EMIGONDUUC

DUGTOn DEVICES
onment. As As there are billions of
v
onfom cluster or band calledfirst-orbit electrons in the solid with
e l sw h i c h f o r
a 6-5
energy bamd.
different energy levels form the second Similarly, the billions of
ightly differ slightly different energy
h
MPORTA ENERGY BANDS IN energy band and second-orbit electrons
so on.
4
SSed earlier, when the atom is inSOLIDS
As discus

ASm converted into

latedatom.are
a
solid, the individual energy levels (K. L,
alids, but
ands in solic
corresponding
more concerned energy bands. Although there are
we are
Metc.) of an

pans with the number of a

nce band. The electrons in the following: energy
outermost orbit of an atom are
nder condition
normal of the atom, valence band contains known as valence electrons.
and may be filled comipletely or partically the clectrons of highest energy. This
Tfhe energ
band which possesses the valence
electrons is called valence band
Conduction band. In some of the materials (e.g. metals), the
(See Fig 4).
valence electrons are loosely
hed to the nucleus and can be detached very easily. These
attache

are responsible for the conduction of current. For this electrons are known as free electrons
and
omduction electrons.
reason, these electrons are also known as

The energy band which possesses the conduction

or free) electrons is called conduction band (See Fig. 4). BAND
ENERGY
If a substance has empty conduction band, it means EMPTY CONDUCTION
BAND
curent conduction is not possible in that substance and is
known as insulator,
(ün Forbidden energy gap. The energy gap between FORBIDDEN
the valence band and conduction band is known as ENERGY
GAP
forbidden energy gap (See Fig. 4).
Forbidden energy gap is a region in which no electron
VALENCE BAND
can stay as there is no allowed energy state. The width of
the forbidden energy gap represents the bondage of valence Fig. 4

electrons to the atom. The greater the forbidden energy gap more tightly the valence eiectrons are
bound to the nucleus. To make the valence electron free, external energy (heat. light radiation etc.)
equal to the forbidden energy gap must be supplied which lifts the electrons from valence bandto
conduction band.
5.CLASSIFICATION OF SOLIDS AND ENERGY BANDS
On the basis of electrical conductivity, the solids may be classified as insulators, conductors
and semiconductors. Their electrical behaviour can be explainedbeautifullywith the help ofeneg
0ands. To ascertain the electrical behaviour, the valence and conduction bands are of particular
portance, whereas the electrons in the lower energy band are tightly bound to the nucleus and play
no
part in the conduction process.
which do not allow the passage of
Insulators: The substances (like wood, glass, mica etc.)
C through them are known as insulators. The valence band of the these substances is full
the conduction band is completely empty. Moreover, the forbidden energy gap between
ereas
ICe band and conduction band is very large (8 eV and more) as shown in Fig. 5 (a). Therefore,
 BASIC ELECTRICAL &ELECTRONICS ENGINEE
6-6 to push the valénce electro ERING
electric filed is required rons to
very high
large amount ofenergyie. eouhe
a
a
why
tions,
such materials, under ordinary conditions,
do not
not
conduct at
conduction band. This is the
reason,

insulators.
all and are designated BAND
BAND
BAND ENERGY
ENERGY
ENERGY
fif1

CONDUCTION BAND CONDUCTION BAND

cONDUCTION BAND

11eV
FORBIDDEN
8 eV ENERGY VALANCE BAND ZIDIZLDnh
GAP VALANCE BAND
ZTIIIIIIIIZA
(a) (b)i C)
Fig.
5
(n Conductors: The substances (like copper, aluminium, silver etc.) which allow the passage
of current through them are known as conductors. The.valence band of these substances overlap
the conduction band as shown in Fig. 5 (6), Due to this overlapping, a large number offree electrons
are available for conduction: This is the reason, why a slight potential difference applied across such
substances causes a heavy flow of current through them.
(i) Semiconductors: The substances (like carbon, silicon, germanium etc.) whose electrical
conductivity lies in between the conductors and insulators are known as senmiconductors. A]though,
the valence band ofthese substances is almost filled and conduction band is almost empty as in case
of insulators. But the forbidden energy gap between valence band and conduction band is very small
(nearly 1 eV) as shown in Fig. 5(¢). Therefore, comparatively a smaller electric field (much smaller
than insulators but much greater than conductors) is required to push the valence electrons to the
conduction band. This is reason, why such materials, under ordinary condition, do not conduct current
and behave as an insulator. However, even at room temperature, some heat energy is imparted to the
valence electrons and afew ofthem (about one electron for 101 atoms), cross over to the conduction
band imparting minor conductivity to the semiconductors. As the temperature is increased, more
valence electrons cross over to the conduction band and the conductivity of the material increases.
Thus, these materials have negative temperature co-cfricient ofresistance.
6.PROPERTIES OF SEMICONDUCTORS
The sutbstances (like carbon, silicon, germanium, selenium, sulphur etc.) which have resistiviy
(1 to 05 ohm-m) in between conduclors and insulators are known as senmiconductors.
However, it is not the resistivity alone that decides whether a substance issemiconductor or
not.Infact, semiconductors have a number ofpeculiar properties, mentioned below which distinguish
them from insulators, conductors and resistance materials
The resistivity of a semiconductor is less than an insuiator but more than
)
Cconductor While studying this property. the reader may ask, "Why not such materials
classified as resistance materials?" The answer is readily available if we study the folOW
table carefully :
 6-7
EMICONDUCTO DEVICES
Substance Nature Resistivity
S. No 1-72 x 10-" 2m
Copper Conductor
Germanium Semiconductor 0-63 2m
2. Insulator
9x 10 Qm
Glass
3 Resistance material 1x 104 Qm
A.
Nichromne to
that the resistiVity of germanium (semiconductor)
is quite high as compressed
Table shows It also shows that the
but it is quite low when compared with glass (insulator).
o0Der (conductor) (one ofthe resistance materials
is much higher than the resistivity of nichrome
copper
as a
esistivity or germanium This clearly shows that electrically germanium cannot be regarded
vine highest resistivity). considered as a semiconductor.
insulator or e v e n a resistance material. It is only
The negative
conductor,
Semiconductor have negative temperature
co-efficient of résistance.
in temperature
(ii) resistance than resistance decreases with the rise
co-etficient of means, lOw
temperature semiconductors behave
like an insulator at very
to this property, the
and vice-vera. According
act as a conductor at high temperatures,bot st a semiconductor,
temperatures but gallium etc.) is added to
suitable metallic impurity (like arsenic, It is this property,
(ii) PYhena the semiconductor appreciably.
conducting properties of transistor, thyristor,
diac, triac
it changes th current solid state devices (e.g. diode,
various
which is exploited to develop
discussed later in detail.
etc.) This property will be
electrons. This
7.BONDs IN SEMICONDUCTORS
valence
the bonding action of
the atoms are held together by its last orbit by
In every element, the tendency of each atom to complete
is
due to the fact that it Therefore, to complete
bonding action is elements, the last orbit is incomplete.
in it. In most of the the atom may lose,
acquiring 8 electrons with other atoms. In this process,
active to enter into bargain
it, the atoms become with other atoms.
gain or share valence electrons

CORES OF
Si OR Ge

ATOM

(si)

Fig.7
Such bonds are called co-
Fig.6 electrons.
are formed by
_haring of valence
bonds
number of valence
electrons and the contributed1
In semiconductors, contributes equal
each atom
valent bonds. In this case, formation of the bond.
the engaged in the
atoms
atoms. A silicon (or germanium)
electrons are shared by s e m i c o n d u c t o r (Ge or Si)
bonds among silicon atom
Fig. 6 shows the
co-valent
outermost orbit). It is tendency of each
electrons (the electronsin third or last orbit.
atomhas* 4 valence in the second and 4 in the
Two in the first orbit,
8 last orbit.
and 4 in the fourth or
in the second, 18 in the third
electrons.
has 14
Asilicon atom
electrons. Two in the first orbit, 8
ARermanium atom has 32
 MICONDUC7
JCTOR DEVICI
fsilicon are arranged in an
of
silicon

The
atomS

orderly pattern and form a 6-11

15 S the energy band diagram crystalline structure as shown
Fig.1 6
sh
10 talso
eV. It
of silicon. The
needs a small energy to lift the
also need; forbidden energy gap in this
material is
e V i

ie
yhal
1.I
electrons from valence band to quite
conduction band.
BAND
ENERGY
(si
CONDUCTION BAND

- Si

| 1:1 ev

VALANCE BAND

Si

2nd ENERGY BAND

Si

Fig. 15 Fig. 16

electrons are lifted to conduction

Therefore, even at room temperature, a minute quantity of valence
electric field is applied. However,at room temperature,
band and constitute current conduction ifa high
case of silicon are quite less than germanium.
the number of electrons lifted to the conduction band in
devices are preferred over germanium
This is the reason why silicon semiconductor
devices.
SEMICONDUCTORS
10.EFFECT TEMPERATURE ONTHE CONDUCTIVITY OF
OF
electrical conductivity of semiconductors appreciably.
The change in temperature, changes the
Let us see how conductivity changes with the change in temperature.
are
absolute zero temperature, all the electrons of semiconductors
) At absolute zero: At whereas, the valence
inner orbit electrons are bound to the nucleus,
The
eid ightly by theiratoms.
bonds. Therefore, at this
temperature, no free electron
CICctrons are bound by the forces of co-valentsemiconductor crystal behaves like a perfect insulator.
s available in the semiconductor. Hence, the
shown in Fig. 17, no
across the semiconductoras
potential difference is applied
, I some
Current will flow through the circuit.
temperature can also be explained with the
at absolute zero
zero temperature, is
Dehaviour of a
semiconductor
The valence band, at absolute
shown in Fig. 18. electron can
C an energy band diagram conduction band is totally empty. Moreover, no valence
PoIetely filled, whereas, the free since no energy is supplied
to the semiconductor'crystal
band to become as an insulator at
nde conduction gap. Thus, the semiconductor behaves
they cannot cross the forbidden energy
this mperature due to empty conduction band..
 CS ENGINE
ELECTAONICS ENGINEERING
BASIC ELECTRICAL &
6-12 BAND
ENERGY

CONDUCTION BANOD

FORBIDDEN
-PURE SE MICONDUCTOR
ENERGY GAP
AT ABSOLUTE
ZERO TEMPERATURE

LVALENCE BAND

NO CURRENT FLOW

LOWER ENERGY BAND

Fig. 17 Fig. 18
(in Above absolute zero: When the temperature ofsemi-conductor is raised, some of its co-
valent bonds break due to the thermal energy supplied to it. The breaking ofbonds sets those electrons
free which were engaged in the formation ofthese bonds. Thus, at higher temperatures, a few free
clectrons exist in the semiconductors and they no longer behave as a perfect insulator.
Now, ifsome potential difference is applied across the semiconductor, as shown in
Fig. 19, a tiny
current will flow through the circuit, minute
because a quantity of free electrons exist in the
semiconductor. It may be noted here that at roem temperature, the current
through semiconductor
a
is too small to be of any practical value.
The behaviour of asemiconductor, at the
temperature above absolute zero, can also be explained
with the help of an energy band
diagram shown in Fig. 20. When, temperature of a semiconductor is
raised, the heat energy suppled to it will lift some of the valence electrons to the
The higher the temperature, the conduction band.
greater the number of valence electrons kicked up to the conduction
band and larger the current it can conduct.
BAND
ENERGY

CONDUCTION BAND
PURE SEMICONDUCTOR
AT RAISED TEMPERATURE

HOLE

MINUTE CURRENT FLOW LVALENCE BANDD

LOWER ENERGY BAND

Fig. 19
Whenever an electron Fig. 20
isjumped-up to the conduction band, a hole is created in the
11. HOLE valence band.
When an
energy is supplied to a
level, the departing electron leaves a semiconductor, a valence electron is lifted to a higher enerey
vacancy in the valence band. This
vacancy is called a hore
 cONDUCTOR DEVICES 6-13
bamd because of ifting of am electron from valence
Thus, a
vacancy lefi the valence
band is known as hole.
h a m dt o conduction
cona

wd to
12.ELECTRON-HOLE PAIRS
external (heat) cnergy is supplicd to a semiconductor, the valence clectrons
are
enever, some
W the valence hand
4.un
tied-up to the
t conduction band one after the other leaving behind a vacancy in
number of lectrons to be lified from valence
band to the conduction band depends
hole. The onc clectron is lifted to the
called

external energy supplied

ofex the semiconductor. If
to only
the quantity of
the quantity electron-hole
pon hole is created in the valence band. Thus, cach time an
ction band, then one

a i ris formed.
valence band) acts as
to note that hole (i.e, vacancy created by the clectron in the
Iis important co-valent bonds.
has strong tendency to attract the
electrons from the nearby
It
nsitive charge.
apo
13.RECOMBINATION OF ELECTRON-HOLEE
of the valence band are
external energy is supplied to a semiconductor, the electrons
When some in the valence band.
conduction band and become free leaving behind holes (vacancies) in
lited to the electrons are moving are larger
than valence band orbits
band orbits in which free
The conduction atom may intersect
the hole
formed. Occasionally, the conduction band orbit of one
which holes are conduction-band electron falls
into a hole. This merging
Because of this, generally the hole
orbit of another. recombination. When recombination takes place,
and a called
hole is
ofafree electron disappears.
does not move elsewhere, just
it hole.
in a semiconductor. This would eventually fill every
The recombinatión occurs continuously electrons up to the
holes by lifting valence
producing news
However, incoming heat energy keeps pair and their
recombination
electron-hole pair. This creation ofelectron-hole
conduction band forming
between creation and the
disappearance of an electron-
on continuously. The average time microseconds, depending
goes nanoseconds to several
as lifetime
which varies from a

holepair is known the semiconductor etc.

upon various
factors like crystal structure of
14. HOLE CURRENT the co-valent
to a pure semiconductor,
When some external energy (heat energy) is supplied to conduction band leaving
electrons are lifted-up
electron-hole pairs. The
bonds are brokenforming the free electrons
band. Under the influence ofelectric field,
valence
behind vacancies (holes) in the the hole current, also
flows in the semiconductor
time another current,
Constitute current. Atthe same
the
asexplained below:
in semiconductors (say Silicon). Suppose
of hole-current covalent bond
ig. 21 shows the phenomenon thermal energy, creating
a hole in the
free due to
Valence electron at Jhas becomes for the electron. Since
some

is a strong centre ofattraction

atd. Ihe hole, having positive charge, valence electron from nearby covalent bond (say
semiconductor, a
Cicctricfield is applied across the another valence electron (say
at L)
This create a hole at K. Then,
a A) comes to fill in the hole at.J. at L which is further
filled by the
the hole at K, thus creating a hole
turn may leave its bond to fill
n bond.
Clectron (say at M) from thenearby covalent valence band) from positive
charged vacancy in the
his movement of the hole (positively constitules hole-current.
erminal of the supply to the negative
lerminal through semiconductor
 ENGINEERIN
BASIC E L E G T H A L

6-14 BAND
ENERGY

CONDUCTION BAND

HOLE MOVEMENT

VALENCE BAND

M L K
ELECTRON MOVEMENT

21 Flg. 22
Fig.
Although. in this case also, electron moves from one covalent band to the other to fill the h
the valence band, but it is still known as *hole-current. It is because, the movement of the elcctr.
iron is
only due to the existence of hole in the valence band causing current.
The same phenomenon can be explained with the help
ofenergy band diagram shown in Fig, 222.
15. INTRINSIC SEMIcONDUCTOR
An extremely pure semiconductor is called intrinsic semiconductor
On the basis band phenomenon, an intrinsic semiconductor at absolute
of energy zero temperature
is shown in Fig. 23. Its valence band is
completely filled and the conduction band is completely empty,
When some heat energy is supplied to it (i.e. its
temperature is raised say to room temperature
some of the valence electrons are
lifted to conduction band leaving behind holes in the valence band
as shown in
Fig. 24. The electrons reaching at the conduction band are free to move at random.
holes created in the crystal also move at random in The
the crystal. This behaviour of
shows that they have negative semiconductors
temperature co-efficient of resistance i.e. the resistivity decreases or
conductivity increases with the rise in tèmperature.
BAND
ENERGY BAND
ENERGY
CONDUCTION BAND
FREE ELECTRONS

-VALENCE BAND
HOLE

LOWER ENERGY BAND

LOWER ENERGY BAND

Fig. 23
The hole-current is constituted the Fig. 24
attached by flow of valence
in the covalent bonds, electrons in the valence band. As these idly
compared to the current constitutedtherefore, their movement is very slow. clectrons
by the free electrons which However, the hole-current is votya a
jump inot the conduction band.
 NGINEERiNe
BASIC ELEGTHI

6-16
en ,
t
ep ,
=e
(n , tPA,)
at always Dre

semiconductors,
the clectrons and holes are
present in c
In case of intrinsic
ofintrinsic
semiconductors
qua
concentration: Therefore, conductivity

intrinsic concentration.
called
p
n and is
where pure germanium
mobilitics of free
electrons amd
noles m
are 38 and
ara

ExampleI. The
018 mW's The coresvmding
are"
values for pure
sileon VIJ md U0)
tively. " ng
respectively
Delermine
the value of in1rimsi msis/ivin for hoth germnium ennd silicom. 25 10h
(Assume n, for e 2 . 5

Dranw n e conclusion from the resul.n

sults
m' at o o m temperatne).
and for Si /5 10 ^

Solution: Intrinsic conductivity ofsemiconductor

1602 10-19c
cn, (, 1,) where e
=

() For GGe: =0 38 m/Vs:/, *0-18 m/Vs; u,

=
2-5 10/m
o I602 10x 2:5 x 10"(0 38 + 0:18)=2.243 2 m

Intrinsic resistivity of germaniun,

=
0-4458 Qm (Ans.)
P 2.243
, =0-13 m"/Vs:4,=0-05 m>/Vs;n, = 1:5 x 10/m
(ii) For Si:
o = 1-602x 10-1 x 1-5 x 10' (0-13 +0-05) =04325 x 10-30m

Intrinsie resistivity ofsilicon,

P
o0-4325x10-3
2312 *
10" 2m (Ans.)
Conclusion: Silicon has higher resistivity because it has
larger value of forbidden energy gan
and contains fewer electron-hole
pairs at the room temperature.
18.SILICONVERSUS GERMANIUM
In the early days
of semiconductor devices,
semiconductor material. But now-a-day, it is
germanium was considered to be the best
rarely used in new designs of semiconductor
Silicon is considered to be the best for the
preparation of semiconductor devices. It devices
room
temperature, a silicon crystal has almost no * free electron is, because at
Because of this property, the semiconductor compared with a
germanium crystal.
devices made from silicon material
performance than the semiconductor devices made from give far better
Thus, silicon has totally overshadowed germanium.
e.g. diodes, transistors, thyristors etc. germanium in the fabrication of semiconductor devices
19. EXTRINSIC SEMICONDCUTOR
Although an intrinsic semiconductor is
but as it is, it is not
useful for the
capable to conduct a little current even at room
small amount of suitable preparation of various electronic devices.To make it temperaturea
'impurity is added. It is then called extrinsic
Doping: The process by which controlled
conductive,
(impure) semiconductor.
as doping. impurity is added toa semiconductor is
The electon-hole
K71OW
pairs are the root
cause of
performance of the device. Since the formation of leakage current in solid state devices. This
small, the performance of silicon devices is electron-hole pair in silicon materail at leakage current aric
far better than the room
temperature is
germanium devices. neg
SEMICONDUCTOR DEVICES
6-17
The amount and type of such impurities have to be closely controlled during the preparation of
onc impurity atom is added to 10 semiconductor.
trinsic semiconductors. Generally,
extri
to which an
atomsof a
Thus, a semiconductor inmpurity ad controlled rate is added to make it conductive
as an extrinsic semiconductor
ic kznown

As discussed earlier, the purpose of adding impurity in the semiconductor crystal is to increase
dhe number
of free electrons or holes to make it conductive. If a pentavalent impurity (having 5
is added to a pure semiconductor a large number of free electron will exist in it.
alence electrons)
3 valence electrons) is added, a large number of holes will exist
Whereas, iftrivalent impurity (having
the type of impurity added, extrinsic semiconductor may be
in the semiconductor. Depending upon

classified as: (i) N-type semiconductor. (i) P-type semiconductor

20. N-TYPE SEMICONDUCTOR
When a small amount of pentavalent impurity is added to a pure semiconductor providing
semiconductor thus formed is known as N-
a large nunmber of free electrons in it, the extinsic
type semiconductor.

The addition of pentavalent impurities such as arsenic (atomicnumber 33) and antimony (atomic
semiconductor crystal. Such imourities
number 51) provide a large number offree electrons in the
which produce N-type semiconductors are known as donor impurities
because their each atom
as explained below:
donates one free electron to the semiconductor crystal
When a small amount of pentavalent impurity like arsenic (At No. 332,8,
18, 5) having five
atom of the impurity fits in the germanium
valence electrons is added to germanium crystal, each
from covalent bonds with four germanium atoms
crystal in such a way that its four valence electrons
as shown in Fig. 26. Whereas, the fifth
electron of the impurity (arsenic) atom finds no place in
one free electron in the germanium
covalent bonds and is thus free. Hence, each arsenic atom provides
has a large number of atoms, therefore,
crystal. Since, an extremely small amount of arsenicimpurity
t provides millions
offree electrons for conduction. shown in Fig. 27. With the addition of
The energy band diagram of N-type semiconductor is
are made available in the conduction band.
pentavalent impurity, a large number of free electrons
not fit in the convalent bonds ofthe erystal i.e. fifth
hese electrons are the free electrons which did
BAND
ENERGY

-CONDUCTION BAND

-PENTAVALENT
IMPURITY ATOM

Ge se VALENCE BAND
FREE ELECTRON

Ge .Ge .Ge

Fig. 26 Fig. 27
 BASIC ELECTRICAL &ELECTRONICS ENGINEE
6-18 minutc quantity of
arc also availa.
Irec clectrons are
INEERIG
availablc in
atom. 1lowever, a
is imna
the
electrons ofcach arscnic when thermal encrgy
al room temperature
conduction band which
are produced
hole-clectron pairs. The lollowing points are worth noting
germanium erystal forming by of pentavalent
the addition
(i)A large number of free electrons
are

clectrons arc
ade availablc
made available byy the gencration of
of hole- mpurity.
hol.Purty
of free
(i) A minute quantity to the semiconductor crvst
roonm temperature is impartcd
thermal energy at
pairs when the valence band.
leave behind holes in is
by the pentavalcnt impurity far exccedin.
electrons s

The number of frce electrons provided the

(iii) over holes that the material is call

number of holes. It is due to this predominance clectrons

of
semiconductor (N-stands for negative) nmaterial.
type
21. CONDUCTIONTHROUGH-NTYPE SEMICONDUCTOR
number of free clectrons FRE E
In N-type semiconductors, a large ELECTRONS N-TYPE
are available in the
conduction
(donated by the inpurity atoms)
across this type of
band. When a potential difference is applied
semiconductor as shown in Fig. 28, the free electrons are directed
towards the positive terminal, constituting electric current. As HOLE
the flow of current through the crystal is constituted by
free
electrons which are carriers of negative charge, therefore,
this |L
type of conductivity is called negative or N-type conducthvity. Fig. 28
It may be noted that conduction through N-type semi-conductor
is similar to that of conduction through metals like copper.
It is seen that at room temperature, electron-hole pairs are formed. These holes which are
available in minute quantity in valence band also constitute a little current in thesemiconductorsas
shown in Fig. 28. Although for all practical purposes, this current is neglected.
22. P-TYPE SEMICONDUCTOR
When a small amount of trivalent impurity is added to a pure semiconductor providing o
large number of holes in it, the extrinsic semiconductor thus formed is known as P-type
semiconductor.
The addition of trivalent impurities such as gallium (atomic number 31) and indium (atomic
number 49) provide a large number of holes in the semiconductor crystal. Such impurities which
produce P-1type semiconductors are known as acceptor inpurities because each atom of them
create one hole which can accept one electron. This is explained below:
When a small amount oftrivalent impurity like gallium (At. NO. 31 -> 2, 8, 18, 3) having three
valence electrons is added to germanium crystal, cach atom of the impurity fits in the germaniun
Cystal in such a way that its three valence electrons form covalent bonds with three surrounding
germanum atoms as shown in Fig. 29. In the fourth covalent bond, only germanium atom contribuc
one valence electron while gallium atom has no valence electron to contribute, as all its three valence
electrons are already cngaged in the covalent bonds. Hence, the fourth covalent bond is incomplete
having one clectron short. This missing electron is called a hole. Thus, each gallium atom provides
one hole in the germanium crystal. Since an extremely small amount of gallium impurity has a large
number of atoms, therefore, it provides millions of holes in the semiconductor.
SEMICONDUCTORDEVICES
6-19
SEM The
erpy band diagram ofP-type semiconductor is shown in Fig. 30. With the addition of
t impurity, a large
trivalen
number of holes
(the vacant spaces in the covalent bonds which can accept
lectrons)
elec
are made available
are made : in the crystal. However, a minute quantity of frec electrons are also
in the conduction band
I which are produced when thermal
available cncrgy at room temperatureis
imparted to the germaniur erystal forming hole-clectron pairs. But the holes are much more in
number than the conduction band electrons. The following points may be noted carcfully:

Ge Ge Ge BAND
ENERGY
TRIVALENT
IMPURITY ATOM CONDUCTION BAND

Ge
-HOLE VALENCE BAND

Ge Gee Ge

Fig. 30
Fig. 29
()A large number ofholes are made available by a addition of trivalent impurity.
(i) A minute quantity of hole-electron pairs are formed at room temperature because of heat
energy imparted to the semiconductor crystal. The free electrons (minute quantity) thus formed are
lited to conduction band leaving behind holes in the valence band.
(in) The number of holes provides by the trivalent impurity is far exceeding the number of free
electrons. It is due to this predominance of holes over electrons that the material is called P-type
semiconductor (P-stands for positive) material.
P-TYPE
23.CONDUCTIONTHROUGH PTYPE SEMICONDUCTOR HOLES
In P-type senmiconductors, a large number of holes are created
bythe trivalent impurity. When a potential difference is applied across
thistypeofsemiconductor, as shown in Fig. 31, the holes available|
in the valence band are directed towards the negative ELECTRON
terminal,
15tituting electric current. As the current flowing through the|
Crystal is by holes, which are carriers of positive charge, therefore,
this type of conductivity is called positive or P-type conductivity. Fig.31
In fact, in P-type conductivity, In fact, in P-type conductivity, the valence electrons move from one
cOvalent bond to another (this gives a look as if holes are moving) unlike the N-type where current
cOnduction is by free electrons available in the conduction band.
1s important to note here that conductivity of N-type semiconductor is nearly double to
available in the conduction band in N-type
4 of P-type semiconductor as the electrons
ICONductor are much more mobile than holes available in the valence band in P-type
Emiconductor. The poor mobility ofthe holes is because they are more bound to the nucleus as they
conduction band.
AVailable in the valence band which is nearer to the nucleus as compared to
 BASIC ELECTRICAL &ELECTRONICS ENGINiEr
6-20
It may be added here that even at room teinperature, clectron-hole pairs are formed T G
ese free
electrons which are available in minute quantity also constitute a little current ins neglected
thre
semiconductors as shown in Fig. 31. However, for all practical purposes, this current is nor P-type
24.CHARGE ON N-TYPE AND P-TYPE SEMICONDUCTORS
It has been diseussed above that in N-type semiconductors, conduction is due to free e
donated by the pentavalent impurity atoms. These clectrons are the excess clectrons with Ons
to
the number of electrons needed to fill the covalent bonds in the semiconductor crystal. Thed
electrons does not create any charge on the N-type semiconductor, Since impurity atoms aswell

germanium atoms all are electrically neutral themselves.

Similarly, in P-type semiconductors, conduction is due to holes created by the trivalent imu
atoms. These holes are the positively charged vacant spaces
npurity
which can accept the electrons. The hole
just represent the deficit of electrons with regard to the number of electrons needed to fill the covalen
ent
bonds in the semiconductor erystal. This deficit of electrons do not create any charge on the P-tvne
semiconductor, since impurity atoms as well as germanium atoms all are electrically neutral themselves
Thus, it follows that N-type as well as P-ype semiconductors are electrically neutra.
25. MAJORITY AND MINORITY CARRIERS
When a small amount of pentavalent impurity is added to a pure semiconductor, it
provides a
large number of free electrons in the crystal forming N-type semiconductor. However, it may be
recalled that even at room temperature, some of the covalent bonds break,
releasinga small number
of hole-electron pairs. Thus, an N-type semiconductor contains a large number of free electrons
(i.e.
the electrons provided by pentavalent impurity added and a share of hole-electron
pairs) only a
but
few number of holes. Therefore, in N-type semiconductor, the most of the current conduction
is due
to free electrons available in the semiconductor.
Thus, in N-type semiconductor, the electrons are the majority carriers, whereas, the holes
ure the minority carriers [See Fig. 32 (a)].
MAJORITY CARRIERs MAJORITY CARRIERS MINORITY CARRIERS
/(Free Electrons) (Holes) (Free Electrons)
. MINORITY CARRIERS oo o o oo
(Holes) o oo o o o
O -N-TYPE P-TYPE O o o o o

(a) (6)
Fig. 32
Giving the similar explanation, it can be concluded that in P-type semiconductor, the holes are
the majority carriers, whereas, the electrons the minority carriers. [See Fig. 32
are
(6)]
26. P-NJUNCTION
When a P-type semiconductor is suitably joined to an N-lype semiconductor, the conuci
surface so formed is called P-N junction.
All the semiconductor
(solid state) devices contain one or more PN-junctions. The PN-Juncuo
is, in fact, the control element for
semiconductor devices. To understand the working of varo
semiconductor devices, it is essential for the readers to have a
thorough knowledge of formation
a
PN-junction and its properties.