Semiconductor Physics Interview Questions and Answers:
1. Why copper is a good conductor?
Ans. In case of copper the electrons in the conduction band are very large. An electron in the conduction band experiences almost negligible nuclear attraction. In fact, an electron in the conduction band does not belong to any particular atom, but it moves randomly throughout the solid. Due to overlapping of valence and conduction bands, a slight pd across conductors causes the electrons in conduction band, called the free electrons, to constitute electric current. So copper is a good conductor of electricity.
2. Classify materials into conductors, semiconductors and insulators.
Ans. According to the free electron theory, the total energy which is equal to the kinetic as well as the potential energy is assumed to be zero, is related to the wave number by the relation E = h2k2/2m , where k = 2π/λ
In the free electron theory, when one considers the interaction of the electron with the period potential of the lattice, one finds that band gaps occur at Brillouin zone boundaries. In case the lower valence band is half filled, the material is said to be a good conductor. There is overlapping of zones/bands. This also refers to case where the material behaves like a metal or good conductor. At higher temperature (KBT ≥ EG) an appreciable number of electrons get excited from the top of the valence band into the bottom of the conduction band. The material is then said to exhibit intrinsic semi conductivity: the magnitude of conductivity depends on the density and mobility of electrons in the conduction band and holes in the valence band. if the band gap is very large so that thermal excitation of electrons from valence to conduction band is negligible even at high temperatures, the material is deemed to be as an insulator.
3. What is the basis for classifying a material as a conductor, semiconductor or a dielectric? What is the conductivity of perfect dielectric?
Ans. Conductors possess high conductivity whereas the characteristic property of insulating materials (or dielectrics) is poor conductivity. Semiconductors occupy an intermediate position between conductors and insulators. Though there is no rigid line separating the conductors from semiconductors and semiconductors from insulators, but still according to resistivity the materials of resistivity of the order from 10-8 to 10-3, 10-3 to 106 and 106 to 1018 ohm-metres may be classified as conductors, semiconductors and dielectrics respectively. Another classification is based on temperature coefficient of resistivity. Metals have positive temperature coefficient of resistivity; semiconductors have small negative temperature coefficient of resistivity and insulators have large negative temperature coefficient of resistivity.
4. What is Fermi level?
Ans. Fermi level is simply a reference energy level. It is the energy level at which the probability of finding an electron ‘n’ energy units above it in the conduction band is equal to the probability of finding a hole ‘n’ energy units below it in the valence band.
5. Why is silicon more widely used for semiconductor material?
Ans. There are many semiconductor materials such as germanium, silicon. grey crystalline tin, selenium, tellurium, boron etc. available but silicon (Si) and germanium (Ge) are the two most widely used semiconductor materials in electronics. The reason for this is that energy required to break their covalent bonds (i.e., the energy required to release an electron from their valence bonds) is very small (1.12 eV for silicon and 0.72 eV for germanium). Both elements have the same crystal structure and similar characteristics.
The silicon semiconductor devices have, in general, higher PIV and current ratings and wider temperature range than germanium semiconductor devices and, therefore, silicon is preferred over germanium in the manufacture of semiconductor devices.
6. What will happen on number of free electrons in a semiconductor on increasing temperature?
Ans. On increasing temperature, the concentration (number) of free electrons increases.
7. What is the effect of temperature on conductivity of a semiconductor?
Ans. The resistance of a semiconductor decreases with the increase in temperature, so the conductivity of a semiconductor increases with the increase in temperature.
8. What is the conductivity of a perfect dielectric?
Ans. Conductivity of a perfect dielectric is zero.
9. Why the energy levels of an atom become energy bands in a solid.
Ans. For the single isolated atom, there are discrete energy levels, 1s, 2s, 2p, 3s, that will be occupied by the electrons of atom. All the atoms of a solid can have identical electronic schemes for their energy levels. If atoms are isolated from one another then the electrons fill the levels in each atom independently but when isolated atoms are brought together to form a solid, the energy levels are considerably affected because of interaction between nearer atoms. There will be a coupling between the outer-shell electrons of the atoms resulting in a band of closely spaced energy states, instead of widely separated energy levels of the isolated atom.
10. What is the importance of valence shell and valence electrons?
Ans. The outermost shell of an atom is called the valence shell and electrons in this shell are called the valence electrons. Formation of energy bands occur owing to overlapping of energy levels of these valence electrons in valence shells. With the decrease in interatomic distance between the atoms in a crystal, the energy levels of electrons in outermost shells of atoms overlap to form energy bands.
11. What is the forbidden energy gap? What is its magnitude for Ge and Si?
Ans. The energy gap between the valence band and conduction band is known as forbidden energy gap. It is a region in which no electron can stay as there is no allowed energy state. Magnitude of forbidden energy gap in germanium and silicon is 0.72 eV and 1.12 eV respectively at 300 K and 0.785 eV and 1.21 eV respectively at absolute zero temperature.
12. Is hole a fundamental particle in an atom?
Ans. Hole is not a fundamental particle in an atom. Holes may be thought of as positive particles, and as such, they move through an electric field in a direction opposite to that of electrons.
13. Define a hole in a semiconductor.
Ans. When an energy is supplied to a semiconductor, a valence electron is lifted to a higher energy level, the departing electron leaves a vacancy in the valence band. This vacancy is called a hole. Thus, a vacancy left in the valence band because of lifting of an electron from valence band to conduction band is known as hole.
14. What is hole current?
Ans. The movement of the hole (positively charged vacancy in the valence band) from positive terminal of the supply to the negative terminal through semiconductor constitutes hole current.
15. Does a hole in a semiconductor contribute to a flow of current? If yes, how and if no, how?
Ans. A hole in a semiconductor contributes to the flow of current. Hole may be thought of as a positive particle and moves through an electric field in a direction opposite to that of an electron. When an external field is applied to a pure semiconductor, the conduction through the semiconductor is by both free electrons and holes. The free electrons in the conduction band move toward the positive terminal of the battery while the holes in the valence band move towards the negative terminal of the battery.
16. What is intrinsic semiconductor?
Ans. An intrinsic semiconductor is one which is made of the semiconductor material in the extremely pure form (impurity content not exceeding one part in 100 million parts of semiconductors).
17. Name any three materials which are most widely used as semiconductors.
Ans. Silicon, germanium and boron.
18. What type of semiconductor results when silicon is doped with (a) donor impurities (b) acceptor impurities?
Ans. (a) Donor impurities- N-type extrinsic semiconductors. (b) Acceptor impurities- P-type extrinsic semiconductors.
19. Which of the two semiconductor materials Si or Ge has larger conductivity at room temperature? Why?
Ans. Since energy required in transferring electrons from valence band to conduction band is more in case of silicon than that in case of germanium. the conductivity of Ge will be more than that of Si at room temperature.
20. Why does a pure semiconductor behave like an insulator at absolute zero temperature?
Ans. For a pure semiconductor at a temperature of absolute zero (- 273.15°C) the valence band is usually full and there may be no electron in the conduction band and it is difficult to provide additional energy required for lifting electron from valence band to conduction band by applying electric field. Hence, the conductivity for a pure semiconductor at absolute zero temperature is zero and it behaves like an insulator.
21. What is the main factor for controlling the thermal generation and recombination?
Ans. Temperature, because with the increase in temperature, concentrations of free electrons and holes increase and the rate of recombination is proportional to the product of concentrations of free electrons and holes and also the rate of production of electron-hole pairs (thermal generation) increases with the rise in temperature.
22. Define mean lifetime of a carrier.
Ans. The amount of time between the creation and disappearance of a free electron is called the lifetime. It varies from a few nanoseconds to several microseconds depending how perfect the crystal is and other factors.
23. In which bands do the movement of electrons and holes take place?
Ans. Free electrons move in conduction band while holes in valence band.
24. What is the mechanism by which conduction takes place inside the semiconductor.
Ans. Conduction occurs in any given material when an applied electric field causes electrons to move in a desired direction within the material. This may be due to one or both of two processes, electron motion and hole transfer. In case of former process, free electrons in the conduction band move under the influence of the applied electric field. Hole transfer involves electrons which are still attached to the atoms i.e., those in valence band.
25. What do you mean by mobility of a carrier?
Ans. Mobility of charge carriers is essentially a measure of the ease with which the carriers can move under the influence of an electric field. The drift velocity per unit electric field is known as mobility of charge carriers.
26. What do you mean by drift velocity and mobility of a free electron?
Ans. The average velocity of a free electron is known as drift velocity
whereas mobility of an electron is defined as the drift velocity per unit electric field.
27. Define mobility of a carrier. Show that the mobility constant of electron is larger than that of a hole.
Ans. Mobility is defined as the average particle drift velocity per unit electric field.
The mobility of electrons is more than that of holes because the probability of an electron having the energy required to move to an empty state in the conduction band is much greater than the probability of an electron having the energy required to move to the empty state in valence band. The mobility of electron is about double that of an hole.
28. Clearly distinguish between conductivity and mobility.
Ans. Conductivity is a measure of ability of material to conduct electric current while mobility of a charge particle characterizes how quickly it can move through a metal or semiconductor.
29. What is diffusion current?
Ans. The diffusion of charge carriers is as a result of a gradient of carrier concentration (i.e., the difference of carrier concentration from one region to another). In this case concentrations of charge carriers (either electrons or holes) tend to distribute themselves uniformly throughout the semiconductor crystal. This movement continues until all the carriers are evenly distributed throughout the material. This type of movement of charge carriers is called the diffusion current.
30. Define drift current in a semiconductor.
Ans. The steady flow of electrons in one direction caused by applied electric field constitutes an electric current, called the drift current.
31. What is meant by Fermi level in semiconductor? Where does the Fermi level lie in an intrinsic semiconductor?
Ans. Fermi level in a semiconductor can be defined as the maximum energy that an electron in a semiconductor have at absolute zero temperature. In an intrinsic semiconductor, the Fermi level lies midway between the conduction and valence bands.
32. Differentiate between intrinsic and extrinsic semiconductors.
Ans. An intrinsic semiconductor is one which is made of the semiconductor material in its extremely pure form.
When a small amount of impurity is added to a pure semiconductor crystal during the crystal growth in order to increase its conductivity, the resulting crystal is called the extrinsic semiconductor.
33. Differentiate between N-type and P-type semiconductors.
Ans. When a small amount of trivalent impurity (such as boron, gallium, indium or aluminium) is added to a pure semiconductor crystal during crystal growth, the resulting crystal is called the P-type semiconductor.
When a small amount of pentavalent impurity (such as arsenic, antimony, bismuth or phosphorus) is added to a pure semiconductor crystal during crystal growth, the resulting crystal is called the N-type semiconductor.
34. What do you mean by donor and acceptor impurities?
Ans. Donor impurities (such as arsenic, antimony, bismuth or phosphorus), when added to a pure semiconductor lattice, form N-type extrinsic semiconductor. The pentavalent impurities are called the donor type impurities as such impurities donate electrons to the lattice.
Acceptor impurities (such as boron, gallium, indium or aluminium), when added to a pure semiconductor lattice, form P-type extrinsic semiconductor. The trivalent impurities are called the acceptor type impurities because such impurities accept electrons from the lattice.
35. What is doping?
Ans. The electrical conductivity of intrinsic semiconductor, which has little current conduction capability at ordinary room temperature and so is of little use, can be increased many times by adding very small amount of impurity (of the order of one atom per million atoms of pure semiconductor) to it in the process of crystallization. This process is called the doping.
36. What is the effect of temperature on extrinsic semiconductors?
Ans. With the increase in temperature of an extrinsic semiconductor, the number of thermally generated carriers is increased resulting in increase of concentration of minority carriers. At temperature exceeding critical temperature the extrinsic semiconductor behaves like an intrinsic semiconductor but with higher conductivity.
37. What are the charge carriers in N-type and P-type semiconductors?
Ans. Free electrons in N-type semiconductors and holes in P-type semiconductors are the charge carriers.
38. For the same order of doping, why does N-type semiconductor exhibit larger conductivity than P-type semiconductor?
Ans. Since the mobility of electrons is greater than that of holes, for same level of doping, N-type semiconductor exhibits larger conductivity.
39. State law of mass action.
Ans. Under thermal equilibrium conditions, the product of concentration n of free electrons and concentration p of holes is constant and is independent of the amount of doping by donor and acceptor impurities. i.e., n p = n2i. This is known as mass action law.
40. Define the term intrinsic concentration.
Ans. Intrinsic concentration of holes or free electrons is called the intrinsic concentration, ni, which varies with temperature according to the relation ni = A0 T3e-EG0/kT.
where A0 is a constant and independent of temperature, k is a Boltzmann constant in eV per K and EG0 is the energy gap in eV at absolute zero temperature, T is the temperature in degrees absolute and ni is the concentration in number/m3. A0 is taken as 2.33 x 1043.
41. Explain the recombination and trapping process in a semiconductor.
Ans. Recombination is a process of merging of a free electron and a hole. The recombination rate is proportional to the product of the concentrations of free electrons and holes. As per need of classical mechanics. momentum must be conserved in an encounter of two particles i.e. momentum must be zero after recombination. So, according to law of conservation, the colliding, electron and hole must have the same magnitude of momentum and have the opposite direction of movement.
The most important mechanism in silicon or germanium through which recombination of holes and electrons occur is the mechanism involving recombination centres or traps which contribute electronic states in the energy gap of the semiconductor material. These new states are associated with imperfections in the crystal. Specifically, metallic impurities in the semiconductor are capable of introducing energy states in the forbidden gap. Recombination is affected not only by volume impurities but also by surface imperfections in the crystal.
Gold is extensively employed as a recombination agent in semiconductor manufacture. Thus the designer of the semiconductor device can obtain desired carrier lifetimes by introducing gold into silicon under controlled conditions.
42. Give the continuation equation for electrons.
Ans. Continuation equation indicates that the rate of thermal generation of electrons equals the rate of electrons lost due to recombination under equilibrium condition.
43. Describe Hall effect.
Ans. When a specimen (metal or semiconductor) is placed in a transverse magnetic field and a direct current is passed through it, then an electric field is induced across its edges in the perpendicular direction of current as well as magnetic field. This phenomenon is called the Hall effect.
44. “Hall effect has played a decisive role in revealing the mechanism of conduction in semiconductors.” Explain the statement.
Ans. The conductivity of a semiconductor does determine the mobility and density of charge carriers separately. But in case there is only one type of charge carriers (either electrons or holes), the density of charge carriers can be found from a measurement of the Hall coefficient of the material. Hence, the given statement is justified.
45. What properties of a semiconductor are determined from a Hall effect?
Ans. The Hall effect may be used to determine (i) whether a semiconductor is of N-type or P-type (ii) the carrier concentration and (iii) measure the conductivity of a material and then compute mobility.
46. What is Hall coefficient? Where is it used?
Ans. Hall coefficient RH = 1/ρ = μ/σ
where ρ is the charge density, μ it is the mobility and σ is the conductivity of specimen.
It is used to determine mobility of specimen.
47. What do you understand by negative I tall constant?
Ans. Hall constant RH will be negative for N-type semiconductors and for metals as charge carriers (majority in N-type) are electrons, charge on which is negative and, therefore, charge density will be negative. So, Hall constant RH will he negative for N-type semiconductors and for metals.
48. What is a P-N junction?
Ans. The contact surface between the layers of P-type and N-type semiconductor pieces placed together so as to form a P-N junction is called the P-N junction.
49. What is potential barrier in a diode? How it gets established?
Ans. The free electrons crossing the P-N junction create negative ions on the P-side by giving some atoms one more electron than their total number of protons. The electrons also leave positive ions behind them on the N-side. As negative ions are created on the P-side of the junction, the P-side acquires a negative potential. Similarly, the positive ions are created on N-side and the N-side acquires a positive potential. The negative potential on the P-side prevents the migration of any more electrons from the N-type material to the P-type material. Similarly, the positive potential on the N-side prevents any further migration of holes across the boundary. Thus, the initial diffusion of charge carriers results in a barrier potential at the junction.
50. How do the transition region width and contact pontential across a P-N junction vary with the applied bias voltage?
Ans. When the P-N junction is forward biased, the transition region width is reduced and the contact potential is also reduced with the increase in applied bias voltage.
When the P-N junction is reverse biased, the transition region is widened, and the contact potential is increased with the increase in applied bias voltage.
51. Which type of charges are present on the two opposite faces of the junction?
Ans. Positive charge on N-side and negative charge on P-side of the junction.
52. What types of carriers are present in space charge region?
Ans. No mobile carrier is present in the space charge region.
53. Why is space-charge region called the depletion region?
Ans. The region around the junction is completely ionized on formation of P-N junction. As a result, there are neither free electrons on the N-side nor the holes on the P-side. Since the region around the junction is depleted of mobile charges, it is called the depletion region.
54. Why an electric field is produced in a depletion region of a P-N junction?
Ans. The separation of positive and negative space charge densities in a P-N junction results in an electric field.
55. What is space-charge width?
Ans. Tile space-charge region extends into the N and P-regions from the metallurgical junction. The distance is known as the space charge width.
56. What is the cause and effect of the depletion layer in a PN junction diode?
Ans. On formation of P-N junction, the complete ionization of the region around the junction is the cause of formation of the depletion layer in a P-N junction diode. The effect of depletion layer is that it prevents the respective charge carriers from crossing the barrier region.
57. What is a compensated conductor?
Ans. A semiconductor containing both donor and acceptor impurity atoms in the same region is called a compensated conductor.
58. How is compensated conductor formed?
Ans. Compensated conductor is formed by diffusing donor impurities in a P-type material or by diffusing acceptor impurities in an N-type material.
59. The electric field in the space-charge region decreases with forward bias and increases with reverse bias. Why?
Ans. Because applied electric field opposes built-in field.
60. What is knee voltage?
Ans. The forward voltage, at which the current through the P-N junction starts increasing rapidly, is called the cut-in or knee voltage.
61. What do you understand by reverse saturation current of a diode?
Ans. Reverse saturation current of a diode is due to minority carriers and is caused when the diode is reverse biased. Only a very small voltage is required to direct all available minority carriers across the junction, and when all minority carriers are flowing across, further increase in bias voltage will not cause increase in current. This current is referred to as a reverse saturation current.
62. Why is silicon preferred to germanium in the manufacturing of semiconductor devices?
Ans. Silicon is preferred to germanium in the manufacturing of semiconductor devices because such devices have higher peak inverse voltage and current ratings and wider temperature range than germanium one.
63. Define peak inverse voltage?
Ans. Peak inverse voltage is the maximum voltage that can be applied to the P-N junction without damage to the junction. If the reverse voltage across the junction exceeds its peak inverse voltage (PIV), the junction may get destroyed owing to excessive heat.