Electronics and Communication Engineering

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SEMICONDUCTORS & DIODES 70
CONDUCTORS, SEMICONDUCTORS AND INSULATORS
i) For Conductors like Copper, Aluminum, Gold, and Silver their atomic structure
contain large number of free electrons, there is no energy gap between valence
band and conduction band as it is illustrated by fig. As soon as voltage is
applied, electrons move through conductor. The energy gap is called as
“forbidden gap”. When an electron of valence band gets extra it jumps into the
conduction band. Actually electron becomes free from nuclear force when it goes
into the conduction band.
ii) Insulators sometimes called as bad conductors in which free electrons are
absent. There is a large energy gap (5 to 10eV) between the conduction and the
valence band. All the electrons of such material are tightly bounded by their
nucleus; no free electron is available for carrying the current. Common examples
are glass, mica, wood, air etc.
iii) Semiconductors, whose conductivity lies in between conductors and insulators
are neither conductors nor insulators therefore they are called as
“Semiconductors”.
INTRINSIC SEMICONDUCTORS
Semiconductors in pure crystal form without any mixing of other substances are
called as “intrinsic semiconductors”. Silicon and Germanium atoms are tetravalent
atoms (tetra – means four) because there are four valence electrons in the last orbit
as shown in fig.
SEMICONDUCTORS & DIODES

Fig. shows an intrinsic semiconductor crystal structures. Each valence electron
forms a covalent bond with adjacent atom. That means all valence electrons form
covalent bonds, no electron is therefore free for carrying the current thus at
absolute zero temperature, semiconductor is an insulator. When extra energy is
supplied to this crystal either by applying voltage or applying light these electrons
become free from covalent bond, they may go into the conduction band from
valence band. The energy required to break covalent bond is 1.12 Volt for Silicon
and 0.75 eVolt for Germanium. When electron jumps into the conduction band it
leaves an empty space called as a „hole‟.
The formation of electron hole pair is simultaneous. The number of electrons
and holes are equal such generation of electron hole pair is called „thermal
generation‟.
EXTRINSIC SEMICONDUTORS
Most of the active components are manufactured by using extrinsic – N – type and
P-type semiconductors. When other atoms or impurity either trivalent or
pentavalent is added to intrinsic semiconductor to increase either number of
electrons or holes such type of semiconductor is called as „extrinsic‟
semiconductor. The process of adding such impurity to a pure crystal of
semiconductor is called as „doping‟. A doped semiconductor is an extrinsic
semiconductor. The two types of semiconductors can be formed

(i) N-type Semiconductor
(ii) P-type Semiconductor
(i) N-type Semiconductor
A semiconductor with majority electrons is called as N-type semiconductor
(N-Negative electrons). N-type Semiconductor is formed by adding pentavalent
atoms to an intrinsic semiconductor. The doping atoms are Arsenic (As),
Antimony (Sb) or Phosphorus (P); they have 5 valence electrons, each
pentavalent atom has 5 electrons in the outermost orbit.
Now 4 of them will form covalent bonds with Silicon or Germanium atom
therefore one extra electron becomes free electrons as shown in fig (1.3). Since
this electron is far away from the nucleus and it is free from covalent bond, it is
available as conduction band electron for carrying the current.
Thus in N-type semiconductor we get more electrons in addition to thermally
generated electron-hole pairs. Here the pentavalent atom is known as „donor‟
atom because it donates electron to Silicon or Germanium. In N-type
semiconductor electrons are called as majority charge carries and holes are
minority charge carriers.

A Semiconductor with excess number of holes (holes –positive- „P‟) is called
as P-type semiconductor. Trivalent (Tri-three valence electrons) atoms are added
to a pure crystal of semiconductor to form this semiconductor. Trivalent
impurity as Aluminum (Al), Boron (B), Gallium (Ga) or Indium (In) are used as
„acceptors‟. Since each trivalent atom has three valence electrons, they will form
covalent bond with silicon atom but the fourth Silicon atom unable to form
covalent bond with Boron atom, because Boron do not have fourth valence
electron. Thus an empty space called as hole is created because of trivalent atom
as shown in fig (1.3b).
Doping with a number of trivalent atoms P-type semiconductor is formed where
more holes are available for carrying the current. This hole can accept one
electron therefore trivalent impurity is called as „acceptors‟. Thermally generated
electron – holes are there, so electrons are few called as „minority‟ carriers and
holes are called as „majority‟ carriers.
COMPARISON
N-type Semiconductor P-type Semiconductor
1) Doped with Pentavalent atoms. 1) Doped with trivalent atoms.
2)Electrons are majority charge
carriers.
2) Holes are majority charge carriers.
3)Holes are minority carriers. 3) Electrons are minority carriers.
4) Pentavalent atoms give electron
therefore called as donor atoms
4) Trivalent atoms create hole,
therefore called as acceptor atoms.
5) Antimony, Arsenic, Phosphorus
impurity is required
5) Aluminum, Boron, Indium,
Gallium impurity is required

P-N JUNCTION DIODE
Semiconductor diode is similar to a valve diode; P-type semiconductor is joined
with N-type semiconductor so it forms a P-N junction diode. P-type material is
acting as anode and N type as cathode. Fig. shows this structure and symbol of
P-N diode.
Barrier Potential
As soon as junction is formed some of the electrons from N-region near the
junction cross the junction and fill some of the holes of P-region as shown in
above fig. Since the atoms of N-region loose electrons they become positively
charged ions and since the atoms of P-regions accept electrons they become
positively charged ions and since the atoms of P-region accept electrons they
become negatively charged ions. Thus opposite types of ions are produced
(Positive ions on N-side and negative ions on P-side) near the junction as shown
in above fig. These ions oppose further flow of electrons from N to P region and
flow of holes from P to N region. The opposing force of ions is called as barrier
potential and this region is called as „depletion region‟. This barrier potential is
a 0.7V in case of silicon diode and 0.3V in case of Germanium diode.
Forward Bias
Bias means to apply voltage across P-N junction. In Forward bias, anode is
connected to positive terminal and cathode to negative terminal of the battery.
Fig. shows forward bias connections and it shows maximum current through the

diode. In forward bias the resistance of diode becomes very low. Diode is acting
as a close switch.
Let us see how current flows through P-N junction. The negative terminal of
the battery will repel electrons at N-region those move towards the junction as
shown in fig. and they overcome potential barrier because battery voltage is
more than 0.7V or 0.3V for Silicon and Germanium respectively. They electrons
move through P-region to the left side of the junction and then collected by the
positive terminal. Similarly because of this holes are attracted by the negative
terminal and flows in opposite direction.
Reverse Bias
This is another way of connecting a diode, P-region (anode) to negative
terminal and N-region (Cathode) to positive terminal of the battery, and it is
called as „reverse bias‟. In this condition since reverse battery voltage is applied
across the junction the width of potential barrier increases. This barrier will not
allow the movement of majority carriers from N to P region. The current through

diode is not possible which indicates a very high resistance or diode is acting as
an open switch as shown in the fig.
Practically, very small current flows through diode, which is called as „reverse
leakage current‟ because of minority carries. P-region has few minority electrons
(generated by thermal electron-hole pairs) those are repelled by negative terminal
and they flow through the junction towards right side through N – region and
then towards positive terminal of the battery.
a) Forward Characteristics
When P-N junction diode is connected in forward bias its N-region has free
electrons refer fig. (a), in conduction band and P-region has holes in Valence
band. P-region has slightly higher energy level. Let us consider single electron
movement from –ve terminal to the +ve terminal of the battery.
i) It is repelled by negative terminal so it travels through N-region as a free
electron in conduction band.

ii) Near the junction it jumps from conduction band it recombines with hole &
becomes valence electron in Valence band.
iii) Now it is in P-region it travels through P-region as a valence electron and
finally it is collected by positive terminal of the battery.
b) Reverse Characteristics
In reverse bias as explained earlier free electrons in N-region go away from the
junction, and holes in P-region move away from the junction. As shown in the
fig. the leaving electrons near the junction leave more +ve ions and the leaving
holes near the junction leave more –ve ions. It results in wider depletion layer.
Refer energy level diagram, in reverse bias conduction band electrons and
valence band holes go away from the junction the width of barrier or depletion
layer gets wider till it becomes equal to the battery voltage.
BLOCK DIAGRAM OF POWER SUPPLY

1. TRANSFORMER
The function of transformer is to transfer 230 V AC into required AC
voltage. It depends upon load, which is to be connected across power supply.
For example, a tape recorder of 12 V is to be connected across it then a step
down transformer of 12 V is connected. Another function of transformer is to
provide isolation between primary and secondary winding. Due to this
separation, power supply becomes shockproof.
2. RECTIFIERS
“The process of converting bi-directional AC voltage into unidirectional
DC is known as rectification” and the circuit used for this process is known as
rectifier. This circuit is also known by common name “Battery eliminator”. Three
circuits are commonly used for rectification
i) Half wave rectifier
ii) Center tap full wave rectifier
iii) Bridge full wave rectifier
I. HALF WAVER RECTIFIER
An AC Voltage consists of two half cycles as shown in the fig. (1.13) upper
half cycle is known as positive half cycle and low cycle is known as negative
half cycle.

i) In First half cycle or positive half cycle of AC voltage, suppose transformer
secondary terminal „A „ is positive with respect to terminal „B‟ refer fig. (a)
diode becomes forward biased it is acting as a closed switch. The current
flows through the circuit its flow is from terminal A through RL to terminal B
as shown in fig (a). Thus same output voltage is developed across RL similar to
half cycle of AC input.
ii) In the next cycle (negative half cycle) transformer terminal „B‟ is positive with
respect to terminal „A‟ now observe the fig. (b) diode becomes reverse biased; diode

is acting as an open switch thus current through circuit is not possible it is
blocked by the diode. The output voltage
 out L LV I R 0 R 0 V     is zero refer waveform fig (b)
Thus diode will conduct only at positive half cycle and it rectifies negative half
cycle hence it is known as half wave rectifier.
Peak – Inverse – Voltage (PIV)
Peak inverse voltage is an important rating of rectifier, diode should withstand
for this voltage when it is in reverse bias. In negative half cycle when terminal „B‟
of transformer is positive it is at cathode of diode, refer fig (1.16) maximum
inverse voltage is peak voltage given as „Vp‟.
Advantages
1) Only one diode and normal transformer is required
2) Transformer is not essential for high voltage rectifier.
3) The circuit is simple and low cost.
4) PIV is only „Vp‟
Disadvantages
1) Output of half wave rectifier is less because only half cycle appears in the
output.
2) There is wastage of negative half cycle.
3) Since ripple frequency is low there are more ripples that generate poor quality
DC.
Ripple Factor
Ripple is unwanted, it is the part of AC in DC output of rectifier. As shown in
the waveform output of half wave rectifier is pulsating DC it is not a constant
voltage DC this is known as “ripple”.

II. CENTER TAP FULL WAVE RECTIFIER
To utilize negative half cycle; one more diode is connected with special type
of transformer called as canter tap transformer. In canter tap transformer the
middle terminal is tapped so that its upper terminal (A) and low terminal (B)
become opposite in phase. e.g. if the terminal A is +ve the terminal B is –ve with
respect to canter tap.
Working
In canter tap rectifier diodes are conducting in alternate cycle so that
current through „RL‟ flows in the same direction for both half cycles.
i) Suppose in first half cycle terminal A is positive then B is negative with respect
to terminal „C‟. Then diode D1 becomes forward biased and D2 reverse biased
observe fig (a) current flows from terminal A – D1 – through RL to terminal
center tap „C‟ thus output voltage across RL is developed as shown.

ii) In the next half cycle of AC, polarity of AB get reversed A becomes negative and
B positive with respect to terminal „C‟. The diode D1 becomes reverse biased
and D2 forward biased. Current flows from terminal B through D2 through RL to
terminal „C‟ thus output voltage is developed which is Similar to half cycle and
it is in the same direction. Fig. Shows input AC and full wave DC output.
Advantages of C.T. Rectifier
1) More DC voltage than half wave.
2) Ripple frequency is doubled because two cycles are developed it is 2 fin. If it is
50Hz AC, for full wave rectifier then ripple frequency is 100 Hz, therefore more
ripple frequency, it means less ripple than half wave. Ripple factor is 0.48.
Disadvantages
1) Center tap transformer is essential which increase the cost.
2) PIV = 2 Vp therefore the cost of diode is more.
3) Still ripple is there, output is full wave pulsating DC.
III. BRIDGE FULL WAVE RECTIFIER
This is more popular rectifier circuit because the output is full wave DC and
center tap transformer is not required. Fig. shows the circuit diagram of Bridge
rectifier.
Working
Refer below figures for both half cycles of AC, current through LR is flowing in
the same direction like canter tap rectifier. But in bridge rectifier in each half
cycle two diodes are in forward bias and other two diodes are in reverse bias.

i) Suppose in first half cycle of AC, terminal „A „ is positive with respect to
terminal „B‟ then diode 1D and 3D become forward biased and 2D , 4D become
reverse biased. Now refer fig. current direction is from terminal „A‟ through 1D
through LR through 3D to terminal B, thus output voltage is developed across
LR  L O L LR V =I ×R .
ii)Next half cycle, terminal „B‟ is positive with respect to terminal „A‟. Now refer
fig. diode 2D , 4D become forward biased and 1D , 3D become reverse biased.
Current through circuit is from terminal „B‟ through 2D through LR through
4D to terminal „A‟ that indicates current through LR is in the same direction as
in first half
cycle.

Advantages
1) More DC output voltage than half wave.
2) Ripple is less because ripple frequency is in2f = 100 Hzand triple factor is 0.48.
3) Center tap transformer is not required that means low cost of transformer.
4) Peak inverse voltage is only PV because maximum secondary voltage of
transformer is PV , in bridge rectifier PIV appears across two diodes.
Disadvantages
1) In Bridge rectifier, only drawback is 4 diodes are required but it is not much
important.
2) Similarly, another drawback in bridge rectifier is for each half cycle two diodes
are conducting and both diodes are in series with LR that drops output voltage
by (0.7V+0.7V=1.4 volts) where 0.7V drop takes place across diode if it is silicon
diode. It reduces the output voltage by 1.4V. It is significant in low voltage power
supply.
COMPARISON OF RECTIFIERS
The points of comparison
(Parameters)
Half Wave
Rectifier
C.T. Full Wave
Rectifier
Bridge
Rectifier
1) Number of diodes One Two Four
2) Need of C.T. transfer Not required Essential Not required
3) PIV of diode PV 2 PV PV
4) Average of DC output
voltage
PV / 2 PV / 2 PV /
5) Ripple frequency Fin (50Hz) 2fin (100Hz) 2fin (100Hz)
6) Current through diode OI = DC LI or I DC LI /2 or I /2 DC LI /2 or I /2
7) Ripple factor 1.21 0.48 0.48

SOLVED PROBLEMS
1) What is the frequency of ripple in output for 50Hz 230V rms line with half wave
rectifier and 60Hz 110V rms line input with bridge rectifier?
Solution
1. For Half wave rectifier 2. For Bridge rectifier
ripple frequency = fin ripple frequency = 2 fin
= 50 Hz == 2 60Hz = 120 Hz
2) A transformer has turns ratio 12:1 its primery is connected to 220 V, 50 Hz
power line calculate DC output Voltage, PIV of diode and ripple frequency for half
wave and Bridge full wave rectifier.
Solution Mains Voltage is given as 220 V but it is RMS Voltage. It must be
converted in to Peak Voltage.
2rmsPeak primeryvoltage V 
220 2 311V  
Therefore peak secondary voltage can be calculated from turn‟s ratio
1 1
2 2
N V
N V

1 2
2
1
311 1
( )
12
V N
V Peak SecondaryVoltage
N
 
  
Let us say (VP)=25.9 V Peak Secondary Voltage
(a) For half wave rectifier
25.9
8.249PV
Vdc V
 
  
[log25.9 log3.14] [1.4133 0.4969]  
[0.9146]
log[0.9146] 8.249Anti Volts
PIV = VP = 25.9 V

Ripple frequency = 50 Hz.
(b) For Bridge full wave rectifier:
2 2 25.9
16.50PV
Vdc V
 

  
[log2 log25.9 log3.14]  
[0.3010 1.4133 0.4969]  
[1.2174]
log[1.2174] 16.50Anti V 
25.9 ; 2 2 50 100PPIV V V Ripple frequency fin Hz      

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