MOVE FAST WITH PHYSICS-XII

14.1 Elemental and Compound Semiconductors. Formation of Energy Bands in Solids, Energy
Bands of Metals, Insulators and Semiconducto rs, Intrinsic Semiconductors and their
Mechanism of Conduction, Doping, Extrinsic Semiconductors : Formation of n-type
and p-type Semiconductors and their Energy Band Diagrams
14.1.1 Elactronic devices Any device whose action is based 14.1.4 Formation of energy bands in solids In an isolated
occupy well defined
on the controlled flowbranch
of electrons through atom, the electrons discrete
electronic device. JThe of physics that itdeals
is called an levels.
with the But due to interatomic interactions in a crystal, the energy
are forced to
study of these electronic devices is called electronics. The electrons of the outer shells have energies
electronic devices are of two types: different from those in isolated atoms. Each energy level
() Vacuum tubes. These include vacuum diode, triode, splits into a number of energy levels forminga continuous
tetrode, etc. In a vacuum tube, electrons obtained from a band.
heated cathode are controlled by varying voltages between
its different electrodes. These devices are bulky, consume Forbidden
high power, operate generally at high voltages, have energy gap
limited life and low reliability.
C.B.
(i) Solid-state electronic devices. In such devices, the
charge carriers flow through solid-state semiconductors. These
devices include junction diodes, transistors arnd integrated 3s
circuits. These are small in size, consume low power, V.B.
operate at low voltages, have long life and high reliability.
Energy
14.1.2 Classification of solids on the basis of their -2p°
resistivity values : 25
A. Metals. They have very low resistivity or high Crystal lattice spacing
-1s
conductivity.
p10-10 Qm; G10 -10 Sm-1 a b C d
Interatomic spacing, r ’
B. Insulators. They have high resistivity or low
conductivity. Fig. 14.1 Formation of energy bands in silicon.
p10 m; g10 Sm Consider a single crystal of silicon having N atoms.
C. Semiconductors. They possess resistivity or conduc- Each atom has electronic configuration :1s²2s²2,'3s'3p'.
At large interatomic separations (r = d) in the outer
tivity intermediate to metals and insulators. shells,
p10 -10 Qm; G105 -10° Sm1
Nenergy levels of 3 s-type are flled with2Nelectrons while
Nenergy levels of 3p-type are filled with 2Nelectrons and
Some distinguishing features of the semiconductors : remaining 2N energy levels of 3p-type are empty.
() Semiconductors have a much higher resistivity
than metals.
As interatomic separation reduces (r=c<d), the
valence electrons of neighbouring atoms begin to interact.
() Semiconductors have a temperature coefficient of The energies of 3s and 3p levels get modified. We have
resistivity (a) that is both negative and high. That is now Ndifferent levels of 3s-type and 3N different levels of
the resistivity of semiconductors decreases rapidly 3p-type. The energy gap between 3s- and 3p-levels
with temperature. decreases. We have a very large number (4N) of closely
(ii) Semiconductors have a considerable lower Spaced energy levels in a very small energy range whidt
number density n of charge carriers than metals. form an energy band.
14.1.3 Classification of semiconductors on the basis of As r reduces further (r =b> a), the energy gap between
their chemical composition : 3s-and 3p-levels disappears. We have a set of continuously
A. Elemental semiconductors : Si and Ge. distributed 4N energy levels.
At equilibrium band of 2N filled
energy levels (valenceseparation
B. Compound semiconductors : Examples are : (r=a), the
bandof
() Inorganic : CdS, GaAs, CdSe, InP, etc. band) gets Separated from thel gap
2N empty energy levels (conduction band) by an energy
(ii) Organic polymers Polypyrrole, polyaniline, poly An enormously large number of energy levels closely
thiophene, etc. band.
spaced in a very small energy range constitute an energy
 CHAPTER 14 : SEMICONDUCTOR ELECTRONICS 809
The allowed energy bands are
which energy levels cannot exist. separated
These
by regions in (ü Semiconductors. The empty conduction band i
are called band gaps or energy gaps. The forbidden regions separated from the filled valence band by asmal
highest energy band
occupied by the valence electrons is called energy gap (E, <3 eV) Some electrons of the
and the next empty allowed band is calledthethevalence band valence band easily get thermally excited to the
band. conduction conduction band and can conduct electricity. Sc
14.1.5 Distinction between metals, insulators and semi semiconductors acquire small conductivity even at
room temperature.
conductors on the basis of band theory:
For Si, E, =1.17 eV and for Ge, E, =0.74 eV
Metals. In metals, either the conduction band is 14.1.6 Fermi level and Fermi energy The highest energy
partially filled (in Li, Na, K, etc.) or the valence and level it teconduction band filled up with electrons
conduction bands partly overlap (in Be, Mg, Zn, etc.) at
zero is called Fermi level and the energy correspondingabsolute
to the
Here E, =0. Fermi level is called Fermi Energy. For intrinsic semicon
This makes available a large number of free ductors, Fermi level lies in the middle of the forbidden
electrons for electric conduction. So metals have energy gap.
high conductivity or low resistivity.
14.1.7 (d Intrinsic semiconductors The pure semicon
Insulators. Here the conduction band is empty and ductors in which
the valence band is filled. The forbidden energy by the thermally the electrical conductivity is totally governed
gap is large excited electrons and consequently created holes
(E, >3 eV). Electrons cannot be and in which no
excited from the valence band to the conduction conductivity are impurity atoms are added to increase their
called intrinsic semiconductors and their
band even by applying a strong electric field. conductivity is called intrinsic conductivity.
Therefore, no electrical conduction is possible.
For diamond, E, =6 eV. (b) Valence bond model of intrinsíc semiconductors In
a crystal of germanium, each Ge atom is tetrahedrally
Partially filled bonded to four neighbouring Ge atoms, as shown in
conduction band Fig. 14.3. Such astructure with all bonds intact exists at
low temperature.

Filled valence
bandotn
()opieoootoveb
Overlapping
conduction band
Filled valence
band

(ii)
(a) Metals

Empty
conduction
band
E, >3eV
Filled
Fig. 14.3 Covalent bonding in Si or Ge.
valence The symbol + 4 represents inner core of Ge or Si.
band All bonds are intact at low temperature.
(b) Insulators
As the temperature increases, the thermalenergy of the
Empty
conduction
valence electrons increases. As shown in Fig. 14.4, an
band electron may break away from the covalent bond and
IE,<3eV becomes free to conduct electricity. This electron leaves
Filled
valence behind a vacancy in the covalent bond (at site 1). This
band vacancy of an electron with an effective positive electronic
() Semiconductors charge is called a hole. It behaves as an apparent free particle
Fig. 14.2 Energy band diagrams.
with acharge +e
 810 MOVE FAST WITH PHYSICS-XI|

Thermally generated (a)Energy band diagram of intrinsic

free electron
ATE0K, the valence band of a semiconductor i semiconductor
completely filled with electrons while the conduction band
is empty, as shown in Fig. 14.6(a). Hence an intrinsic
Hole at
site 1
semiconductor behaves like an insulator at T=0K. A.
(electron higher temperatures (T >0 K), some electrons of H
vacancy) valence band gain sufficient thermal energy and jump to
Site 1
the conduction band, creating an equal number of holes in
the valence band. These thermally excited electrons
:Site 2

(+4 energy
Electron

E,
Fig. 14,4 Generation of a hole at site 1 and liberation of a free
electron due to thermal energy at moderate temperature. E,

Aseach free electron creates one hole, so in an intrinsic

semiconductor, the number density of free electrons (n, ) is (a) at T=0K (b) at T>0 K
equal to the number density of holes (n, )and each is equal (behaves like insulator)
to the intrinsic charge carrier concentration (n,).
Fig. 14.6 Generation of a hole at site 1 and liberation of a free
(c) Mechanism of copduciion in intrinsic electron due to thermal energy at moderate temperature.
semiconductors Electrical conduction in an intrinsic occupy the lowest possible energy levels in the conduction
semiconductor is due to electron-hole pairs. Fig. 14.4 band. Therefore, the energy band diagram of an intrinsic
shows a hole at site 1. An electron at site 2 of the semiconductor at T >0 Kis of the type shown in Fig. 14.6(b).
neighbouring covalent bond may jump to the site 1. After Clearly, the number of electrons in conduction band is
such a jump, a hole is created at site 2 and site1 gets equal to the number of holes in valence band.
occupied by an electron as shown in Fig. 14.5. Apparently, (e) Limitations of intrinsic semiconductors in
the hole has moved from site 1to site 2. Under the action of
an applied electric field, the holes move in the direction of
developing semiconductor devices :
the electric field (due to jumping of bound electrons in the (i) Intrinsic semiconductors have low intrinsic charge
reverse direction from one atom to another). So they act as carrier concentration (of hole and electrons)
positive charge carriers and give rise to a hole current I,. 10 m So they have low electrical
The thermally generated free electrons give rise to an conductivity.
independent electron current I,. The total current is (ii) As intrinsic charge carriers are always thermally
I= Electron current + hole current = I, +I, generated, so flexibility is not available to control
their number.
(iii) For intrinsic semiconductors, n, = 1,. They cannot
have predominant hole or electron conduction.
This puts a limit to the usefulness of such
materials.
Site 1
14.1.8 Doping The process of deliberate addition ofa desirable
inmpurity to a pure semiconductor so as to increase its
conductivity is called doping. The impurity atoms added are
Site 2 called dopants and the semiconductors doped with the impurity
atoms are called extrinsic or doped semiconductors.
Essential requirements for a doping process :
() The semiconductor material should be of very
high purity, 99.9999% or more.
Fig. 14.5 Apparent movement of a (i) The dopant atom should neatly replace
hole in an intrinsic semiconductor.
semiconductor atom.
 CHAPTER 14 : SEMICONDUCTOR ELECTRONICS 811

() The size of the dopant atom should be almost the electrons in forming four covalent bonds with
same as that of the semiconductor atom. For this neighbouring Si atoms while the fifth electron is loosely
the atoms of third and fifth group of the periodic bound to the impurity atom. A very small amount of
table are most suitable.
ionisation energy (<0.01 eV for Ge and 0. 05 eV for Si) is
(0) The dopant atoms should not distort the crvstal required to detach this electron. At room temperature, the
lattice. thermal energy is enough to set free this electron. The
(0) The concentration of dopant atoms shonld be dopant atom gets converted into an ionised +ve core. As
small, about 1part per million. each pentavalent impurity atom donates one extra electron
for conduction, it is called a donor. These semiconductors
Methods of doping have free electrons contributed by donors and generated
() By adding the impurity atoms to an extremely by the thermal process while the holes are only due to
pure sample of a molten semiconductor. thermal generation. Hence, the electrons are the majority
(ii) By heating the crystalline semiconductor in an charge carriers and holes are the minority charge carriers. As
most of the current is carried by the negatively charged
atmosphere containing dopant atoms or their electrons.
molecules so that the dopant atoms diffuse into impurites soarethe semiconductors doped with donor type
the semiconductor. known as n-type semniconductors.
(ii) By bombarding the semiconductor with the ions of For -type such semiconductors, n, >> ,:
dopant atoms, the dopant atoms can be implanted B. Formation of p-type semiconductor. Such a
into the semiconductor. semiconductor is obtained by doping the tetravalent
semiconductor Si (or Ge) with trivalent impurities such as
Two types of dopants In, B, Al or Ga. As shown in Fig. 14.8, the impurity atom
() Pentavalent dopants such as As, Sb and P. These are uses its three valence electrons in forming covalent bonds
also called donors. with three neighbouring Si atoms and one covalent bond
(i) Trivalent dopants such as In, B and Al. These are with a neighbouring Si atom is left incomplete due to the
also called acceptors. deficiency of one electron. An electron from the neigh
14.1.9\ Extrinsic semiconductors A semiconductor obtained
bouring Si-Si covalent bond can slide into this vacant bond,
creating a vacancy or hole in that bond. This hole is now
by doping putesemiconductor with acceptor or donor impurity available for conduction. The trivalent impurity atom
atoms so as to increase its conductivity is called an extrinsic becomes negatively charged when all its valence bonds get
semiconductor. Extrinsic semiconductors are of fwo types : filled. The trivalent impurity atom is called an acceptor because
() rtype semiconductors. These are the semiconductors it creates a hole which can accept an electron from the
obtained by doping Ge or Si with pentavalent dopants. neighbouring bond. Obviously, there are holes created by
(i) the acceptor atoms in addition to the thermally generated
rtupe semiconductors. These are the semiconauCtors holes
while the free electrons are only due to thermal
obtained by doping Ge or Si with trivalent dopants.
generation. Hence, holes are the majority charge carriers and
A. Formation of 1-type semiconductor. This electrons are the minority charge carriers. The semicon
semiconductor is obtained by doping the tetravalent ductors doped with acceptor type impurities are called
semiconductor Si (or Ge) with pentavalent impurities such p-type semiconductors, because most of the current in these
as As, Por Sb of group Vof the periodic table. As shown in semiconductors is carried by holes which have effective
Fig. 14.7, when a pentayalent impurity atom substitutes positive charge.
the tetravalent Si atom, it uses four of its five valence For ptype semiconductors, n, >>n,.

.0. 0. Unbonded 'free'

electron
donated by
pentavalent
(+5 valency)
atom

O. Elecirön
Fig. 14.7 Formation of n-type semiconductor by Fig. 14.8 Formation of p-type semiconductor by
doping tetravelent Si with pentavalent impurity. doping tetravelent Si with trivalent impurity.
 number
14.1.10 Thermodynamic relation between the extrinsic Electron
energy E
densities of electrons and holes for an --Ep
semiconductor When conduction electrons and holes are e0.01 eV
E E, 0.01 - 0.05 eV
created in a semiconductor, a process of destruction occurs
simultaneously in which electrons and holes recombine EA
with each other. At equilibrium, the rate of generation of E E
charge carriers is equal to the rate of destruction of charge -
carriers.
For an extrinsic semiconductor, (a) (b)
Rate of recombination < n,n, Fig. 14.9 (a) Energy band diagram of -type semiconductor at T>0 K
or, rate of recombination = Rn,n, ..(1)
(b) Energy band diagram of p-type semiconductor at T>0 K.
where R is aconstant known as recombination coefficient. covalent bond i.e., a very small energy (001-0.05 eV) i
required by an electron of the valence band to move inte
For an intrinsic semiconductor, n,=", =, so the this hole. Hence the acceptor energy level E, lies slightl
equation (1) becomes above the top of the valence band, as shown in Fig. 14.9(b)
Rate of recombination =Rn,2 ...2) At room temperature, many electrons of the valence bang
get excited to these acceptor energy levels, leaving behind
As long as the lattice structure of the semiconductor egual number of holes in the valence band. These holes car
remains the same, the rates of recombination given by conduct current. Thus the valence band has more holes
equations (1) and (2) for extrinsic and intrinsic than electrons in the conduction band.
semiconductors must be equal.
Hence Rn,n, =Rn; Or
14.1.12 Holes The vacancy or absence of an electron in th
...3) bond of akovalently bonded crystal is calleda hole. In terms of
The above equation implies the following acts about band theory, whenever an electron is removed from the
Htype and ptype semiconductors : completely filled valence band of a semiconductor, a
() For an n-type semiconductor, n, is necessarily vacancy 1s left behind in the valence band. This vacany
greater than n, and yet its product with n, remains serves as a positive charge carrier and is called a hole.
equal to n. This is possible only if n, becomes less Characteristics of holes :
than n,. This implies that the number of holes gets () A hole is just a vacancy created by the removal of
Suppressed in rtype semiconductors. an electron from a covalent bond of
semiconductor.
(i) For a p-type semiconductor, n, is necessarily (ii) It has the same mass as the (removed) electron.
greater than n, and yet its product with n, remains (ii) It is associated with a positive charge of
equal to n. This is possible only if n, becomes less magnitude e.
than n,, Thus the number of eleçtrons is
suppressed in a p-type semiconductor. (iv) The energy of a hole is higher, the farther below it
is from the top of the valence
14.1.11 Energy band diagram of n-type band.
In rtype semiconductors, the extra (fifth) sefmiconductor 14.1.13 Variation of conductivity of a
electron
weakly attracted by the donor impurity. A very small is very with temperature The conductivity of a semiconductor15
energy (z001eV) is required to free this electron from the given by semiconductor
donor impurity. When freed, this electron will
the lowest possible energy level in the occupy
the energy of the donor electron is conduction band i.e.,
slightly less than E. As the temperature increases, the
Thus the donor energy level E, lies just below the bottom of the of electrons and holes decrease due tomobilitiesu,
the
and
conduction band, as shown in Fig. increase in their
14.9(a).
temperature this small energy gap is easily covered by At room collision frequency. But due to the small energy gP
the semiconductors, more
thermally excited electrons. The conduction band has
e and more electrons [n ce
electrons (than holes in more from the valence band cross over to the conduction band.
valence band) as they have been The
contributed both by thermal increase
excitation and donor impurities. that the in carrier concentrations, n, and n issolarge
Energy band diagram of p-type semiconductor In decrease in the values of u, and has no
-type semiconductors, each acceptor impurity creates a iniiuence. The overall effect is that the conductivity
ole which can be easily filled by an electron of Si-Si temperature. increases or the resistivity decreases with the increase of
 CHAPTER 14 : SEMICONDUCTOR ELECTRONICS 819

conduction band has electrons as the majority charge 6. (a) Refer answer to Q. 1(a) on page 816.
carriers. When an external electric field is applied,
(b) See Figs. 14.11(c) and (b).
these electrons drift in the opposite direction of the
field. 7. Refer answer to Q. 3 and 4 above.
Conduction in ptype semiconductors : Role of
acceptor levels. With a very small supply of energy, Long Answer Questions II
the electrons from the valence band jump to acceptor 1. (a) Refer answer to Q. 1(a) on page 816.
energy level leaving behind holes in the valence band. (b) Refer answer to Q. 1on page 818.
When an electric field is applied, electrons from the 2. Refer to Art. 14.1.9. See Figs. 14.9(1) and (b).
neighbouring covalent bonds move into these holes, 3. See Figs. 14.2(b) and (c).
creating new holes in the direction of the field. Thus
(a) Whern a pure semiconductor is doped with
the holes act as positive charge carriers in the valence acceptor impurity atoms, it results in the
band.
formation of an additional energy (acceptor)
5. The pure semiconductors (Ge or Si) in which the level just above the valence band. Electrons from
electrical conductivity is totally governed by electrons the valence bond easily jump to this level leaving
thermally excited from the valence band to the
holes behind, which act as majority charge
conduction band are called intrinsic semiconductors. carries in these ptype semiconductors.
They have equal number densities of free electrons
and holes.
(b) When a pure semiconductor is doped with
donor impurity atoms, it results in the forma
A
tetravelent semiconductor of Si or Ge doped with tion of an additional energy (donor) level just
trivalent impurity atoms of B, Al or In is called a below the bottom of conduction band. Electrons
ptype semiconductor. It has n, >>1,. from the donor level easily jump to the conduc
In a ptype semiconductor, the trivalent impurity tion band. Hence electrons acts as majority
atom shares its three valence electrons with the three charge carries in these -type semiconductors.
tetravelent host atoms while the fourth bond remains
unbonded. The impurity atom as a whole is electrical
neutral. Hence the ptype semiconductor is also neutral.

14.2 Semiconductor Diode : p-n junction, Formation of Depletion Region and Potential Barrier,
Working of a p-n junction in Forward and Reverse Biasing, V-I Characteristics of a Junction
Diode, Junction Diode as a Half-Wave and Full-Wave Rectifiers
junction towards the pside and electron towards the
14.2.1 p-n junction Itis asingle crystal of Ge or Si doped in nside. So no charge carriers are left in the small region
such Tanner that one half portion of it acts as ptype
semiconductor and the other half as ttype semiconductor. near the junction as shown in Fig. 14.12(0).
diffusion
We take a thin p-type silicon (pSi) semiconductor wafer Electron drià sElectron
and add to it a small quantity of pentavalent impurity. A
The
part of the pSi wafer gets converted into rSi wafer. 1

wafer now contains pregion and Hregion with Depletion region

metallurgical junction between the two regions. Hole diffusion-’
+Hole drift
Formation of depletion region and potential barrier
in ajunction diode. Two important processes involved
()

during the formation of p-n junction are diffusion and

drift. Due to the concentration gradient across the p and
Irsides of the junction, holes diffuse from p-side to rside
and electrons diffuse from mside to p-side of the junction. X
The diffusion of holes from p’nside sets up a layer of -ve
of electrons
ions on the pside of the junction. The diffusion -side of (b)
ions on the
Irom n’p side sets up a layer of +ve layer in a p-n junction,
the junction.This sets up an electric field near the junction Fig. 14.12 (a) Formation of depletion
hole left near the (b) Barrier potential
from n pside. This field pushes any
820 MOVE FAST WITH PHYSICS-XI

the junction which 14.2.3 Working of a p-n junction An external

wapotential
The sace charge region on either sidehasof only immobile ions difference can be applied to a p-n junction in fuo
gets devoid ef mobile charge carriers and (a) Foruard biasing. If the positive terminal of a batteryyis
is called deletion layer.
The accumulation of negative charges in the pregion connected to the p-side and the negative terminal to the n-side,
biased.
positive charges in the region sets up a potential then the p-n junction is said to be forward
and a barrier and is As shown in Fig. 14.14(a), here the applied voltage V
difference across the junction. This acts as
which opposes opposes the barrier voltage V
called barrier potential V, [Fig. 14.12(b)]holes
the further diffusion of electrons and across the
junction. Only those electrons and holes which have energy
atleast eV, are able to cross this barrier and some diffusion p-n

takes place.
This difusion of majority charge carriers across the junction
gies rise to an electric current from p’nside and is called R

hesion current.
The barrier potential V sets up a barrier field E, in the (a) (6)
direction n’p side. The barrier field E pushes the
Fig. 14.14 (a) Reduced depletion layer,
electrons of p-side towards the rside and holes of n -side (b) Symbolic representation, for a forward biased p-n junction.
14
towards the pside.
The current set up by the minority charge carriers under the As a result of this,
influence of barrier field E from n’pside is called drift () the effective barrier potential decreases to(V-V)
CurTent. and hence the energy barrier across the junction
decreases,
The drift current and diffusion current are in opposite CUr
directions. In equilibrium state, the diffusion current is (ii) the majority charge carries i.e., holes from pside
equal to the drift current and there is no net flow of charge and electrons from rside begin to flow towards
across the junction. the junction,
(iii) the diffusion of electrons and holes into the
The barrier potential V, depends on () the nature of the depletion layer decreases its width, and
semiconductor, () temperature, and (ii) the amount of
doping. (iv) the effective resistance across the p-n juncion
decreases.
14.2.2 Semiconducor diode and its circuit symbol A
semiconductor diode is basically a pn junction with Alowing When Vexceeds Va, the majority charge carriers star
easily across the junction and set up a la
metallic contacts provided at the ends Fig. 14.13(0)| for the current (s mÀ). called forward current, in the circuit.
application of an external voltage. It is a two terminal
increase in applied voltage.
device. The circuit symbol of a pn junction diode is shoumn Current increases with the
in Fig. 14.13(b). The direction of the arrow is from pregion (b) Reverse biasing. If the positive terminal of a battery b
to region. The arrow indicates the direction in which the connected to the n-side and negative terminal to the p-side, th
conventional current can flow easily (when the diode is the pn junction is said to be reverse biased.
forward biased). The pside is known as the anode and the andthe
Pside is known as the cathode.
:
As shown in Fig. 14.15(a), the applied voltage V
barrier potential V,B are in the same direction.
poter
staty
Metallic Metallic vo
Contact
Depletion
region
contact n
Catrer
Anode o - Cathode
This,
(b) (a) (b)

Fig. 14.1 (a) Semiconductor diode,

(b) Symbol for a p-n junction diode. ig. 14.15 (a) Increased depletion layet,pnjunction
(b) Symbolic
representation, for a reverse biased
 ELECTRONICS 821
CHAPTER 14 :SEMICONDUCTOR

features of the graph. (i) The V-I graph is not

Important
As aresult of this, straight line ie, a junction diode does not obeyOhm's lauw.
increases to (V, +V)and hence a increases very slowly almost
0 the barrier potential Initially, the current
increases,
the energy barrier across the junction from the
(i) diode crosses a certain
negligibly,till the voltage across thecut-ín voltage Before this
carriers move away or
value, called the threshold-oltageplays a dominant role in
() the majority charge the width of the depletion layer
junction, increasing voltage, the depletion charge carriers.
layer, very controlling the motion of increases
p-n junction becomes voltage, the diode current
() the resistance of the (iii) After the cut-in
even
increase
for a very small junction
in
large, and rapidly (exponentially),
across the junction due to the diode bias voltage. The resistance across the
() no current flows
majority charge carriers. becomes quite low.
always Fig. 14.18 shows the
However, at room temperature there are
(b) Reverse bias characteristic. characteristic
minority charge carriers like holes in experimental arrangement for studying
present some p-region. The reverse biasing
-region and electrons in setting a current, called
Voltmeter (V)
pushes them towards junction,the external circuit in the
in
reverse or leakage current,
minority charge carriers are
opposite direction. As the majority charge carriers, p-n Micrometer
than the
much less in number (zuA).
(uA)
small
hence the reverse current is
p-n junction diode. A graph
14.2.4 V-I characteristics of a
Rheostat
flowing through a p-n junction (potentiometer) Switch
showing the variation of current is called the voltage-current or
across it
with the voltage applied
V-lcharacteristic of ap-n junction Fig. 14.16 shows the
V-I characteristic
14.18 Circuit for studyingdiode.
characteristic. Fig.
(a) Forward bias studying the characteristic of a reverse biased
arrangement for Here a
experimental forward biased. A battery is junction when it is reverse biased.
junction'when it is curve of a p-n
measure the small
currents
curve of ap-n microammeter is used to
Voltmeter (V) diode. A V-I graph of thereversetype
through the reverse biased called
obtained. It is
shown in Fig. 14.19 is
Milliammeter
characteristic of the junction diode.
p-n Breakdown
(mA) voltage V(Reverse bias)
-3 2 1 0

Rheostat -0.5
(potentiometer) Switch Reverse -1.0
conduction
- 1.5
Breakdown
Circuit for studying V-I characteristic region
Fig. 14.16 biased diode.
of a forward
of a junction diode.
Fig. 14.19 Reverse characteristic
Connected acroSs the p-n (mA)
Milliampere (i) When the diode is
Important features of the graph.
current, about a few
junction diode through a 80 Si-diode/AI small
reverse biased, a very almost remains constant with
potentiometer (or rheo 70
values microamperes flows, which current. It
stat). For different 60 called reverse saturation
value of bias. This small current is the
of voltages, the graph 40
due to the drift of minority charge carriers across
is
Current is noted. A and 30
junction.
between V
voltage across the p-n junction
20
is plotted
in Fig 14.17. () When the reverse value, the reverse current
, asshown voltage-Current reaches a sufficiently high
This Cut-in Volts
value. This voltage at which
foruard voltage VForward bias) suddenly increases to a large Zener
graph is called of the junction diode occurs is called
characteristic. characteristic of a breakdown the diode.
Fig.14.17 Forward breakdown voltage or peak-incerse voltage of
junction diode.
822 MOVE FAST WITH PHYSICS-XIl
Working. When a.c. is supplied to the primary, the
characteristic of a supplies desired
Figure 14.20 shows the complete V-l secondary of the transformer
pu junction. Obviously, it is a
unidirectional current
resistance
the positive half cyde of
voltage across Aand B. Duringthe end
alternating
characteristic. Ajunction diode offers a very small positive and Bis negative. The
a.c., the end Ais
when forward biased and has a very large resistance when diode D is forward biased and a current I
flows throue
only
reverse biased ie, the diode can conduct current well the input voltage
. As increases
R,I also increases or decreases, the curren.
into
in one direction. This property is used to convert a.c. or decreases and so does output voltage
dc The conversion of a.c. into d.c. is called rectification.
across the load R,. Output
IR, )waveform voltage
across K, is of
Ge-diode
Si-diode (=
same as the positive half wave of the input.
Milliampere
During the negative half cycle, the end A becomes
80

60
negative and B positive. The diode is reverse biased and
E 50
40 no current flows. No voltage appears across R,. So only
30 the half wave is rectified, as shown in Fig. 14.22. This
process is called half-wave rectification and the arrangement
20
v(Reverse bias) 10
-2
2
used is called a half-wave rectifier
-0.5 Volts
V(Forward bias) Voltage
atA
-10
Cut-in voltage
-1.5 Input a.c.
Breakdown
Microampere
(uA)
across
R
Voltage
Fig. 14.20 Complete V-I characteristic of a junction diode.
Output
14.2.5 Dynamic resistance of a junction diode The Ivoltage
dynamic or ac-resistance of the diode is defined as the ratioof
the small change in applied voltage AV to the corresponding
change in current Al. It is givern by AV Fig. 14.22 Waveforms of input a.c. and output
AI voltage obtained from a half-wave rectifier.
14.2.6 Rectifier The process of converting an alternating
Current into a direct current is called rectification and the device 14.2.8 Junction diode as a full wave rectifier The input
used for this process is called rectifier. a.c. signal is fed to the primary coil P of the transformet.
Principle of arectifier. When ap-n junction diode is The two ends A and Bof the secondary S are connected to
forward biased, it offers a low resistance and when it is the p-ends of diodes D, and D,.The secondary is tapped
reverse biased, it has a high resistance ie, it conducts at its central point Twhich is connected to the n-ends of the
current well only in one direction. This unidirectional two diodes through the load resistance R,, as shown
property of a diode enables it to be used as a rectifier. When Fig. 14.23.
ac. signal is fed to adiode, the diode is
forward biased
during the positive half cycle and a current flows through
it. During the negative halfcycde, the diode is Centre-tap
transformer
and it does not conduct. Thus the signal gets reverse biased
rectified.
14.2.7 Junction diode as a half-wave rectifier
primary coil of the transformer is connected to the The a.c. tap
mains and the secondary coil is connected in R
the junction diode Dand load resistance R,. series with S

D
Transformer
Fig. 14.23 Full wave rectifier circuit.
AC
input A
Working, At any instant, the voltages at the end
with
(inputof D,) and end B(input of D,)of the secondary each
respect to the centre tap T will be out of phase with
Fig. 14.21 Half-wave rectifier creuit.
other. Suppose during the positive half cycle of a.c. input
the end Ais positive and the end Bis negative withrespect
 ELECTRONICS 825
CHAPTER 14 SEMICONDUCTOR
atA
Waveform
A.C. Input at A
to the centre tap T. Then the diode D, gets forward biased
and conducts current along the path AD, XYTA, as
indicated by the solid arrows. The diode D, is reverse
biased and does not conduct. During the negative half
cycle, the end A becomes negative and the end Bbecomes atB
Waveform | A.C. Input at B
positive with respect to the centre tapT. The diode D, gets
reverse biased and does not conduct. The diode D,
conducts current along the path BD, XYTB, as indicated
by broken arrows. As during both half cycles of input a.c. waveform Output voltage
same direction
the current through load R, flows in the Due to Due to Due to Due to i

(X’), so we get a pulsating d.c. voltage across R,, as

aeross
R. D, D,
D,
load
shown in Fig. 14.24. Since output voltage across the Output
input a.c.,
resistance R, is obtained for both half cycles of
rectification and the
this process is called full wave
arrangement used is called full-wave rectifier and output
Fig. 14.24 Waveforms of input a.c. rectifier.
voltage obtained from a full wave

As Ssess Yourself
1mark each 14. In the circuit diagram of Fig.
14.25, which bulb out of
Very Short Answer Questions B, and B, willglow and why ?
[CBSE OD 17]

? Give its pictorial symbol. D, D,

1. What is a junction diode
2. What is depletion region in a p-n junction ?
[CBSE D 02 ; ISCE 97]| +9V
B,
important processes involved in the
3. Name the two [CBSE OD 16, 16C]
formation of a n junction.
the thickness of depletion layer in a p-n
4. What is Fig. 14.25
junction ?
called a junction diode ? 15, Can we measure the potential difference of a p-n
also
5. Why is a p-n junction barrier set up across a junction
across its
junction by connecting a sensitive voltmeter
6. Why does a potential terminals ?
diode ?
diode, its pside is 16. Under what condition does a p-n junction work as an
a junction
7. To forward bias terminal a battery and rside to the
open switch ?
connected the +ve necessary ? 17. What is an ideal junction diode ?
of the battery. Is it
-ve terminal diode, its pside is 18. Define resistance of a junction
diode. [CBSE D 03]
a junction
8. To reverse bias
terminal of the battery and -side 19. The resistance of a p-n junction is of the order of a few
connected to the -ve battery. Is it necessary ? ohms. Is the p-n junction forward or reverse
biased ?
terminal of the
to the + ve
biasing of a p-n junction diode so 20, How much is the resistance of a p-n junction when it is
of
9. Name the type offers very high resistance. reverse biased ?
that the junction current in a reverse
[CBSE D 05C] 21. What is the order of reverse
pn
when a forward bias is applied to biased p-njunction ?
10. What happens [CBSE OD 15]
22. Draw the voltage-current characteristic of a p-1
junction ? (CBSE OD 04)
width of the depletion region of a p-n junction diode in forward bias.
11. How does the
the reverse bias applied to it 23. Can a slab of ptype semiconductor be physically
junction vary, if semiconductor slab to form
[CBSE OD 02, 08] joined to another rtype