4. Valance band
The band of energies occupied by the valance electron is called valance band.
The electron in the outermost orbit of an atom are known as valance electrons.in
Normal atom and based posses the electron of higher energy.
This band may be completely of partial filled. electrons can be moved from one
valance band by the application of external energy.
Conduction band
The band of energies occupied by conduction electrons is known as conduction band. This is
the uppermost band, all electrons
In the conduction band are free electrons. The conduction band is empty for insulator and
partially filled for conductors
5. Forbidden energy gap
The gape between the valance band and the conduction band on energy level diagram
Known as forbidden energy gap. Electrons are never found in the gape. Electrons may jump
From back and forth from the bottom valance band to the top conduction band.
6. II.SEMICONDUCTORS
In semiconductors, electron is loosely bound to the nucleus hence requires
less energy for separating them from the nucleus. Semiconductors are
materials whose electrical resistivity lies between insulator and conductor
e.g. germanium and silicon.
The resistivity of semiconductors lie between 10 ohm to 1000 ohm meter at
room temperature.
The forbidden gape is very small equal to 1 eV, the energy band diagram
of a semiconductors .
The conductivity increase with temperature. As the temperature is
increased, some of the valance electron acquire thermal greater than
forbidden energy gape and hence moves into the conduction band.
7. III.INSULATOR
The valance band is full but the conduction band is totally empty .so free
electrons from conduction band is not available. In an insulator , the
energy gape between valance band and conduction band is very large
and approximately equal to 5 eV or more. Hence electron cannot jump
from valance band to conduction band. So, a very high energy is required
to push the electrons to the conduction band.
The resistivity of insulator lie between 10000 to n10(17) ohm meter at a
room temperature .
An insulator does not conduct at room temperature because there are no
conduction electrons in it, an insulator may conduct if its temperature is
very high or if a high voltage is applied across it. This is known as
breakdown of the insulator.
8. V. CONDUCTOR
Conductors are characteristised by high electrical conductivity .these are
the solids. In which plenty of free electrons are available for electrical
conduction . Ex. Silver , copper, iron.
In general electrical resistivity of conductor is very low and is of the order
of 10 (-6) ohm cm.
There is no forbidden gap and the conduction band and valance band
are overlapping each other ,so a slight potential difference the conductor
causes the free electrons to constitute electric current.
Due to the absense of forbidden gape ,there is no structure to elablish
holes. The total current in conductor is simply a flow of electrons for
conductor , the energy gape is of the order of 0.01 eV .
9. VI. PN Junction diode
p-n JUNCTION:
p-n junction = semiconductor in which impurity changes abruptly from p-type to n-type ;
“diffusion” = movement due to difference in concentration, from higher to lower concentration;
in absence of electric field across the junction, holes “diffuse” towards and across boundary into n-
type and capture electrons;
electrons diffuse across boundary, fall into holes (“recombination of majority carriers”);
formation of a “depletion region” (= region without free charge carriers)
around the boundary;
charged ions are left behind (cannot move):
negative ions left on p-side net negative charge on p-side of the junction;
positive ions left on n-side net positive charge on n-side of the junction
electric field across junction which prevents further diffusion.
10. What is a PN Junction?
A PN junction is a device formed by joining p-type ( doped with B, Al) with n-
type (doped with P, As, Sb) semiconductors and separated by a thin junction is
called PN Junction diode or junction diode.
11. Electronic Symbol …..the triangle shows indicated the direction of
current
Depletion layer forms an insulator
between the 2 sides
P type N type
12. Forward biased PN junction
It forces the majority charge carriers to move across the junction ….decreasing the width of
the depletion layer.
Once the junction is crossed, a number of electrons and the holes will recombine .
For each hole in the P section that combines with an electron from the N section, a covalent bond
breaks and an electron is liberated which enters the positive terminal
Thus creating an electron hole pair.
Current in the N region is carried by ….electrons
Current in the P region is carried by …. Holes.
13. Reverse biased PN junction
If the + of the battery is connected to the n-type and the – terminal to the p-type,
the free electrons and free holes are attracted back towards the battery,
hence back from the depletion layer, hence the depletion layer grows.
Thus a reverse biased PN junction does not conduct current
14. Diode
diode = “biased p-n junction”, i.e. p-n junction with
voltage applied across it
“forward biased”: p-side more positive than n-side;
“reverse biased”: n-side more positive than p-side;
forward biased diode:
the direction of the electric field is from p-side towards n-
side
p-type charge carriers (positive holes) in p-side are
pushed towards and across the p-n boundary,
n-type carriers (negative electrons) in n-side are pushed
towards and across n-p boundary
current flows across p-n boundary
15. PN junction can basically work in two modes, (A battery is connected to the
diode )
forward bias mode ( positive terminal connected to p-region and negative
terminal connected to n region)
reverse bias mode
negative terminal connected to p-region and positive terminal connected to n
region
16. VOLTAGE –CURRENT (V-I) CHARACTERISTICS OF PN
JUNCTION DIODE
The curve drawn between voltage across the junction along X axis and current through
the circuits along the Y axis.
They describe the d.c behavior of the diode.
When it is in forward bias, no current flows until the barrier voltage (0.3 v
for Ge) is overcome.
Then the curve has a linear rise and the current increases, with the increase
in forward voltage like an ordinary conductor.
Above 3 v , the majority carriers passing the junction gain sufficient energy to knock out
the valence electrons and raise them to the conduction band.
Therefore , the forward current increases sharply .
17. With reverse bias
potential barrier at the junction increased. …junction resistance
increase…and prevents current flow.
However , the minority carriers are accelerated by the reverse voltage
resulting a very small current (REVERSE CURRENT)….in the order
of micro amperes.
When reverse voltage is increased beyond a value ,called breakdown voltage,the
reverse current increases sharply and the diode shows almost zero resistance .It is
known as avalanche breakdown.
Reverse voltage above 25 v destroys the junction permanentaly.
18. Thus the P N junction diode allows the electrons flow only when
P is positive .
This property is used for the conversion of AC into DC ,Which is
called rectification
Automatic switch
When the diode is forward bias ,the switch is
CLOSED.
When it is reverse biased , it is OPEN
19. APPLICATIONS
….as rectifiers to convert AC into DC.
As an switch in computer circuits.
As detectors in radios to detect audio signals
As LED to emit different colours.
20. When the diode is forward bias ,the switch is CLOSED.
When it is reverse biased , it is OPEN
Automatic switch
21. Working of a PN junction
Forward Bias
Reverse Bias
Zener or
Avalanche
Breakdown
Voltage
Current
I-V characteristic of
a PN junction diode.
• PN junction diode acts as a rectifier as seen in the IV characteristic.
• Certain current flows in forward bias mode.
• Negligible current flows in reverse bias mode until zener or avalanche
breakdown happens.
23. 1.What is rectifier?
One of most widely used electronic circuit to convert AC voltage to
DC voltage.
Since the rectifier circuit uses diodes to convert ac voltage to dc, its
also called a converter circuit.
All power that supply to a modern factory is alternating current, so it
is important to have circuit that can convert the ac power to dc
power since most solid-state device require a source of dc power to
operate.
24. 2.Single phase half wave rectifier
The transformer feeding a resistor as its load with a
rectifier inserted in the circuit.
The rectifier will conduct each time its anode is positive
with respect to its cathode.
So when the end of the secondary winding shown + is
positive, the diode acts as a short-circuit and the +
appears across the load.
Current flows around the secondary circuit for the time
that the diode is conducting.
The waveform appearing across the load is shown in red
on the graph.
25. 3.Two diode full wave rectifier
This is two half-wave rectifiers combined - it uses
a center-tapped secondary winding and one
additional diode.
Each side of the centre-tap has the same
number of turns as our previous example - and
each "works" for half the cycle as our half-wave
rectifier did.
The "top half" of the secondary works with one
diode like the half-wave circuit we have just
considered.
When the polarity of the secondary changes,
the upper diode shuts off and the lower diode
conducts.
26. 4. Four diode Bridge Rectifiers
This uses one single winding as the secondary and four diodes
- two are conducting at any one time.
The operation is simple: Parallel-side diodes conduct at the
same time. Note that the two + points are connected by a
diode - same as in the two previous cases. The other end of
the load returns to the transformer via the other parallel diode.
When the polarity changes, the other two diodes conduct.
The output waveform is the same as the full-wave rectifier
example shown before.
27. Another use of semiconductor technology is in the fabrication of
transistors, devices that amplify voltages or currents in many kinds of
circuits. The first transistor was developed in 1948 by John Bardeen,
William Shockley, and Walter Brattain (Nobel Prize, 1956). As an
example we consider an npn-junction transistor, which consists of a
thin layer of p-type semiconductor sandwiched between two n-type
semiconductors. The three terminals (one on each semiconducting
material) are known as the collector, emitter, and base. A good way
of thinking of the operation of the npn-junction transistor is to think of
two pn-junction diodes back to back.
Transistors
Figure 11.22: (a) In the npn transistor, the base is a p-type material, and the emitter and
collector are n-type. (b) The two-diode model of the npn transistor. (c) The npn transistor
28. Consider now the npn junction in the circuit shown in Figure 11.23a. If the
emitter is more heavily doped than the base, then there is a heavy flow of
electrons from left to right into the base. The base is made thin enough so that
virtually all of those electrons can pass through the collector and into the output
portion of the circuit. As a result the output current is a very high fraction of the
input current. The key now is to look at the input and output voltages. Because
the base-collector combination is essentially a diode connected in reverse bias,
the voltage on the output side can be made higher than the voltage on the input
side. Recall that the output and input currents are comparable, so the resulting
output power (current × voltage) is much higher than the input power.
Figure 11.23: (a) The npn transistor in a voltage amplifier circuit. (b) The circuit has been
modified to put the input between base and ground, thus making a current amplifier. (c) The
same circuit as in (b) using the transistor circuit symbol.
Transistor
29. Field Effect Transistors (FET)
The three terminals of the FET are known as the
drain, source, and gate, and these correspond to
the collector, emitter, and base, respectively, of a
bipolar transistor.
Figure 11.25: (a) A schematic of a FET. The two gate regions are connected internally.
(b) The circuit symbol for the FET, assuming the source-to-drain channel is of n-type
material and the gate is p-type. If the channel is p-type and the gate n-type, then
