VEL TECH HIGH TECH DR.RANGARAJANDR.

SAKUNTHALA
ENGINEERING COLLEGE
B.E/B.TECH DEGREE EXAMINATIONS, APRIL / MAY 2025
21PH23T ENGINEERING PHYSICS – II Q.P CODE: E25273
PART – A (5 X 2 = 10 MARKS)
1. What are Fermi Energy And Fermi Level?
Fermi Energy:
Energy of The State, Probability of Electron occupation is ½ at any Temperature Above 0k.
Fermi Energy Level:
An Energy Level, Probability of Electron Occupation is 1/2 at any Temperature Above 0k.
2. What is mean collision time?
The average time between collisions of a particle, like an electron, molecule, or atom, with
other particles within a substance, such as a gas or a conductor
3. What is forward bias?
When the p-type is linked to the positive terminal of the battery and the n-type to the
negative terminal, the p-n junction is said to be forward-biased. When the p-n junction is forward
biased, the built-in electric field and the applied electric field are in opposite directions. When both
electric fields are put together, the resulting electric field is smaller than the built-in electric field.
As a result, the depletion zone grows narrower and less resistive. The resistance of the depletion
zone becomes negligible when the applied voltage is large.
4. What is intrinsic carrier concentration or density of intrinsic carriers?
The concentration of free electrons in the conduction band and free holes in the valence
band within a pure semiconductor material, where the number of electrons equals the number of
holes.
5. What is mean by hysteresis loop and what do you infer from it?
It represents the magnetic flux density and the magnetizing field strength.

PART-B (3x 12= 36 Marks)

consider two cross-sections A and B in a

Q =1/2 𝑛𝑣𝑘𝑑T

or
6 (b). Derive effective mass of electron (me*) and negative mass of electron (mh*) EFFECTIVE
MASS OF ELECTRON (me*) -12 marks
The effective mass plays an important role in conduction process of semiconductors and
insulators since they have full or almost filled valence bands. We can find that the effective mass m* is
negative near the zone edges of almost filled valence bands. Physically speaking the electrons in these
regions are accelerated in a direction opposite to the direction of the applied field. This is called the
negative mass behaviour of the electrons. The electrons with the negative effective mass is considered
as a new entity having the same positive mass of that of an electron but with positive charge. The new
entity is given the name "hole". The holes are not real particles like electrons or positrons, but it is a
way of looking at the negative mass behaviour of the electrons near the zone edge.
7 (a). What is p-n junction diode describe its formation and working? Also discuss its I-V
characteristics and applications?
p-n Junction Diode- Definition, Formation, Characteristics, Applications
The electrical conductivity of a semiconductor material is between that of a conductor, such as
metallic copper, and that of an insulator, such as glass. Its resistivity decreases as the temperature rises,
whereas metals have the reverse effect. By adding impurities (doping) into the crystal structure, its
conducting characteristics can be changed in beneficial ways. A semiconductor junction is formed
when two distinct doped areas occur in the same crystal. Diodes, transistors, and most contemporary
electronics are built on the behavior of charge carriers such as electrons, ions, and electron holes at
these junctions. Silicon, germanium, gallium arsenide, and elements along the periodic table’s so-
called metalloid staircase are examples of semiconductors. Gallium arsenide is the second most
common semiconductor after silicon, and it is used in laser diodes, solar cells, microwave-frequency
integrated circuits, and other applications. Silicon is a crucial component in the production of most
electrical circuits. p-n Junction Inside a semiconductor, a p-n junction is an interface or a border
between two semiconductor material types, namely the p-type and the n-type.
The semiconductor’s p-side, or positive side, has an excess of holes, whereas the n-side, or
negative side, has an excess of electrons. The doping process is used to produce the p-n junction in a
semiconductor. Formation of p-n Junction When we utilize various semiconductor materials to form a
p-n junction, there will be a grain boundary that will prevent electrons from moving from one side to
the other by scattering electrons and holes, which is why we employ the doping procedure. For
example, consider a p-type silicon semiconductor sheet that is very thin. A portion of the p-type Si will
be changed to n-type silicon if a tiny quantity of pentavalent impurity is added. This sheet will now
have both a p-type and an n-type area, as well as a junction between the two. Diffusion and drift are
the two sorts of processes that occur following the creation of a p-n junction. As we all know, the
concentration of holes and electrons on the two sides of a junction differs, with holes from the p-side
diffusing to the n-side and electrons from the n-side diffusing to the p-side. This causes a diffusion
current to flow across the connection. When an electron diffuses from the n-side to the p-side, it leaves
an ionized donor on the inside, which is stationary. On the n-side of the junction, a layer of positive
charge develops as the process progresses. When a hole is moved from the p-side to the n-side, an
ionised acceptor is left behind on the p-side, causing a layer of negative charges to develop on the p-
side of the junction. The depletion area is defined as a region of positive and negative charge on each
side of the junction. An electric field direction from a positive charge to a negative charge is generated
due to this positive space charge area on each side of the junction. An electron on the p-side of the
junction travels to the n-side of the junction due to the electric field. The drift is the name given to this
motion. We can observe that the drift current runs in the opposite direction as the diffusion current.
Biasing conditions for the p-n Junction Diode In a p-n junction diode, there are two operational
regions: 1. p-type 2. n-type The voltage applied determines one of three biasing conditions for p-n
junction diodes.
There is no external voltage provided to the p-n junction diode while it is at zero bias. Forward
bias: The p-type is linked to the positive terminal of the voltage potential, while the n-type is
connected to the negative terminal. Reverse bias: The p-type is linked to the negative terminal of the
voltage potential, while the n-type is connected to the positive terminal. Forward Bias The p-n junction
is said to be forward-biased when the p-type is connected to the positive terminal of the battery and the
n-type to the negative terminal. The built-in electric field at the p-n junction and the applied electric
field are in opposing directions when the p-n junction is forward biased. The resulting electric field is
smaller than the built-in electric field when both electric fields are added together. As a result, the
depletion area becomes less resistant and thinner. When the applied voltage is high, the resistance of
the depletion zone becomes insignificant. At 0.6 V, the resistance of the depletion area in silicon
becomes absolutely insignificant, allowing current to flow freely over it. Reverse Bias. The p-n
junction is said to be reverse-biased when the p-type is linked to the negative terminal of the battery
and the n-type is attached to the positive side. The applied electric field and the built in electric field
are both in the same direction in this situation. The resultant electric field is in the same direction as
the built-in electric field, resulting in a more resistive, thicker depletion zone. If the applied voltage is
increased, the depletion area gets more resistant and thicker.
The impurity concentrations are denoted by the letters ND and NA. The intrinsic concentration
is denoted by ni Flow of current in p-n junction diode When the voltage is increased, electrons move
from the n-side to the p-side of the junction. The migration of holes from the p-side to the n-side of the
junction occurs in a similar manner as the voltage rises. As a result, a concentration gradient exists
between the terminals on both sides. There will be a movement of charge carriers from higher
concentration regions to lower concentration regions as a result of the development of the
concentration gradient. The current flow in the circuit is caused by the movement of charge carriers
inside the p-n junction. V-I Characteristics of p-n Junction Diode A curve between the voltage and
current across the circuit defines the V-I properties of p-n junction diodes. The x-axis represents
voltage, while the y-axis represents current. With the help of the curve, we can see that the diode
works in three different areas, which are:
1. Zero bias
2. Forward bias
3. Reverse bias
There is no external voltage provided to the p-n junction diode while it is at zero bias,
which implies the potential barrier at the junction prevents current passage. When the p-n junction
diode is in forwarding bias, the p-type is linked to the positive terminal of the external voltage, while
the n-type is connected to the negative terminal. The potential barrier is reduced when the diode is
placed in this fashion. When the voltage is 0.7 V for silicon diodes and 0.3 V for germanium diodes,
the potential barriers fall and current flows. The current grows slowly while the diode is under
forwarding bias, and the curve formed is non-linear as the voltage supplied to the diode overcomes the
potential barrier. Once the diode has crossed the potential barrier, it functions normally, and the curve
rises quickly as the external voltage rises, yielding a linear curve. When the PN junction diode is under
negative bias, the p-type is linked to the negative terminal of the external voltage, while the n-type is
connected to the positive terminal. As a result, the potential barrier becomes higher. Because minority
carriers are present at the junction, reverse saturation current occurs at first. Applications of p-n
Junction Diode When the p-n junction diode’s arrangement is reverse-biased, the diode may be
utilised as a photodiode since it is sensitive to light. It has the potential to be utilised as a solar cell.
The diode can be utilised in LED lighting applications when it is forward-biased. Many electric
circuits utilize it as a rectifier.
(or)
7(b). Obtain an expression for density of holes in the valence band of an intrinsic semiconductor with
suitable energy diagram? -12 marks
density of Holes (Hole Concentration) In Valence Band

𝐻𝑜𝑙𝑒 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛, 𝑝 = ∫− 𝐸 𝑣
∞ 𝑍(𝐸)[1 − 𝐹(𝐸)] 𝑑E

intrinsic Carrier Concentration

We can write, n = p = ni

n p = ni × ni

n p = ni2

8(a). What are soft and hard magnetic materials explain with the concept of the hysteresis in
ferromagnetic materials?
soft magnetic materials
Easily magnetized and demagnetized.

High permeability

Hard magnetic materials

Cannot be easily magnetized and demagnetized.

Low permeability

(or)
8(b). Explain the properties of superconductors in detail.
The phenomenon of sudden disappearance of electrical resistance in a material suddenly, when
it is cooled below a certain temperature known as Superconductivity.
PROPERTIES:
Zero Resistance:
The electrical resistance of the superconductor is zero below the transition temperature (Tc). It
is quickest test to prove the superconductivity.
Effect of Magnetic Field:
The minimum magnetic field strength required to destroy the superconducting property is
known as critical magnetic field (Hc.)
Effect of Electric Current:
A very high electrical current passing through a superconducting material destroys its
superconducting property.
Persistent Current:
An induced current which flows through a superconducting ring without any decrease in its
strength as long as the material is in superconducting state is called persistent current
Meissner Effect:
The Meissner effect is the expulsion of a magnetic field from a superconductor during its
transition to the superconducting state. When the material is cooled below its transition temperature
i.e., T<TC, the magnetic flux (H<H) originally present in the specimen as shown in fig.

PART-C (1x 14=14 Marks)

9(A). Elucidate the postulates and drawback of Drude and Lorentz theory. Also explain what is
electrical conductivity and thermal conductivity and prove wiedmann franz law

When an electrical field (E) is applied to an electron of charge ‘e’ of a metallic rod, the electron
moves in opposite direction to the applied field with a velocity vd. This velocity is known as drift
velocity. Lorentz force acting on the electron F = eE

Wiedmann Franz Law

The ratio of between the thermal conductivity (K) and electrical conductivity
of a metal is directly proportional to the absolute temperature (T) of the metal.

𝐾 /𝜎 = 𝐿T

(or)

9(B). Derived density of electrons in conduction band and derive density of holes in the valence band

𝑍 (𝐸)𝑑𝐸 = 4𝜋/ℎ 3 (2𝑚𝑒 ∗) 3/2𝐸 1/2 𝑑E