ring
.



Figure

2
.
0

show

silicon

doped

with

aluminum

(Al)
.

The

element

Al

has

atomic

number

13
,

which

means

three

outer

electrons
.


In

the

covalent

bonds

of

Al

and

Si

atoms,

there

are

seven

electrons

instead

of

eight
.

The

one

missing

electron

in

covelent

bond

can

be

considered

as

a

free

positive

charge,

called

a

hole
.



With

many

aluminum

atoms

added,

the

doping

provide

many

hole

charges
.

The

hole

are

free

charges

that

can

move

with

relative

ease

to

produce

electric

current
.


The

result

is

P
-
type

doped

silicon
.

PN Junction

Electron jumping across

The junction to recombine

With a hole

Depletion Region

Junction

P
-
type material

Figure 3.0:

The PN Junction

N
-
type material

PN Junction (cont.)


When
p
-
type

semiconductor is placed next to an
n
-
type

semiconductor, thus forming a “junction.”


At the junction some of the free electrons from the n
-
type
semiconductor cross the junction and fill the holes.


This movement of electrons across the junction leaves the
region in
n
-
type

semiconductor adjacent to the junction
with excess positive charge.


The electrons filling the holes in the region adjacent to the
junction on the
p
-
type

semiconductor create excess
negative charge.


Figure 3.0

shows the dynamics of a
p
-
n junction
. This
oppositely charged region on both sides of the junction,
known as the “depletion region,” creates a potential barrier,
which is 0.7 volts for Silicon based semiconductor and 0.2
for Germanium based semiconductor.

Forward Biased PN Junction

Figure 4.0:

A forward biased PN Junction

With sufficiently high potential

In forward
-
biased junction, free
electrons will jump across the
depletion region to form a flow
of electrical current


Electron jumping across

The junction to recombine

With a hole

Depletion Region

Junction

Forward Biased PN Junction


If we connect the positive and negative terminals of a
power supply to the p
-
type and n
-
type semiconductors,
respectively, the junction is said to be
forward
-
biased.


Figure 4.0

shows the dynamics of the junction under this
condition.


When the potential applied across the
p
-
n junction

is
sufficiently high (greater than 0.7 volts for Silicon based
semiconductor and greater than 0.2 volts for the
germanium based semiconductor), the free electrons from
the
n
-
type

material will be able to jump across the
depletion region and go towards the positive terminal of the
power supply.


This causes the
Id
to flow through the p
-
n junction, from
positive terminal to the negative terminal of the power
supply.

Reversed Biased PN Junction

Figure 5.0:

A reversed biased PN Junction

Free electrons moving towards the positive
terminal leaving a wider positively charged
depletion region (no free electrons) on the N
-
type material


Hole moving towards the negative
terminal leaving a wider negatively
charged depletion region (no holes)
on the P
-
type material

Junction

Depletion Region widens

P
-
type material

N
-
type material

Reversed Biased PN Junction


If we connect the positive and negative terminals of a power
supply to the n
-
type and p
-
type semiconductors, respectively,
the junction is said to be
reverse
-
biased.


Figure 5.0

shows the dynamics of the junction under this
condition.


When the potential applied across the
p
-
n junction

with this
polarity, the free electrons in the
n
-
type

semiconductor are
attracted towards the positive voltage and move away from
depletion region.


The holes on the
p
-
type

semiconductor are attracted to the
negative terminal of the power supply.


This, in effect, widens the depletion region and essentially
makes it impossible to have a current flow from positive
terminal to the negative terminal of the power supply.


In this way, a
p
-
n junction

(diode) essentially acts like a
valve, in which air can be pumped in one direction but cannot
come out (flow) in the other direction.

Half Wave Rectifier

Figure 6.0

Half Wave Rectifier


The behavior of the p
-
n junction under forward
-
and reverse
-
biased conditions isvery useful in several applications.


Consider one such application known as
rectification
.


Rectification is a process of converting an AC signal to a DC
signal.


Figure 6.0

shows a simple circuit of a rectifier.


This particular rectifier is called
half
-
wave rectifier
as it only
produces the positive half cycle of the input signal at the
output of the rectifier.


Note that during the positive half cycle, the diode is forward
biased and the current
ID
flows through the diode.


During the negative half cycle, however, the diode becomes
reverse
-
biased and prevents the current flow through the
diode.