Semi Conductor

CLASSIFICATION OF METALS, CONDUCTORS AND

SEMICONDUCTORS
On the basis of conductivity
On the basis of the relative values of electrical conductivity or
resistivity the solids are broadly classified as:
(i) Metals: They possess very low resistivity (or high conductivity).
(ii) Semiconductors: They have resistivity or conductivity
intermediate to metals and insulators.
(iii)Insulators: They have high resistivity (or low conductivity).

Semiconductors which could be:

(i) Elemental semiconductors: Si and Ge
(ii) Compound semiconductors: Examples are:
• Inorganic: CdS, GaAs, CdSe, InP, etc.
• Organic: anthracene, doped pthalocyanines, etc.
• Organic polymers: polypyrrole, polyaniline, polythiophene, etc.

ENERGY BANDS IN SOLIDS

 In case of a single isolated atom, there are single energy levels
in case of solids, the atoms is arranged in a systematic space
lattice and hence the atom is greatly influenced by
neighbouring atoms.
 The closeness of atoms results in the intermixing of electrons of
neighbouring atoms of course, for the valence electrons in the
outermost shells which are not strongly bounded by nucleus.
 Due to intermixing the number of permissible energy levels
increases or there are significant changes in the energy levels.
 Hence in case of a solid, instead of single energy levels
associated with the single atom, there will be bands of energy
levels.
Valence Band, Conduction Band & Forbidden Energy
Gap


The band formed by a series of energy levels containing the

valence electrons is known as valence band. The valency band
may be defined as a band which is occupied by the valence
electrons or a band having highest occupied band energy.
 The conduction band may also be defined as the lowest unfilled
energy band.
 The separation between conduction band and valence band is
known as forbidden energy gap. There is no allowed energy
state in this gap and hence no electron can stay in the
forbidden energy gap.

1.2 Insulators, Semiconductors and Conductors

 On the basis of forbidden band, the insulators, semiconductors
and conductors are described as follows:

1.2.1 Insulators
 In case of insulators, the forbidden energy band is very wide.
  Due to this fact electrons cannot jump from valence band to
conduction band. In insulators the valence electrons are bond
very tightly to their parent atoms. Increase in temperature
enables some electrons to go to the conduction band.

1.2.2 Semiconductors
 In semiconductors, the forbidden band is very small.
Germanium and silicon are the examples of semiconductors. A
semiconductor material is one whose electrical properties lies
between insulators and good conductors. When a small
amount of energy is supplied, the electrons can easily jump
from valence band to conduction band. For example when the
temperature is increased the forbidden band is decreased so
that some electrons are liberated into the conduction band.


12..3 Conductors
 In case of conductors, there is no forbidden band and the
valence band and conduction band overlap each other. Here
plenty of free electrons are available for electric conduction. A
slight potential difference across the conductor cause the free
electrons to constitute electric current. The most important
point in conductors is that due to the absence of forbidden
band, there is no structure to establish holes. The total current
in conductors is simply a flow of electrons.
2. SEMICONDUCTORS
 Thus a substance which has resistively in between conductors
and insulators is known as semiconductor.
 Semiconductors have the following properties.
(i) They have resistively less than insulators and more than
conductors.
(ii) The resistance of semiconductor decreases with the increase in
temperature and vice versa.
(iii) When suitable metallic impurity like arsenic, gallium etc. is added
to a semiconductors, its current conducting properties change
appreciably.

2.1 Effect of temperatue of Semiconductors

 At very low temperature (say 0 K) the semiconductor crystal
behaves as a perfect insulator since the covalent bonds are
very strong and no free electrons are available. At room
temperature some of the covalent bonds are broken due to the
thermal energy supplied to the crystal. Due to the breaking of
the bonds, some electrons become free which were engaged in
the formation of these bonds.
 The absence of the electron in the covalent bond is
represented by a small circle. This empty place or vacancy left
behind in the crystal structure is called a hole. Since an electron
unit negative charge, the hole carries a unit positive charge.

2.2 Mechanism of conduction of Electrons and Holes

 When the electrons are liberated on breaking the covalent
bonds,they move randomly through the crystal lattice.
 When an electric field is applied, these free electrons have a
steady drift opposite to the direction of applied field. This
constitute the electric current. When a covalent bond is
broken, a hole is created. For one electron set free, one hole is
created.
 This thermal energy creates electron-hole pairs-there being as
many holes as free electrons. These holes move through the
 crystal lattice in a random fashion like liberated electrons.
When an external electric field is applied, the holes drift in the
direction of applied field. Thus they constitute electric current.
 There is a strong tendency of semiconductor crystal to form a
covalent bonds. Therefore, a hole attracts an electron from the
neighbouring atom. Now a valence electron from nearby
covalent bond comes to fill in the hole at A. This results in a
creation of hole at B. The hole has thus effectively shift from A
to B. This hole move from B to C from C to D and so on.
 This movement of the hole in the absence of an applied field is
random. But when an electric field is applied, the hole drifts
along the applied field.

2.3 Carrier Generation and Recombination

 The electrons and holes are generated in pairs. The free
electrons and holes move randomly within the crystal lattice. In
such a random motion, there is always a possibility that a free
electron may have an encounter with a hole. When a free
electron meets a hole, they recombine to re-establish the
covalent bond. In the process of recombination, both the free
electron and hole are destroyed and results in the release of
energy in the form of heat.
 The energy so released, may in turn be re-absorbed by another
electron to break its covalent bond. In this way a new electrol-
hole pair is created.
 Thus the process of breaking of covalent bonds and
recombination of electrons and holes take place
simultaneously.
 When the temperature is increased, the rate of generation of
electrons and holes increases. This is turn increases, the
densities of electrons and hole increases. As a result, the
conductivity of semiconductor increases or resistivity
decreases. This is the reason that semiconductors have
negative temperature coefficient of resistance.
2.4 Pure or Intrinsic Semiconductor

 A semiconductor in an extremely pure from is known as

intrinsic semiconductor or a semiconductor in which electrons
and holes are solely created by thermal excitation is called a
pure or intrinsic semiconductor. In intrinsic semiconductor the
number of free electrons is always equal to the number of
holes.
2.4.1 Extrinsic Semiconductors

 The electrical conductivity of intrinsic semiconductor can be

increased by adding some impurity in the process of
crystallization. The added impurity is very small of the order of
one atom per million atoms of the pure semiconductor. Such
semiconductor is called impurity or extrinsic semiconductor.
The process of adding impurity to a semiconductor is known as
doping.
 The doping material is either pentavalent atoms (bismuth,
antimony, arsenic, phosphorus which have five valence
electrons) or trivalent atoms (gallium, indium, aluminium,
boron which have three valence electrons). The pentavalent
doping atom is known as donor atom because it donates one
electron to the conduction band of pure semiconductor.
 The doping materials are called impurities because they alter
the structure of pure semiconductor crystals.

2.4.2 N–Type Extrinsic Semiconductor

 When a small amount of pentavalent impurity is added to a

pure semiconductor crystal during the crystal growth, the
resulting crystal is called as N-type extrinsic semiconductor.
 In case of N-type semiconductor, the following points should be
remembered
 (i) In N-type semiconductor, the electrons are the majority
carriers while positive holes are minority carriers.
 (ii) Although N-type semiconductor has excess of electrons but
it is electrically neutral. This is due to the fact that electrons are
created by the addition of neutral pentavalent impurity atoms
to the semiconductor i.e., there is no addition of either
negative changes or positive charges.
 ne >> nh

2.4.3 P–Type Extrinsic Semiconductor

 When a small amount of trivalent impurity is added to a pure

crystal during the crystal growth, the resulting crystal is called a
P-type extrinsic semiconductor.
 In case of P-type semiconductor, the following points should be
remembered
 (i) In P-type semiconductor materials, the majority carriers are
positive holes while minority carriers are the electrons.
 (ii) The P–type semiconductor remains electrically neutral as
the number of mobile holes under all conditions remains equal
to the number of acceptors.
ne << nh

 The electron and hole concentration in a semiconductor in

thermal equilibrium is given by
2.5 P–N Junction Diode

 When a P-type material is intimately joined to N-type, a P-N

junction is formed. In fact, merely-joining the two pieces a P-N
junction cannot be formed because the surface films and other
irregularities produce major discontinuity in the crystal
structure.
 Therefore a P-N junction is formed from a piece of
semiconductor (say germanium) by diffusing P-type material to
one half side and N-type material to other half side.When P-
type crystal is placed in contact with N-type crystal so as to
form one piece, the assembly so obtained is called P-N junction
diode.
2.5.1 Forward Bias
 When external d.c. source is connected to the diode with p–
section connected to +ve pole and n–section connected to –e
pole, thejunction diode is said to be reverse biased.

2.5.2 Reverse Bias

 When an external d.c. battery is connected to junction diode
with P–section connected to –ve pole and n–section connected
to +ve pole, the junction diode is said to be reverse biased.
 P–N JUNCTION is such a device (any way) which offers low
resistance when forward biased and behaves like an insulator
when reverse biased.
2.6 Junction Diode as Rectifier
An electronic device which converts a.c. power into d.c. power is
called a rectifier.

2.6.1 Principle
Junction diode offers low resistive path when forward biased and
high resistance when reverse biased.

2.6.2 Arrangement
The a.c. supply is fed across the primary coil (P) of step down
transformer. The secondary coil ‘S’ of transformer is connected to
the junction diode and load resistance RL. The output d.c voltage is
obtained across RL.
2.6.3 Theory
 Suppose that during first half of a.c. input cycle the junction
diode get forward biased. The conventional current will flow in
the direction of arrow heats.
 The upper end of RL will be at +ve potential w.r.t. the lower
end.
 The magnitude of output across RL during first half at any
instant will be proportional to magnitude of current through
RL, which in turn is proportional to magnitude of forward bias
and which ultimately depends upon the value of a.c. input at
that time.
 Thus output across RL will vary in accordance with a.c. input.
 During second half, junction diode get reverse biased and
hence no–output will be obtained. Thus a discontinuous supply
is obtained.

2.7 Full Wave Rectifier

 A rectifier which rectifies both halves of a.c. input is called full
wave rectifier.

2.7.1 Principle
 Junction Diode offers low resistive path when forward biased
and high resistive path when reverse biased.

2.7.2 Arrangement
 The a.c. supply is fed across the primary coil (P) of step down
transformer. The two ends of S–coil (secondary) of transformer
are connected to P-section of junction diodes D1 and D2. A load
resistance RL is connected across the n–sections of two
diodesand central tapping of secondary coil. The d.c. output is
obtained across secondary.
2.7.3 Theory
 Suppose that during first half of input cycle upper end of s-coil
is at +ve potential. The junction diode D1 gets forward
biased,while D2 gets reverse biased. The conventional current
due to D1 will flow along path of full arrows.
 When second half of input cycle comes, the conditions will be
exactly reversed. Now the junction diode D2 will conduct and
the convensional current will flow along path of dotted arrows.
 Since current during both the half cycles flows from right to left
through load resistance RL, the output during both the half
cycles will be of same nature.
 The right end of RL is at +ve potential w.r.t. left end. Thus in full
wave rectifier, the output is continuous.

SPECIAL PURPOSE p-n JUNCTION DIODES

Zener diode

 Zener diode is fabricated by heavily doping both p-, and n- sides

of the junction.
 Reverse bias
 Due to this, depletion region formed is very thin (<10–6 m) and
the electric field of the junction is
 extremely high (~5×106 V/m) even for a small reverse bias
voltage of about 5V.
 The I-V characteristics of a Zener diode is shown in Fig.
14.21(b).
 It is seen that when the applied reverse bias voltage(V) reaches
the breakdown voltage (Vz) of the Zener diode, there is a large
change in the current.
 Note that after the breakdown voltage Vz, a large change in the
current can be produced by almost insignificant change in the
reverse bias voltage.
  In other words, Zener voltage remains constant, even though
current through the Zener diode varies over a wide range.
 This property of the Zener diode is used for regulating supply
voltages so that they are constant.

Zener diode as a voltage regulator

 We know that when the ac input voltage of a rectifier
fluctuates, its rectified output also fluctuates. To get a constant
dc voltage from the dc unregulated output of a rectifier, we use
a Zener diode.

 The unregulated dc voltage (filtered output of a rectifier) is

connected to the Zener diode through a series resistance Rs
such that the Zener diode is reverse biased.
 If the input voltage increases, the current through Rs and Zener
diode also increases.
 This increases the voltage drop across Rs without any change in
the voltage across the Zener diode.
 This is because in the breakdown region, Zener voltage remains
constant even though the current through the Zener diode
changes.Similarly, if the input voltage decreases, the current
through Rs and Zener diode also decreases.
 The voltage drop across Rs decreases without any change in the
voltage across the Zener diode.
 Thus any increase/decrease in the input voltage results in,
increase/decrease of the voltage drop across Rs without any
change in voltage across the Zener diode.
 Thus the Zener diode acts as a voltage regulator. We have to
select the Zener diode according to the required output voltage
and accordingly the series resistance Rs.