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INTERNAL ASSESMENT EXAMINATIONS – II
Answer key- PH6251 – Engineering Phiscs - II
Part-A
1. Elemental Semiconductors:
It is made up of elements beloning to the fourth group in the periodic table.(1)
Compound Semiconductors
The Compound Semiconductors are made by combining the third and fifth column elements (or) second sixth column elements. GaAs, GaSb, InP, are the important examples for Compound Semiconductors.(1)
2. Formula RH=VHbIxB Ans: 1.85 x 10-5 (2)
3. Hall Angle
tan∅H = µB (2)
4. Bohr Magneton
The total magnetic moment and the spin magnetic moment of an electron in an atom can be expressed in terms of atomic unit of magnetic moment called Bohr magneton.
1 Bohr magneton = eħ/2m = 9.27 x 10-24 Am2 (2)
5. applications of ferrites
· They are used to produce ultrasonincs by magneto-striction principle.
· Ferrites are used in audio and video transformers.
· Ferrites rods are used in radio receivers to increase the sensitivity.
· Since the ferrites have low hysteresis loss and eddy current loss, they are used in two port devices such as gyrator, circulator and isolator.
Gyrator: It transmits the power freely in both directions with a phase shift of π radians.
Circulator: It provides sequential transmission of power between the ports.
Isolator: It is used to display differential attenuation.(2)
6. Antiferromagnetism
In this the spins are arranged in a antiparallel manner due to the various repulsive interactions among them, resulting in zero magnetic moment. Even when the field is increased, the magnetic moment remain zero.This phenomenon is known as antiferromagnetism.(2)
7. Formula HC = Ho [1-(T2/Tc2)] Ans: 0.02165 Tesla. (2)
8. Meissner effect
When the super conducting material is placed in magnetic field, under the condition when T ≤ TC and H ≤ HC the flux lines are excluded from the material. Thus the material exhibits perfect diamagnetism. This phenomenon is called as Meissner effect.(2)
9. SQUIDS
(Super conducting Quantum Interference devices) are the improved model of Josephson devices. It has high efficiency, sensitivity and quick performance.
Principle : Small change in magnetic field, produces variation in the flux quantum.(2)
Part-B
1.a.(i). P-type semiconductor
Ø P-type semiconductor is obtained by doping an intrinsic semiconductor with trivalent impurity atoms like boron, Gallium, Indium, etc.,
Ø The three valence electrons of the impurity atom pairs with three valence electrons of semiconductor atom and one position of the impurity atom remains vacant, this is called hole as shown in figure.
Ø Therefore the no. of holes is increased with the impurity atoms added to it.
Ø Since holes are produced in excess, they are the majority charge carriers in p-type semiconductor and electrons are the minority charge carriers.
Ø Since the impurity can accept the electrons this energy level is called acceptor energy level (Ea) and is present just above the valence band as shown in figure.
Ø Here the current conduction is mainly due to holes.
CARRIER CONCENTRATION IN P-TYPE SEMICONDUCTOR
Ø For p-type at absolute zero Ef will be exactly between Ea and Ev.
Ø At low temperatures some electron from valence band fills the holes in the acceptor energy levels as shown in figure.
Ø We know the density of holes in the valence band,
nh =2 2πmh*KBTh232.e(Ev-EF)/TKB ……………… (1)
Ø Let Na be the number of acceptor energy levels per cm3 (i.e.,) density of state Z(Ea)dE, which has energy Ea below the conduction band.
Ø If some of the electrons are accepted by the acceptor energy level from valence band say for example if two electrons are accepted to fill the hole sites in the acceptor levels, then two holes will be created in valence band as shown in figure.
Ø Thus in general we can write the density of holes in donar energy level as
N (Ea)dE = Z(Ea)dE.F(Ea)
ne = Na .F(Ea) ..……………. (2)
we know
F(Ea) = 11+e(Ea-EF)/TKB
And
e(Ea-EF)/TKB> 1
1+e(Ea-EF)/TKB = e(Ea-EF)/TKB
F(Ea) = 1e(Ea-EF)/TKB ………………….. (3)
Substituting eqn (3) in (2)
ne = NaNae(Ea-EF)/TKB …………………… (4)
At equilibrium condition
Equating eqn (1) & eqn (4) and taking log on both sides we get
EF =Ev+Ea2+ KBT2logNa22πmh*KBTh2 ……………. (5)
At 0K when T=0K,
We can write the above equation as Ef =(Ev+Ea)/2 ………….. (6)
(ii).VARIATION OF FERMI ENERGY LEVEL WITH TEMPERATURE AND IMPURITY CONCENTRATION IN P-TYPE SEMICONDUCTOR
Ø When the temperature is increased, some of the electrons in the valence band will go to acceptor energy levels by breaking up the covalent bonds and hence the Fermi level is shifted in upward direction for doping level of Nd = 1021 atoms/m3 as shown in figure.
Ø From the figure it can be seen that for the same temperature, if the impurity atoms doping level is increased say Na = 1024 atoms/m3, the hole concentration increases and hence the Fermi level decreases.
Ø Therefore at low temperature the Fermi energy level may be increased upto the level of intrinsic energy level (Ei)
OR
b. Hall Effect
When a conductor carrying a current is placed in a transverse magnetic field, an electric field is produced inside the conductor in a direction normal to both the current and the magnetic field. This phenomenon is known as Hall effect and the generated voltage is called “Hall voltage”.
Hall effect in n-type semiconductor
Ø Let us consider an n type material to which the current is allowed to pass along x-direction from left to right and the magnetic field is applied in Z-direction, as a result Hall voltage is produced in y direction as shown in figure.
Ø Now due to the magnetic field applied the electrons move towards downward direction with the velocity ‘v’ and cause the negative charge to accumulate at face (1) of the material as shown in figure
Ø Here the force due to PD = -eEH ………….. (1)
Ø Force due to magnetic field = -Bev ………….. (2)
At equilibrium (1) = (2), we get
EH =Bv …..……… (3)
Current density Jx = -neev and v=-(Jx/nee) ..………… (4)
Sub eqn (4) in eqn(3) we get
Hall coefficient RH = -(1/nee) … .……….. (5)
The negative sign indicates that the field is developed in the negative y direction.
Hall effect in P type semiconductor
Ø Let us consider an p type material to which the current is allowed to pass along x-direction from left to right and the magnetic field is applied in Z-direction, as a result Hall voltage is produced in y direction as shown in figure.
Ø Now due to the magnetic field applied the holes move towards downward direction with the velocity ‘v’ and cause the negative charge to accumulate at face (1) of the material as shown in figure.
Ø Here the force due to PD = eEH …………….. (6)
Ø Force due to magnetic field = Bev …………….. (7)
At equilibrium (1) = (2), we get
EH =Bv ………..…… (8)
Current density Jx = nhev and v=(Jx/nhe) …………… (9)
Sub eqn (9) in eqn(8) we get
Hall coefficient RH = 1/nh e ………….. (10)
The positive sign indicates that the field is developed in the positive y direction.
Hall coefficient in terms of Hall voltage
If the thickness of the sample is t and the voltage developed is VH, then
Hall voltage VH = EH.t ……………… (11)
Sub the value of EH in eqn (11) we have
EXPERIMENTAL VERIFICATION OF HALL EFFECT
Ø A semiconductor slab of thickness ‘t’ and breadth ‘b’ is taken and current is passed using the battery as shown in figure.
Ø The slab this placed between the pole pieces of an electromagnet so that current direction coincides with x-axis and magnetic field coincides with z-axis.
Ø The Hall voltage is measured by placing two probes at the centre of the top and bottom faces of the slab (y-axis).
Ø If B is magnetic field applied and the VH is the Hall voltage produced, then the Hall coefficient can be calculated from the formula,
RH=VHbIxB
Mobility of charge carriers
Ø In general the Hall Co-efficient can be written as RH=-1ne
Ø The above expression is valid only for conductors where the velocity is taken as the drift velocity.
Ø But for semiconductors velocity is taken as average velocity so RH for an ‘n’ type semiconductor is modified as
RH=-3π8ne=-1.18nee ………………… (1)
We know the conductivity for n type is σe =neeμe
(or) μe =σenee ………………… (2)
From eqn (1) we can write
1ne=-RH1.18 ………………… (3)
Substituting eqn (3) in eqn (2) we get
μe =-σeRH1.18
The mobility of electron is in an n-type semiconductor is
μe =-σeVHb1.18IxB
Similarly for p-type semiconductor, the mobility of hole is
μh =-σhVHb1.18IxB
Hall Angle (θH)
We know if the current (Ex) is applied to a specimen along x direction, magnetic field (B) along z-direction then Hall voltage (EH) is produced along y-direction.
Here the Hall angle can be measured from the formula
tanθH=EHEx ………… (1)
We know EH=RHJxB ………. (2)
EH=VxB ………. (3)
Sub eqn (3) in (1) we get
tanθH=μB
Thus the mobility can be defined as the velocity acquired by the charge carrier per unit electric field.
APPLICATIONS HALL EFFECT
i. It is used to determine whether the material is p type or n type semiconductor.
ii. It is used to find the carrier concentration
iii. It is used to find the mobility of charge carriers
iv. It is used to determine the sign of the current carrying charges.
v. It is used to design magnetic flux meters and multipliers on the basis of Hall voltage
vi. It is used to find the power flow in an electromagnetic wave.
2.a. FERROMAGNETIC DOMAIN
According to Weiss theory, the molecular magnets in the ferromagnetic material are said to be aligned in such a way that, they exhibit a magnetization even in the absence of an external magnetic field. This is called spontaneous magnetization.
Thus according to Weiss hypothesis, a single crystal of ferromagnetic material is divided into large number of small regions called domains. These domains have spontaneous magnetization. But the directions of spontaneous magnetization vary from domain to domain and are oriented in such a way that the net magnetization of the specimen is zero as shown in figure.
Due to this reason the iron does not have any magnetization in the absence of an external field.
Now, when the magnetic field is applied, then the magnetization occurs in the specimen by two ways
1. By the movement of domain walls
2. By rotation of domain walls
(i) By the movement of domain walls
The movement of domain walls takes place in weak magnetic fields. Due to this weak field applied to the specimen the magnetic moment increases and hence the boundaries of domains are displaced, so that the volume of the domains changes as shown in figure.
(ii) By rotation of domain walls
The rotation of domain walls takes place in strong magnetic fields. When the external field is high (strong) then the magnetization changes by means of rotation of the direction of magnetization towards the direction of the applied field as shown in figure.
DOMAIN THEORY OF FERROMAGNETISM
The domain in ferromagnetic solid is understandable from the thermo dynamical principle, (i.e.,) in equilibrium the total energy of the system is minimum. For this, first we consider the total energy of the domain structure and then how it is minimized. The total energy of the domain comprises the sum of following energies. Viz,
1. Exchange energy
2. Anisotropy energy
3. Domain wall energy
4. Magneto-strictive energy
(i) Exchange energy (or) magnetic field energy (or) magneto-static energy
The interaction energy which makes the adjacent dipoles to align themselves is known as exchange energy (or) magnetic field energy. It is the energy required in assembling the atomic magnets into a single domain and this work done is stored as potential energy. The magnetic energy can be reduced by dividing the specimen into two domains as shown in figure. The process of subdivision may be carried further, until the reduction of magnetic energy is less than the increase in energy to form another domain and its boundary. This boundary is called as domain wall (or) Block wall.
(ii) Anisotropy energy
In ferromagnetic crystals there are two direction of magnetization, viz,
(i) Easy direction (ii) Hard direction
In easy direction of magnetization, weak field can be applied and in hard direction of magnetization, strong field should be applied. For producing the same saturation magnetization along both the hard and easy direction, strong fields are required in the hard direction than the easy direction.
Therefore the excess of energy required to magnetize the specimen along hard direction over that required to magnetize the specimen along easy direction is called crystalline anisotropy energy.
(iii) Domain wall energy (or) Bloch wall energy
Bloch wall is a transition layer which separates the adjacent domains, magnetized in different directions. The energy of domain wall is due to both exchange energy and anisotropic energy. Based on the spin alignments, two types of Bloch walls may arise, namely
(i) Thick wall (ii) Thin wall
Thick wall: When the spins at the boundary are misaligned and if the direction of the spin changes gradually as shown in figure, it leads to a thick Bloch wall. Here the misalignments of spins are associated with exchange energy.