UNIT - IV SEMICONDUCTORS AND MAGNETIC MATERIALS

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Ashlie Lester
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1 1. What is intrinsic If a semiconductor is sufficiently pure, then it is known as intrinsic semiconductor. ex:: pure Ge, pure Si 2. Mention the expression for intrinsic carrier concentration of intrinsic semiconductor. The expression for intrinsic carrier concentration of intrinsic semiconductor is given by n i = (N C N V ) 1/2 e E g /2k B T Form the above relation, it is clear that The intrinsic carrier concentration is independent of the Fermi level position. The intrinsic carrier concentration is a function of the band gap E g. n i Depends on the temperature T. 3. What is Fermi level? Locate its position for intrinsic semiconductor. The Fermi level indicates the probability of occupation of energy levels in conduction and valence bands. For an intrinsic semiconductor, hole and electron concentrations are equal, and it indicates that the probability of occupation of energy levels in conduction and valence bands are equal. Thus, the Fermi level lies in the middle of the energy gap E g. E F = E C +E V 2 4. Mention the expression for conductivity of intrinsic semiconductor. The expression for conductivity of intrinsic semiconductor is given by σ = Ae E g /2k B T where A = (N C N V ) 1/2 e μ n + μ p E g = Energy gap of a semiconductor K B = Boltzmann constant T = Temperature 5. What is extrinsic When impurities are added to an intrinsic semiconductor them it becomes an extrinsic semiconductor. Depending upon the type of impurity added to the intrinsic semiconductors, extrinsic semiconductors are two types. n type extrinsic semiconductor p type extrinsic semiconductor Page 1

2 6. Mention the effect of temperature on Fermi energy level. In an n type semiconductor, as T increases, more number of electron hole pairs are formed. At very high temperature T, the concentration of thermally generated electrons in the conduction band will be far greater than the concentration of donor electrons. In such a case, as concentration of electrons and holes become equal, the semiconductor becomes essentially intrinsic and E F returns to the middle of the forbidden energy gap. Hence, it is concluded that as the temperature of the p type and n type semiconductor increases, E F moves towards the middle of the forbidden energy gap. 7. Mention the expressions for Fermi energy levels for n- and p- type For an n type semiconductor, the Fermi energy is E F = E C k B T log N C N D Where N D is concentration of donor atoms. From the above relation, it is clear that Fermi level lies below the conduction band. For a p type semiconductor, the Fermi energy level is E F = E V + k B T log N V N A Where N A is concentration of acceptor atoms. From the above relation, it is clear that Fermi level lies above the valence band. 8. When does the drifting of charge carrier takes place in a Under the influence of an external electric field, the charge carriers are forced to move in a particular direction constituting electric current. This phenomenon is known as the drift. 9. What is the contribution of drifting of charge carriers towards the conductivity of The contribution of drifting of charge carriers towards the conductivity of semiconductor is that the total current density is due to both current density due to electrons and holes. J (drift ) σ drift = = nμ E n + pμ p e 10. When does the diffusion of charge carrier takes place in a Due to non uniform carrier concentration in a semiconductor, the charge carriers moves from a region of higher concentration to a region of lower concentration. This process is known as diffusion of charge carriers. 11. What is the contribution of drifting of charge carriers towards the conductivity of The total current density due to electrons is the sum of the current densities due to drift and diffusion of electrons. Page 2

3 J n = J n drift + J n diffusion = neμ n E + ed n n x Similarly, J p = peμ p E ed p p x 12. Mention Einstein relation for a semiconductor. The relation between mobility μ and diffusion coefficient D of charge carriers in a semiconductor is known as Einstein s relation. D n D p = μ n μ p 13. What is Hall Effect? When a magnetic field is applied perpendicular to a current carrying conductor, a potential difference is developed between the points on the opposite side of the conductor is called Hall Effect. 14. What are the applications of Hall effects? It gives the information about the sign of charge carriers in electric conductor. From this most metals have negatively charged electrons. It can be used to measure the drift velocity of the charge carriers. Hall measurements give the number of charge carriers per unit volume. Strong magnetic field can be measured by the application of Hall Effect. The mobility of the charge carriers can be measured by the conductivity of the material and Hall coefficient. Hall Effect quite helpful in understanding the electrical conduction in metals and semiconductors. 15. What is direct band gap The minimum energy of conduction band and maximum energy of valence band have the same value of wave vector. Ex :: InP, GaAs 16. What is indirect band gap The minimum energy of conduction band and maximum energy of valence band having different value of wave vector. Ex :: Si, Ge 17. What is LED? Light Emitting Diode (LED) is a pn junction device which emits light when forward biased, by a phenomenon called electroluminescence. 18. What is laser diode? Laser diode is a specially fabricated p-n junction device that emits coherent radiations when it is forward biased. 19. What is photo diode? A silicon photodiode is a light sensitive device, also called a photodetector, which converts light signals into electrical signals. Page 3

4 20. What is magnetic susceptibility? It is defined as the ratio of the magnetization produced in a sample to the magnetic field strength. χ = M H 21. What is magnetic permeability? It is a measure of the amount of magnetic lines of forces penetrating through a material. It is defined as ratio the magnetic flux density B in the material to the applied magnetic field intensity H. μ = B H 22. Derive the relation between B, H and M. B = μh = μ o μ r H = μ o μ r H + μ o H μ o H = μ o H + μ o H(μ r 1) = μ o H + μ o M < M = H(μ r 1) > B = μ o (H + M) 23. Derive the relation between χ and µ r? wkt μ r = μ μ o Also B = μ o (H + M) μh = μ o (H + M) μ μ o = 1 + M H μ r = 1 + χ 24. What is Bohr magneton? Bohr magneton is the elementary electron magnetic moment. It is elementary in the sense that no electron can have magnetic moments below it. It is the natural unit for the measurement of atomic magnetic moments. It is denoted by μ B. μ B = e 4πm is a fundamental unit of magnetic moment known as Bohr magneton, its value is 9.27 x Am What is diamagnetic material? The substance which when placed in a magnetic field acquire feeble magnetism opposite to the direction of the field are known as diamagnetic material. 26. What is paramagnetic material? The material which when placed in a magnetic field acquire feeble magnetism in the direction of magnetic field are known as paramagnetic material. Page 4

5 27. What is ferromagnetic material? The substance which when placed in a magnetic field, become strongly magnetized in the direction of field are known as ferromagnetic material. 28. What is an Antiferromagnetic material? In materials that exhibit antiferromagnetism. The magnetic moments of atoms or molecules align in a regular pattern with neighboring spins pointing in opposite directions. 29. What is Ferrimagnetic material? This is a special case of anti-ferromagnetisms. The net magnetization of magnetic sub lattice is not zero since antiparallel moments are of different magnitudes. 30. What is hysteresis? The hysteresis of ferromagnetic materials refers to the lag of magnetization behind the magnetization field. When the temperature of the ferromagnetic substance is less than the ferromagnetic Curie temperature, the substance exhibits hysteresis. 31. What are soft magnetic materials? Soft magnetic materials are also called as permeable magnetic materials, since they allow the flux lines to pass through very easily. They can be easily magnetized and demagnetized. The magnet which is made of soft magnetic material is called soft magnet. 32. What are hard magnetic materials? Hard magnetic materials are also called as permanent magnetic materials because of their ability to retain magnetic field even after the magnetizing force is withdrawn. They are called hard magnetic materials because they cannot be magnetized and demagnetized very easily. When a hard magnetic material is magnetized it is called hard magnet. 33. Mention the applications of soft magnetic materials. Mainly used in electro magnetic machinery and transformer cores. They are also used in switching circuits, microwave isolators and matrix storage of computers, communication equipments. 34. Mention the applications of hard magnetic materials. For production of permanent magnets, used in Magnetic detectors, microphones, flux meters, voltage regulators, damping devices and Magnetic separators, digital computers, transducers, magnetic tapes. Page 5

VALLIAMMAI ENGINEERING COLLEGE

VALLIAMMAI ENGINEERING COLLEGE SRM Nagar, Kattankulathur 603 203 DEPARTMENT OF PHYSICS QUESTION BANK II SEMESTER PH8252 - PHYSICS FOR INFORMATION SCIENCE (Common to CSE & IT) Regulation 2017 Academic Year