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SOLIDS AND SEMICONDUCTOR DEVICES

VERY SHORT ANSWER QUESTIONS

Q-1. The larger the orbit…….is the energy of the electron. (Fill in the blank).

Q-2. Name the minority charge carriers in n-type semiconductors ?

Q-3. Name a device whose characteristics are very close to that of an ideal voltage source.

Q-4. What is the other name for depletion region ?

Q-5. What happens to the potential barrier when a reverse bias is applied to a p-n junction diode ?

Q-6. Name the charge in the space charge region.

Q-7. Which one has higher mobility, electron or hole ?

Q-8. Name a device whose characteristics are very close to that of an ideal current source.

Q-9. What is an ideal voltage source ?

Q-10. What is an ideal current source ?

Q-11. When does avalanche breakdown in a semiconductor occur ?

Q-12. What is the physical meaning of the term ‘recombination’ in semiconductor physics ?

Q-13. Define mobility. What are its units ?

Q-14. Who was awarded the Nobel prize for inventing the transistor and in which year ?

Q-15. Is the ionization energy of an isolated free atom different from the ionization energy Eg for the atoms in a

crystalline lattice ?

Q-16. Doping of silicon with indium leads to which type of semiconductor ?

Q-17. Define the transconductance of a transistor.

Q-18. What is the ratio of the number of holes and the number of conduction electrons in an intrinsic

semiconductor ?

Q-19. Is the ratio of number of holes and the number of conduction electrons in an n-type extrinsic

semiconductor more than, less than or equal to 1 ?

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Q-20. What kinds of biasing are required to the collector and base of a transistor in a common emitter amplifier ?

Q-21. What type of charge carriers are there in a p-type semiconductor ?

Q-22. What type of impurity is added to obtain n-type semiconductor ?

Q-23. How does the collector current change in a junction transistor if the base region has larger width ?

Q-24. What is doping ?

Q-25. Distinguish between n-type and p-type semiconductor.

Q-26. What is an intrinsic semiconductor ?

Q-27. How p-n-p and n-p-n transistors are represented symbolically ?

Q-28. What is an extrinsic semiconductor ?

Q-29. Draw the voltage-current characteristic of a Zener diode.

Q-30. What type of charge carriers are there in a n-type semiconductor ?

Q-31. What is Fermi level ?

Q-32. Name one impurity each, which when added to pure Si produces (i) n-type and (ii) p-type semiconductor.

Q-33. Why is the conductivity of n-type semiconductor greater than that of the p-type semiconductor even when

both of these have same level of doping ?

Q-34. Name two factors on which electrical conductivity of a pure semiconductor at a given temperature

depends.

Q-35. What is Zener breakdown ?

Q-36. How is a sample of an n-type semiconductor electrically neutral thought it has and excess of negative

charge carriers ?

SHORT ANSWER QUESTIONS

Q-37. A change of 7.89 mA in the emitter current of a silicon transistor produces a change of 7.8 mA in the

collector current. What change in the base current is necessary to produce an equivalent change in the

collector current ?

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Q-38. In a p-n junction diode, the increase in forward voltage of 0.19 V increases the forward current by 37.6 mA.

What is the dynamic resistance of the junction ?

Q-39. A Zener diode is specified having a breakdown voltage of 9.1 V with a maximum power dissipation of 364

mW. What is the maximum current that the diode can handle ?

Q-40. What is the junction current at equilibrium in an unbiased p-n junction diode ? Justify your answer.

Q-41. Why radio transistors do not work satisfactorily when used inside a railway carriage ?

Q-42. Distinguish a hole current from an electron current.

Q-43. In a transistor, the emitter is always forward-biased but the collector is reverse-biased, Why ?

Q-44. Explain, in terms of the energy band model of a semiconductor, why the resistance of semiconductors will

decrease when their temperature rises above room temperature. Semiconductor thermometers based on

this effect are known as thermistors.

Q-45. When light strikes a solid, it is possible for an electron in the solid to absorb the energy of a photon in the

beam of light, thereby increasing the kinetic energy of the electron. Explain, in terms of the energy band

model of solids, why metals and semiconductors are opaque, but many insulators are transparent to visible

light.

Q-46. Compare and contrast in a tabular form the common base, common emitter and common collector

transistors, when operated as amplifiers.

Q-47. In the circuit shown in Fig – 26.76, we assume that when the input voltage at the base resistance is 5 V, VBE

is zero and VCE is also zero. What is IB, IC and β ? When the input is zero, then IB is zero. What will be the

output waveform if the input waveform is as shown in the figure ?

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Q-48. In the circuits shown in Fig – 26.78, if the input waveform is as shown in the figure, what will be the output

waveform, (i) across R [in Fig – 26.78 (a)], and (ii) across the diode [in Fig – 26.78 (b)]. Assume that the

diode is ideal.

Q-49. (i) When a p-n junction is forward biased, then the motion of charge carriers across the barrier is due

to……(drift, diffusion), and when it is reversed biased, then the motion of charge carriers is due to

…….(drift, diffusion)

(ii) An ideal p-n junction diode conducts when……biased and does not conduct when…….biased.

(forward, reverse ; forward, reverse)

(iii) The base of a transistor is always…...and…..doped compared to the emitter and collector, (thinner,

thicker ; lightly, heavily)

(iv) In a transistor, as long as VCE > VBE, the IC is almost……..of VCE. (independent, proportional)

(v) For using a transistor as an amplifier, the base-emitter junction is……..biased and the base-collector

junctions is……..biased. (forward, reverse ; forward, reverse)

(vi) An oscillator is nothing but an amplifier…….feedback. (with, without)

Q-50. C, Si and Ge have lattice structure. Why is C insulator while Si and Ge are intrinsic semiconductors ?

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Q-51. An intrinsic semiconductor has 108 m-3 free electrons and is doped with pentavalent impurity atoms of

density 1024 m-3. The free electron density will increase by……….order of magnitude.

Q-52. Why a common emitter transistor amplifier is preferred over a common base transistor amplifier ?

Q-53. In the circuit shown in Fig – 26.80, a voltmeter V is connected across lamp L. What changes would occur at

lamp L and voltmeter V, if the resistance R is reduced ?

Q-54. The ratios of number density of free electrons to holes, 𝑛𝑒

𝑛ℎ , for two different materials A and B, are equal

to one and less than one respectively. Name the type of semiconductor to which A and B belong. Draw

energy level diagrams for A and B.

Q-55. Distinguish between intrinsic and extrinsic semiconductors ?

Q-56. In Fig – 26.83, which of the diodes are forward biased, and which are reverse biased ?

Q-57. Which type of biasing results in very high resistance of a p-n junction diode ? Draw a diagram showing

this bias.

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Q-58. The output of an unregulated DC power supply needs to be regulated. Name the device that can be used

for this purpose and draw the relevant circuit diagram.

Q-59. Draw the circuit of a p-n junction diode with forward bias.

Q-60. Two semiconductor materials X and Y shown in Fig – 26.84 are made by doping germanium crystal with

indium and arsenic respectively. The two are joined end to end and connected to a battery as shown. (i)

Will the junction be forward biased or reversed biased ? (ii) Sketch a V-I graph for this arrangement.

Q-61. Carbon and silicon are known to have similar lattice structures. However, the four bonding electrons of

carbon are present in second orbit while those of silicon are present in its third orbit. How does this

difference result in a difference in their electrical conductivities ?

Q-62. What so the terms ‘depletion region’ and ‘barrier potential’ mean for a p-n junction ?

SHORT ANSWER QUESTIONS

Q-63. An amplifier is represented by a circuit shown in Fig – 26.85 ri is the input resistance of the amplifier and

the voltage Vi is appearing across it. This voltage is amplified by a factor of Av and appears across the load

as voltage V0. An external voltage V8 is applied at the input terminals of the amplifier via series resistor Rs.

What will be the apparent gain Av ‘V0 / Vs of the amplifier ? Express the gain in terms of Av Rs and ri.

Q-64. Fill in the blanks using the word(s) from the list appended with each statement :

(i) As the temperature rises, the resistance offered by metals……..(increases, decreases)

(ii) The conduction band of an insulator is…….empty. (partially, practically)

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(iii) The semiconductors are…….at absolute zero. (conductors, insulators)

(iv) The resistance of semiconductors……with increasing temperature. (decreases, increases)

(v) The forbidden energy gap in case of a semiconductor is……..compared to that of an insulator. (smaller,

greater)

(vi) In an intrinsic semiconductor, the electron and hole concentrations are……(equal, unequal)

(vii) When a block of semiconductor is connected to a battery by a metallic wire, the current flow in the

wire is due to the motion of……..in the wire. (electrons, electrons and holes)

(viii) The doping of an intrinsic semiconductor with certain type of impurity atoms causes an……..in its

electric conductivity. (decrease, increase)

(ix) The movement of charge carriers from a region of higher concentration to a region of lower

concentration is called……...(drift, diffusion)

(x) When a battery is connected to a p-type semiconductor with metallic wire, the current in the

semiconductor is predominantly due to…….(electrons, holes) (xi)

Thomson coefficient for a p-type semiconductor is…….(positive, negative) (xii)

The drift velocity of electrons is expected to be……..(equal to, greater than) holes.

Q-65. Fill in the blanks using the word(s) from the list appended with each statement :

(i) Majority of the solids in nature have…….internal structure (ordered, disordered)

(ii) All crystals are…..(isotropic, anisotropic)

(iii) If the physical properties of a solid do not depend on the directions, the solid is known

as…….(isotropic, anisotropic)

(iv) The structure of NaCl is…….(cubic, hexagonal)

(v) The polycrystalline solids show…..properties, while monocrystals have……..properties. (isotropic,

anisotropic, isotropic, anisotropic)

(vi) Within the liquid crystal phase, some materials show a change in…….with change in temperature.

(colour, shape)

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(vii) Physical properties of amorphous solids are identical in…….directions, and therefore, they are

of…….nature. (all, some; isotropic, anisotropic).

(viii) The liquid crystal display (LCD) are based on the principle that they………(emit light, change the

plane of polarisation of polarised light)

(ix) Ceramics are…….solids (polycrystalline, amorphous).

(x) Ceramic solids are……insulators (generally, always)

(xi) The energy band in solids is an outcome of……..(Pauli’s exclusion principle, Coulomb’s low).

(xii) When two isolated atoms are brought close to each other such that the distance between them

is……..(comparable to, much larger than) lattice spacing, the……..(energy bands, energy levels) of the

outer most electrons are split into…….(very large, very small) number of levels called……..(energy gap,

energy bands).

Q-66. The ionization energy of isolated pentavalent phosphorous atom is very large. How is it possible that when

it goes into silicon lattice position, it releases its fifth electron at room temperature so that n-type

semiconductor is obtained ?

Q-67. Why Si and GaAs are preferred materials for solar cells ?

Q-68. Draw the circuit diagram of a common emitter amplifier using n-p-n transistor. What is the phase

difference between the input signal and output voltage ? State two reasons why a common emitter

amplifier is preferred to a common base amplifier.

Q-69. Zener diodes have higher dopant densities as compared to ordinary p-n junction diodes. How does it affect

the (i) width of the depletion layer, (ii) junction field ?

Q-70. Draw the transfer characteristics of a base-biased transistor in its common emitter configuration. Explain

briefly the meaning of the term ‘active region’ in these characteristics. For what practical use, do we use the

transistor in this ‘active region’ ?

CONCEPTUAL PROBLEMS

Q-71. Give one example of a polycrystal.

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Q-72. Give one example of single crystal.

Q-73. Name some liquid crystal used in light crystal display(LCD)

Q-74. Name three types of crystals.

Q-75. What should be the value of feedback factor for sustained oscillations in an oscillator ?

Q-76. Is there any phase difference between the output and input in common base amplifier ?

Q-77. When a p-n junction diode is formed, what stops all the electrons to flow from n region to p region ?

Q-78. A small portion of indium is incorporated in germanium. Is the crystal n-type or p-type ?

Q-79. Name three physical properties which must be the same along all directions in an isotropic solid.

Q-80. Why a transistor is called current-operated device ?

Q-81. Why the term ‘transistor’ is so named ?

Q-82. Comment on the directions of hole current and electron current.

Q-83. Why amorphous substances have a softening range and not a sharp melting point ?

Q-84. Give three important advantages of semi-conductor devices.

Q-85. Mention one advantage and one disadvantage in the working of a transistor when the temperature is

increased

Q-86. Fig – 26.87 shows a p-n junction diode connected to a battery of emf 4.5 V and an external resistance of

1000 Ω. What is the value of current in the circuit ? Given : barrier potential in the diode = 0.5 V.

Q-87. Do 0 and 1 in digital electronics designate voltage equal to 0 V and 1 V, respectively ? If not, then what are

the commonly used voltage levels ?

Q-88. Can two separate p-n junction diodes placed back to back be used to form p-n-p transistor ?

Q-89. What is LED diode ?

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Q-90. If the emitter and base of n-p-n transistor have same doping concentration, explain how will the base and

collector currents be affected ?

Q-91. A transistor is being used as a common emitter amplifier. What is the value of phase difference, if any,

between the collector-emitter voltage and the input signal ?

Q-92. Define forbidden energy gap in solids.

Q-93. Why are photodiodes used preferably in reverse bias condition ?

Q-94. Draw the energy band diagrams of p-type and n-type semiconductors.

Q-95. Why is a semiconductor damaged by strong current ?

LONG ANSWER QUESTIONS

Q-96. Explain, with the help of a circuit diagram, how the thickness of depletion layer in a p-n junction diode

changes when it is forward biased ?

Q-97. With the help of a labelled circuit diagram, explain how an n-p-n transistor can be used as an amplifier in

common-emitter configuration. Explain how the input and output voltages are out of the phase by 1800 for

a common-emitter transistor amplifier.

Q-98. Explain briefly, with the help of circuit diagram, how V-I characteristics of a p-n junction diode are

obtained in (i) forward bias and (ii) reverse bias. Draw the shape of the curves obtained.

Q-99. What is an oscillator ? With the help of a circuit diagram, explain the working of transistor as oscillator.

Q-100. Discuss the variation of current with voltage in a p-n junction diode when it is forward biased.

Q-101. With the help of a circuit diagram, explain the use of p-n junction diode as a full wave rectifier.

Q-102. With the help of diagrams, distinguish between forward biasing and reverse biasing of a p-n diode.

Q-103. Show the biasing of a photodiode with the help of a circuit diagram. Draw graphs to show variations in

reverse bias currents for different illumination intensities.

Q-104. Explain the working of a transistor as a common emitter amplifier.

Q-105. What are energy bands ? How are these formed ? Distinguish between a conductor, an insulator and a

semiconductor on the basis of energy band diagram.

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Q-106. Explain (i) forward biasing (ii) reverse biasing of a p-n junction diode. With the help of a circuit diagram,

explain the use of this device as a half-wave rectifier.

Q-107. Explain the function of base region of a transistor. Why is this region made thin and lightly doped ? Draw

a circuit diagram to study the input and output characteristics of n-p-n transistor in a common emitter (CE)

configuration. Show these characteristics graphically. Explain how current amplification factor of the

transistor is calculated using output characteristics.

Q-108. What is a p-n junction diode ? With the help of a diagram, explain the formation of potential barrier and

depletion region in it. On what factors does the magnitude of potential barrier of a p-n junction diode

depend ?

Q-109. What is a junction transistor ? With the help of a labelled diagram, explain the use of a junction transistor

as an oscillator.

Q-110. Explain the formation of energy band in solids. Draw energy band diagram for (i) a conductor, (ii) an

intrinsic semiconductor.

Q-111. Distinguish between metals, insulators and semiconductors on the basis of their energy bands.

Q-112. Draw the symbolic representation of a (i) p-n-p, (ii) n-p-n transistor. Why is the base region of transistor

thin and lightly doped ? With proper circuit diagram, show the biasing of a p-n-p transistor in common

base configuration. Explain the movement of charge carriers through different parts of the transistor in

such a configuration and show that IE = IC - IB.

Q-113. State the principle of working of p-n diode as a rectifier. Explain, with the help of a circuit diagram, the use

of p-n diode as a full wave rectifier. Draw a sketch of the input and output waveforms.

Q-114. Explain, with the help of a schematic diagram, the principle and working of a light-emitting Diode. What

criterion is kept in mind while choosing the semiconductor material for such a device ? Write any two

advantages of light-emitting Diode over conventional incandescent lamps.

Q-115. A student has to study the input and output characteristics of a n-p-n silicon transistor in the Common

Emitter configuration. What kind of a circuit arrangement should she use for this purpose ? Draw the

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typical shape of input characteristics likely to be obtained by her. What do we understand by the cut-off,

active and saturation states of the transistor ? In which of these states does the transistor not remain when

being used as a switch ?

Q-116. The given circuit diagram shows a transistor configuration along with its output characteristics. Identify

(i) the type of transistor used and (ii) the transistor configuration employed.

Use these graphs to obtain the approximate value of current amplification factor for the transistor at

VCE = 3 V.