Unit:-1 Semi Conductor Physics · The materials whose electrical conductivity lies between those of...
Transcript of Unit:-1 Semi Conductor Physics · The materials whose electrical conductivity lies between those of...
Unit:-1Semi Conductor Physics
ATOMIC STRUCTURE:
There are two models of atomic structure in use today: the
Bohr model and the quantum mechanical model. Of these two
models, the Bohr model is simpler and relatively easy to
understand.
A model is useful because it helps you understand what’s
observed in nature. It’s not unusual to have more than one
model represent and help people understand a particular topic.
▪ The Bohr model shows that the electrons in atoms are in
orbits of differing energy around the nucleus (think of planets
orbiting around the sun).
▪ Bohr used the term energy levels (or shells) to describe
these orbits of differing energy. He said that the energy of an
electron is quantized
Covalent bonding in silicon and germaniumCovalent bonding in silicon:-
The outermost shell of atom is capable to hold up to eight electrons. The atom which has eight electrons in the outermost orbit is said to be completely filled and most stable. But the outermost orbit of silicon has only four electrons. Silicon atom needs four more electrons to become most stable. Silicon atom forms four covalent bonds with the four neighboring atoms. In covalent bonding each valence electron is shared by two atoms.
Covalent bonding in germanium:-The outermost
orbit of germanium has only four electrons. Germanium
atom needs four more electrons to become most stable.
Germanium atom forms four covalent bonds with the
four neighboring atoms. In covalent bonding each
valence electron is shared by two atoms.
Conductors:-Conductors are generally substances which
have the property to pass different types of energy. In the
following, the conductivity of electricity is the value of
interest.
Insulator:-An electrical insulator is a material whose
internal electric charges do not flow freely; very little
electric current will flow through it under the influence of an
electric field. This contrasts with other materials,
semiconductors and conductors, which conduct electric
current more easily.
Semiconductors:-Semiconductors are solids whose
conductivity lies between the conductivity of conductors and insulators
Intrinsic Semiconductor
A semiconductor, which is in its extremely pure form, is known
as an intrinsic semiconductor. Silicon and germanium are the
most widely used intrinsic semiconductors.
Both silicon and germanium are
tetravalent, i.e. each has four
electrons (valence electrons) in
their outermost shell.
Each atom shares its four
valence electrons with its four
immediate neighbours, so that
each atom is involved in four
covalent bonds.
Extrinsic Semiconductor
Pure semiconductors have negligible conductivity at room
temperature. To increase the conductivity of
semiconductor, some impurity is added. The
intrinsic
resulting
semiconductor is called impure or extrinsic semiconductor.
Impurities are added at the rate of ~ one atom per 106 to 1010
semiconductor atoms. The purpose of adding impurity is to
increase either the number of free electrons or holes in a
semiconductor.
Energy Band Diagram
Conduction
electrons
Energy Band Diagram
Forbidden energy band is small
for semiconductors.
Less energy is required for
electron to move from valence
to conduction band.
A vacancy (hole) remains when
an electron leaves the valence
band.
Hole acts as a positive charge
carrier.
Semiconductors
The materials whose electrical conductivity lies between those
of conductors and insulators, are known as semiconductors.
Silicon
Germanium
Cadmium Sulphide
0.7 eV
0.3 eV
2.4 eV
Silicon is the most widely used semiconductor.
Semiconductors have negative temperature coefficients of
resistance, i.e. as temperature increases resistivity deceases
Intrinsic Semiconductor
nductor is
number of
When
increase
electron
Since s so,
holes (p)Free e
= Intrin
the temperature of an intrinsic semico
d, beyond room temperature a large
-hole pairs are generated.
the electron and holes are generated in pair
lectron concentration (n) = concentration of
sic carrier concentration (ni)
Extrinsic Semiconductor
Two types of impurity atoms are added to the semiconductor
Atoms containing 5
valance electrons
(Pentavalent impurity atoms)
e.g. P, As, Sb, Bi
Atoms containing 3
valance electrons
(Trivalent impurity atoms)
e.g. Al, Ga, B, In
N-type semiconductor P-type semiconductor
N-type Semiconductor
The semiconductors which are obtained by introducing
pentavalent impurity atoms are known as N-type
semiconductors.
Examples are P, Sb, As and Bi. These elements have 5
electrons in their valance shell. Out of which 4 electrons will
form covalent bonds with the neighbouring atoms and the 5th
electron will be available as a current carrier. The impurity atom
is thus known as donor atom.
In N-type semiconductor current flows due to the movement of
electrons and holes but majority of through electrons. Thus
electrons in a N-type semiconductor are known as majority
charge carriers while holes as minority charge carriers.
P-type Semiconductor
The semiconductors which are obtained by introducing trivalent
impurity atoms are known as P-type semiconductors.
Examples are Ga, In, Al and B. These elements have 3
electrons in their valance shell which will form covalent bonds
with the neighbouring atoms.
The fourth covalent bond will remain incomplete. A vacancy,
which exists in the incomplete covalent bond constitute a hole.
The impurity atom is thus known as acceptor atom.
In P-type semiconductor current flows due to the movement of
electrons and holes but majority of through holes. Thus holes in
a P-type semiconductor are known as majority charge carriers
while electrons as minority charge carriers.
The free electron and hole concentrations are related by the
Law of Electrical Neutrality i.e.
Total positive charge density is equal to the total negative
charge density
Let ND = Concentration of donor atoms = no. of positive
charges/m3 contributed by donor ions
p = hole concentration
NA=Concentration of acceptor atoms
n = electron concentration
By the law of electrical neutrality
ND + p = NA + n
Charge carrier concentration in N-type and
P-type Semiconductors
For N-Type semiconductor
NA = 0 i.e. Concentration of acceptor atoms
And n>>p, then
ND + 0 = 0 + n
ND = n
i.e. in N-type, concentration of donor atoms is equal to the
concentration of free electrons.
i
According to Mass Action Law
n.p n2
Dp n2 / n n2 / N
i i
For P-Type semiconductor
ND = 0 i.e. Concentration of donor atoms
And p>>n, then
NA + 0 = 0 + p
NA = p
i.e. in P-type, concentration of acceptor atoms is equal to the
concentration of holes.
According to Mass Action Law
in.p n2
An n2 / p n2 / N
i i
Barrier Formation in P-N Junction Diode
The holes from p-side diffuses to the n-side while the free
electrons from n-side diffuses to the p-side.
This movement occurs because of charge density gradient.
This leaves the negative acceptor ions on the p-side and
positive donor ions on the n-side uncovered in the vicinity of the
junction.
.
Barrier Formation in P-N Junction Diode
Thus there is negative charge on p-side and positive on n-side.
This sets up a potential difference across the junction and hence
an internal Electric field directed from n-side to p-side..
Equilibrium is established when the field becomes large enough to
stop further diffusion of the majority charge carriers.
The region which becomes depleted (free) of the mobile charge
carriers is called the depletion region. The potential barrier
across the depletion region is called the potential barrier.
Width of depletion region depends upon the doping level. The
higher the doping level, thinner will be the depletion region.
Unit:-2Semi Conductor Diode
PN Junction Diode:-P-N junction diode is the most
fundamental and the simplest electronics device. When one side of an intrinsic semiconductor is doped with acceptor i.e, one side is made p-type by doping with n-type material, ap-n junction diode is formed. This is a two terminal device.
Diffusion current:-Diffusion current is a current in
a semiconductor caused by the diffusion of charge carriers (holes and/or electrons). This is the current which is due to the transport of charges occurring because of non uniform concentration of charged particles in a semiconductor.
Diffusion current Drift current
Diffusion current movement caused by variation in the carrier concentration.
Drift current movement caused by electric fields.
Direction of the diffusion current depends on the slope of the carrier concentration.
Direction of the drift current is always in the direction of the electric field.
Forward Bias P-N Junction
When an external voltage is applied to
the P-N junction making the P side
positive with respect to the N side the
diode is said to be forward biased.
The barrier potential difference is decreased by the external
applied voltage. The depletion band narrows which urges
majority carriers to flow across the junction.
A Forward biased diode has a very low resistance.
When an external voltage is applied
to the P-N junction making the P side
negative with respect to the N side
the diode is said to be Reverse Biased.
The barrier potential difference increases. The depletion bandwidens preventing the movement of majority carriers across thejunction.
A Reverse Bias diode has a very high resistance.
Reverse Bias P-N Junction
Conductivity of N and P-type
semiconductor
For intrinsic semiconductor
i q.(n.n p. p )
For N-Type semiconductor (n>>p)
q.n.n
For P-type semiconductor (p>>n)
q.p. p
Diode Applications —Half Wave Rectifier
•Diode converts ac input voltage to a pulsed dc
output voltage.
•Whenever the ac input becomes negative at
diode’s anode, the diode blocks current flow.
– o/p voltage become zero.
•Diode introduces a 0.6V drop so o/p peak is 0.6V
smaller than the i/p peak.
•The o/p frequency is same as the i/p frequency.
Vin
Diode Applications —Full Wave Rectifier• A full-wave rectifier does not block negative swings in the i/p voltage, rather
it transforms them into positive swings at the o/p.
• To gain an understanding of device operation, follow current flow through pairs ofdiodes in the bridge circuit.
• It is easily seen that one pair (D3-Rout-D2) allows current flow during the +ve halfcycle of Vin while the other pair (D4-Rout-D1) allows current flow during the -ve halfcycle of Vin.
– o/p voltage peak is 1.2V below the i/p voltage peak.
– The o/p frequency is twice the i/p frequency.
D1 D3
D2 D4
ParametersCenter tapped full wave
rectifierBridge rectifier
Number of diodes 2 4
Maximum efficiency 81.2% 81.2%
Peak inverse voltage 2Vm Vm
Vdc(no load) 2Vm/π 2Vm/π
Transformer utilization factor
0.693 0.812
Ripple factor 0.48 0.48
Form factor 1.11 1.11
Peak factor √2 √2
Average current Idc/2 Idc/2
Output frequency 2f 2f
The differences between center tapped fullwave rectifier and bridge rectifier:-
Working of Zener Diode
Zener Forward
Zener Reverse
Experimental Determination of Carrier concentration and
Mobility
▪ Consider a semiconductor (P-Type or N-types) in which
current I and Magnetic field B is applied, a force is act on
the charge carriers. This force pushing the charge carriers
towards the back of the semiconductor.
▪ When the mobile carriers (i.e. electrons or holes) are
pushed towards the back, the front becomes depleted and
the semiconductor loss it neutrality.
▪ Now there is an excess of mobile charge carriers at the
back and an excess of opposite charge due to impurity
atom at the front.
If the semiconductor is N-type,
The electron will be in excess at the back surface and the surface
becomes negatively charged with respect to front. This gives rise
to a potential difference called Hall voltage between front and
back
If the semiconductor is P-type,
The hole will be in excess at the back surface and the surface
becomes positively charged with respect to front. The polarity of
Hall voltage is in reverse direction
FILTER CIRCUIT:-
The filter is a device that allows passing the
dc component of the load and blocks the ac component of the
rectifier output. Thus the output of the filter circuit will be a
steady dc voltage.
Shunt Capacitor Filter:-
As the name suggests, a capacitor is used as the filter and this high value capacitor is shunted or placed across the load impedance.
LC or L Section Filter:-
L section filter connected to a full wave bridge rectifier
The inductor will operate as an inductor filter, attempting to
keep the current flowing through the load at a constant
rate. The
reduced ripple voltage from the inductor will then be
filtered by the action of the capacitor as illustrated
Pi Filter:-
AC sees the capacitors as a short circuit and the inductor as a block
DC sees the capacitors as an open circuit and the inductor as a short
circuit
The AC of the will flow through C1 and be blocked by the inductor the
DC will not pass through the capacitor but will pass through the
inductor. Any AC not passed through C1 will then be “cleaned up” by
C2
Types of Diodes
• Light Emitting Diode (LED)
• Avalanche Diode.
• Laser Diode.
• Schottky Diode.
• Zener Diode.
• Photo Diode.
• Varicap Diode or Varactor Diode.
• Rectifier Diode.
ZENER
DIODE It was named after Clarence Zener, who discovered this electrical
property.
It Permits current in the forward direction as well as in the reverse
direction if the voltage is larger than the breakdown voltage or “Zener
knee voltage” or “Zener voltage”.
▪ USES
▪ Zener Diode Shunt Regulator.
▪ Zener Diode as Peak Clipper.
▪ Switching operation.
USES OF ZENER DIODE:-1. Zener Diode Shunt Regulator
2. Meter Protection
3. Zener Diode as Peak Clipper
4. Switching operation.
Light-emitting diode:-
Emits visible light when an electric current passes
through it.
LED’s are essentially pn diodes operated in
forward bias.
Benefits Of LED:
• Low power requirement.
• High efficiency.
• Long life.
Applications:
• Indicator lights.
• LCD panel backlighting.
• Fiber optic data transmission.
• Remote control.
Photo Diode:-
A type of photo detector capable of converting light into either current or voltage.
Designed to operate in reverse bias.
USES :
Used in:
• CD Players.
• Smoke Detectors.
• Light sensors
• Various medical applications, such as detectors
for computer tomography and pulse oximeters.
Breakdown in P-N junction diode
In Electronics, the term “breakdown” stands for release of
electron-hole pairs in excess.
The critical value of the voltage, at which the breakdown of a
P-N junction diode occurs is called the breakdown voltage.
The breakdown voltage depends on the width of the depletion
region, which, in turn, depends on the doping level.
There are two mechanisms by which breakdown can occur at a
reverse biased P-N junction:
1. Avlanche Breakdown (uncontrolled)
2. Zener Breakdown (controlled)
Avalanche breakdown
If the reverse bias is made very high, the thermally generated
electrons and holes get sufficient K.E from applied voltage to
break the covalent bonds near the junction and a large no. of
electron-hole pairs are released. These new carriers, in turn,
produce additional carrier again by breaking bonds. Thus
reverse current then increase abruptly and may damage the
junction by the excessive heat generated.
The avalanche breakdown occurs in lightly doped junctions,
which produce wide depletion region.
The avalanche breakdown voltage increases as the temp. of
the junction increases due to the increased probability of
collisions of electron and holes with crystal atoms.
Zener breakdown
Zener Breakdown occurs at low voltage in heavily doped
reverse biased p-n junction.
Strong electric field directly (without impact of electron) pull out
the electrons from the covalent bond.
Zener breakdown voltage decreases as the temp. of the
junction increases. Since an increase in temp. increase the
energy of valence electron. So escape from covalent bond
become easier for these electrons. Thus a smaller reverse
voltage Is sufficient to pull the valence electron out of the
covalent bonds.
V-I Characterstics of Zener and
Avalanche breakdown
Unit:-3Introduction to Bipolar Transistor
Transistor:-Transistors are basically classified into two types; they are Bipolar Junction Transistors (BJT) and Field Effect Transistors (FET). The BJTs are again classified into NPN and PNP transistors.NPN TransistorNPN is one of the two types of Bipolar Junction Transistors (BJT). The NPN transistor consists of two n-type semiconductor materials and they are separated by a thin layer of p-type semiconductor. Here the majority charge carriers are electrons and holes are the minority charge carriers.PNP TransistorThe PNP is another type of Bipolar Junction Transistors (BJT). The PNP transistors contain two p-type semiconductor materials and are separated by a thin layer of n-type semiconductor.