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Semiconductor is a material whose electrical conductivity is intermediate between that of a conductor and an insulator. As the temperature increases, its resistivity becomes less; mrtals respond in an opposite manner. By adding impurities to the crystal structure, its conducting qualities can be changed in advantageous ways. Diodes, transistors, and the majority of modern electronics are built on the behavior of charge carriers, such as electrons, ions, and electron holes, at these junctions. The semiconductors gallium arsenide, germanium, and silicon are some examples of the Semiconductors in the Periodic Table.
Semiconductor Electronics Class 12 Important Notes PDF
Semiconductor Electronics Class 12 Important Notes
MCQs
Ques 1. What bonds are present in a semiconductor?
- Monovalent
- Bivalent
- Trivalent
- Covalent
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Answer: (d)
Explanation: In a semiconductor, the electrons around each atom are a component of a covalent connection. Two atoms sharing a pair of electrons form a covalent connection. Each atom creates four covalent bonds with its four neighbouring atoms. 8 electrons are thus shared by each atom and the four atoms around it.
Ques 2. The number of electrons in the valence shell of a semiconductor is
- 1
- 2
- 3
- 4
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Answer: (d)
Explanation: The number of electrons in the valence shell of a semiconductor is 4. Covalent bonds are those kinds of bonding. One electron occupies the valence shell in the majority of conductors. On the other hand, semiconductors normally have four electrons in their valence shell.
Ques 3. What happens to the forbidden energy gap of a semiconductor with the fall of temperature?
- Decreases
- Increases
- Unchanged
- Sometimes decreases and sometimes increases
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Answer: (c)
Explanation: The forbidden energy gap of a semiconductor remains unchanged with the fall of temperature. The forbidden energy band gap is the energy difference (measured in eV) between the top of the conduction band and the bottom of the valence band in any material, whether it be a semiconductor, insulator, or metal.
Ques 4. In a P-type semiconductor, the current conduction is due to
- Holes
- Atoms
- Electrons
- Protons
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Answer: (a)
Explanation: In a P-type semiconductor, the current conduction is due to holes. There are more holes than free electrons in p-type semiconductors. Therefore, a p-type semiconductor's conduction is caused by a higher number of holes and a lower number of free electrons.
Ques 5. What is the main function of a transistor?
- Simplify
- Amplify
- Rectify
- All of the above
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Answer: (b)
Explanation: A transistor is a type of solid semiconductor device that performs a variety of tasks, including signal modulation, voltage stabilisation, switching, amplifying, rectifying, and detecting. The transistor functions as a variable current switch that may alter the output current according to the input voltage.
Ques 6. In a semiconductor, what is responsible for conduction?
- Electrons only
- Holes only
- Both electrons and holes
- Neither electrons nor holes
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Answer: (c)
Explanation: In a semiconductor, there are two different types of charge carriers: electrons and holes. A semiconductor's conductivity is the result of adding the conductivities of its holes and electrons. Therefore, both holes and electrons play a role in electrical conduction in a semiconductor.
Ques 7. What happens to the resistance of semiconductors on heating?
- Increases
- Decreases
- Remains the same
- First increases later decrease
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Answer: (b)
Explanation: The valence band conduction band in semiconductors has a gap before it is heated. Because more electrons are available as the semiconductor is heated, the gap shrinks and resistance falls. Consequently, a semiconductor's resistance drops as it gets hotter.
Ques 8. In intrinsic semiconductors at room temperature, the number of electrons and holes are
- Unequal
- Equal
- Infinite
- Zero
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Answer: (b)
Explanation: An intrinsic semiconductor has an equal number of electrons and holes because it is neutral at ambient temperature. The number of excited electrons and the number of holes in intrinsic semiconductors are equal: n = p. In this situation, n = p is still true, and the semiconductor is still intrinsic despite being doped.
Ques 9. Which of the following is not a universal gate?
- NOT
- AND
- OR
- NAND
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Answer: (d)
Explanation: Because it can implement the AND, OR, and NOT functions, the NAND gate is a universal gate. The NAND gate is a Universal Gate: We will demonstrate that the AND, OR, and NOT operations can be carried out just using NOR gates in order to demonstrate that any Boolean function can be implemented using these gates.
Ques 10. A p-type semiconductor is
- Positively charged
- Negatively charged
- Uncharged
- None of the above
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Answer: (a)
Explanation: Because they have a lot of holes and few free electrons, p-type semiconductors are not positively charged. Since the entire number of holes is equal to the total number of acceptor ions, which have opposing charges to each other, they cancel each other out as a result.
Ques 11. Which among the following is the most commonly used semiconductor?
- Silicon
- Carbon
- Germanium
- Sulphur
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Answer: (a)
Explanation: Usually, silicon is found in compounds with other elements. Atoms can be tightly and intricately bound by silicon components. Silicon is widely available, cheap, and simple to obtain. This is the primary factor influencing silicon's position as the most popular semiconductor material.
Ques 12. What happens to the resistance of a pure semiconductor when heated?
- The resistance increases
- The resistance decreases
- The temperature remains the same
- Can’t say
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Answer: (b)
Explanation: A pure semiconductor loses resistance as it becomes hotter. Some covalent bonds in the semiconductor break as the temperature rises as a result of the thermal energy delivered.
Ques 13. How many valence electrons does a pentavalent impurity have?
- 3
- 4
- 5
- 6
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Answer: (c)
Explanation: Five valence electrons make up a pentavalent impurity. One atom of it creates bonds with four atoms of the semiconductor when it is added. Bonding only takes place between four atoms. The fifth electron is unoccupied and free to conduct.
Ques 14. How many valence electrons do trivalent impurities have?
- 2
- 3
- 4
- 5
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Answer: (b)
Explanation: Since trivalent atoms—i.e., dopants with valency 3—have three valence electrons apiece, they are known as trivalent impurities. Examples include Indium, Gallium, Aluminum, Boron, etc. The term "Acceptor impurities" refers to these contaminants. As they take on electrons from Si's and Ge's covalent connections.
Ques 15. Which of the following is created when trivalent impurities are added to a semiconductor?
- Free electrons
- Holes
- Bound electrons
- Valence electrons
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Answer: (b)
Explanation: When trivalent impurities like boron, aluminum, or gallium are added to an inherent semiconductor, "holes"—deficient valence electrons—are produced. B2H6 diborane gas is typically used to introduce boron into silicon material.
Ques 16. Which of the following does a hole in the semiconductor define?
- A free proton
- A free neutron
- A free-electron
- An incomplete part of an electron pair bond
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Answer: (d)
Explanation: The gap in an electron pair bond is known as a hole in a semiconductor. A hole in a covalent bond is the result of an electron being missing.
Ques 17. An electron and a hole in close proximity would tend to _____.
- attract each other
- repel each other
- have no effect on each other
- destroy each other
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Answer: (a)
Explanation: There is an attractive force between an electron and a hole because a free electron tends to fall into these holes. Therefore, holes act like positively charged objects. Due to the negative charge of electrons, holes, and electrons are drawn to one another.
Ques 18. What is the random motion of free electrons and holes due to thermal agitation called?
- Pressure
- Diffusion
- Ionization
- None of the above
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Answer: (b) Diffusion
Explanation: The definition of diffusion is the transfer of individual molecules of material from a region of higher concentration to a region of lower concentration over a semipermeable barrier.
Ques 19. Why is the mobility of free electrons greater than that of holes?
- They are light
- They mutually collide less
- They require low energy to continue the motion
- They carry negative energy
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Answer: (c)
Explanation: They require low energy to continue the motion. Because of this, free electrons are more mobile than holes. They have an adverse charge. They weigh little. Less of them colliding with each other
Ques 20. The relation between the number of free electrons in a semiconductor and its temperature is given as
- n α T
- n α T2
- n α T3/2
- n α T1/2
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Answer: (c)
Explanation: The conductivity of the semiconductor increases as temperature rises because more electrons have the energy to move from the conduction band to the valence band.
Ques 21. Which of the following does the resistivity of a semiconductor depend upon?
- Length of the semiconductor
- Atomic nature of the semiconductor
- Shape and atomic nature of the semiconductor
- Shape of semiconductor
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Answer: (b)
Explanation: Because semiconductors, such as germanium and silicon, have a crystalline structure, their atoms are arranged in what is known as a crystal lattice. Both of these substances establish covalent connections with the atoms nearby because they are tetravalent, having four valence electrons in their outermost shell.
Ques 22. Which of the following statements is true about extrinsic semiconductors?
- The gap between the conduction band and the valence bond is more than 16 eV
- The gap between the conduction band and the valence bond is about 1 eV
- The gap between the conduction band and valence band is 100 eV and more
- The conduction band and the valence band overlap.
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Answer: (b)
Explanation: Conduction and valence bands are separated by an energy difference of 0.07 eV. After gaining a little energy, electrons can migrate easily from the valence band to the conduction band due to the tiny gap. The substance is therefore a conductor.
Ques 23. In a semiconductor
- There are no free electrons at 0 K
- There are no free electrons at any temperature
- The number of free electrons increases with the pressure
- The number of free electrons is more than that in a conductor
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Answer: (a)
Explanation: At 0 K, the conduction band is empty in semiconductors, but the valence band is full. Thus, at 0 K, there are no free electrons available for conduction. Covalent bonds that give a semiconductor a free charge carrier for conduction break when the temperature rises.
Ques 24. Let nh and ne be the number of holes and conduction electrons in an extrinsic semiconductor. Then
- nh > ne
- nh = ne
- nh < ne
- nh ≠ ne
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Answer: (d)
Explanation: A dopant is added to an extrinsic semiconductor. An n-type semiconductor, for instance, is created by doping n-type impurities so that there are more electrons than holes. Therefore, the numerical density of holes does not match that of electrons.
Ques 25. A p-type semiconductor is
- Positively charged
- Negatively charged
- Uncharged
- Uncharged at 0K but charged at higher temperatures
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Answer: (c)
Explanation: Doping causes the band gap to decrease from 1 eV to 0.3 to 0.7 eV, allowing electrons to reach C.B. at these energies at ambient temperature (conduction band).
Ques 26. Electric conduction in a semiconductor takes place due to
- electrons only
- holes only
- both electrons and holes
- neither electrons nor holes
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Answer: (c)
Explanation: Semiconductors are substances that fall between conductors (often metals) and nonconductors or insulators in terms of conductivity (such as most ceramics). A semiconductor has enough free electrons at normal temperatures to carry electricity.
Ques 27. The impurity atoms with which pure silicon may be doped to make it a p-type semiconductor are those of
- Phosphorus
- Boron
- Antimony
- Nitrogen
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Answer: (b)
Explanation: When silicon is doped with boron gas, it becomes p-type silicon, which is conductive and rapidly takes electrons when voltage is applied. Only three of the four nearby silicon atoms can form bonds with boron because it only has three electrons in its outer shell.
Ques 28. The electrical conductivity of pure germanium can be increased by
- increasing the temperature
- doping acceptor impurities
- doping donor impurities
- All of the above
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Answer: (d)
Explanation: A single crystal of pure n-type silicon and a crystal of p-type silicon doped with gold exhibit behavior akin to that of germanium. Germanium and silicon have relative conductivities of 0-64 and 1-45 watt units at normal temperatures.
Ques 29. The resistivity of a semiconductor at room temperature is in between
- 10–2 to 10–5 Ω cm
- 10–3 to 106 Ω cm
- 106 to 108 Ω cm
- 1010 to 1012 Ω cm
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Answer: (b)
Explanation: At ambient temperature, the electrons in semiconductors have enough energy to cross the forbidden gap. As a result, the valence band and conduction band are both partially filled at ambient temperature.
Ques 30. The number of electrons in the valence shell of a pure semiconductor is
- 1
- 2
- 3
- 4
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Answer: (d)
Explanation: The valence electrons in each of the semiconductor elements total four. Carbon (C), silicon (Si), germanium (Ge), and antimony are the most prevalent semiconductor elements (Sb). These are all "Group IV" elements, which means that each of them has four valence electrons.
Ques 31. In a semiconductor, the forbidden energy gap between the valence band and the conduction band is of the order is
- 1 MeV
- 0.1 Mev
- 1 eV
- 5 eV
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Answer: (c)
Explanation: The energy difference (measured in eV) between the top of the conduction band and the bottom of the valence band in any material, whether it be a metal, an insulator, or a semiconductor, is known as the forbidden energy band gap of a semiconductor.
Ques 32. The forbidden energy gap for germanium crystal at 0 K is
- 0.071 eV
- 0.71 eV
- 2.57 eV
- 6.57 eV
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Answer: (b)
Explanation: In semiconductors, the forbidden energy gap has a value of 1 eV. Experts' step-by-step guidance will help you clear up any doubts and achieve exceptional exam results. For germanium crystals, the forbidden energy gap is 0.7 electron volts.
Ques 33. In an insulator, the forbidden energy gap between the valence band and conduction band is of the order of
- 1 MeV
- 0.1 MeV
- 1 eV
- 5 eV
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Answer: (d)
Explanation: Conduction and valence bands are separated by an energy difference of 0.07 eV. After gaining a little energy, electrons can migrate easily from the valence band to the conduction band due to the tiny gap.
Ques 34. What is the resistivity of a pure semiconductor at absolute zero?
- Zero
- Infinity
- Same as that of conductors at room temperature
- Same as that of insulators at room temperature
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Answer: (b)
Explanation: A semiconductor has zero electrical conductivity at absolute zero. As a result, resistivity (= electrical conductivity 1/resistivity) is infinite.
Ques 35. The temperature coefficient of resistance of semiconductor is
- Zero
- Constant
- Positive
- Negative
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Answer: (d)
Explanation: A semiconductor's temperature coefficient of resistance is negative. It implies that resistance decreases as temperature rises.
Ques 36. In a p-type semiconductor, the acceptor valence band is
- close to the valence band of the host crystal
- close to the conduction band of the host crystal
- below the conduction band of the host crystal
- above the conduction band of the host crystal
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Answer: (a)
Explanation: The electrons that travel from one hole to another in a p-type possess the acceptor energy level, which is a property of the holes. Here, for the electron to enter the conduction band, the covalent bonds must be broken. Its energy level is therefore most similar to the valence energy level.
Ques 37. In an n-type semiconductor, the donor valence band is
- above the conduction band of the host crystal
- close to the valence band of the host crystal
- close to the conduction band of the host crystal
- below the valence band of the host crystal
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Answer: (c)
Explanation: This is caused by the atom's bind energy, or the energy needed to bind the electron to the atom. Given that a donor atom can be roughly compared to a hydrogen atom (which has a tendency to give just one electron), its binding energy is less than 0.1eV! In other words, that amount of energy is enough to ionize the donor.
Ques 38. The mobility of free electrons is greater than that of free holes because
- they are light
- they carry a negative charge
- they mutually collide less
- they require low energy to continue their motion
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Answer: (a)
Explanation: Electron mobility in solid-state physics refers to how quickly an electron may move through a metal or semiconductor when being drawn by an electric field. For holes, there is a corresponding quantity called hole mobility. Both electron and hole mobility are collectively referred to as carrier mobility.
Ques 39. Filter circuit
- eliminates a.c. component
- eliminates d.c. the component
- does not eliminate a.c. component
- None of these
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Answer: (a)
Explanation: A filter circuit is a component that keeps the dc component of the rectifier output from reaching the load while removing the ac component. Between the rectifier and the load is where the filter circuit is situated.
Ques 40. In semiconductors, at room temperature
- the conduction band is completely empty
- the valence band is partially empty and the conduction band is partially filled
- the valence band is completely filled and the conduction band is partially filled
- the valence band is completely filled
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Answer: (b)
Explanation: At ambient temperature, only a small fraction of the electrons in the valence band of a semiconductor have enough energy to go into the conduction band, while the majority stay in the valence band. As a result, the conduction band is partially filled and the valence band is partially filled.
Ques 41. At absolute zero, Si acts as
- non-metal
- metal
- insulator
- None of these
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Answer: (c)
Explanation: A semiconductor's valence band is fully filled and its conduction band is entirely empty at low temperatures. As a result, at low temperatures, a semiconductor practically behaves as an insulator.
Ques 42. One serious drawback of semiconductor devices is
- they do not last for a long time.
- they are costly
- they cannot be used with high voltage.
- they pollute the environment.
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Answer: (c)
Explanation: At high voltages, semiconductor devices are inoperable. The fluctuating voltage of a semiconductor device affects the conductivity of the semiconductor (it means the resistivity of the semiconductor device decreases with an increase in temperature). Therefore, this type of problem is a significant disadvantage of semiconducting.
Ques 43. When an impurity is doped into an intrinsic semiconductor, the conductivity of the semiconductor
- increases
- decreases
- remains the same
- becomes zero
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Answer: (a)
Explanation: There are fewer thermally produced charge carriers in pure semiconductors. However, the quantity of charge carriers, like electrons and holes, increases when it is doped with pentavalent or trivalent impurity atoms. Conductivity thus rises.
Ques 44. An electric field is applied to a semiconductor. Let the number of charge carriers is n and the average drift speed be v. If the temperature is increased
- both n and v will increase
- n will increase but v will decrease
- v will increase but n will decrease
- both n and v will decrease
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Answer: (a)
Explanation: The electrons in the conduction band accelerate and gain energy when an electric field is applied across a semiconductor. They go from a low energy state to a high energy state.
Ques 45. If a small amount of antimony is added to the germanium crystal
- it becomes a p-type semiconductor
- the antimony becomes an acceptor atom
- there will be more free electrons than holes in the semiconductor
- its resistance is increased
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Answer: (c)
Explanation: A little quantity of pentavalent antimony transforms germanium crystal into an n-type semiconductor. As a result, the semiconductor will have more free electrons than holes.
Ques 46. By increasing the temperature, the specific resistance of a conductor and a semiconductor
- increases for both
- decreases for both
- increases, decreases
- decreases, increases
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Answer: (c)
Explanation: As the temperature rises, the conductor's resistivity rises as well. As temperature rises, semiconductor resistivity lowers.
Ques 47. A strip of copper and another of germanium are cooled from room temperature to 80K. The resistance of
- each of these decreases
- copper strip increases and that of germanium decreases
- copper strip decreases and that of germanium increases
- each of these increases
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Answer: (c)
Explanation: As thermal agitation diminishes, copper's resistance falls because it is a conductor, whereas germanium is a semiconductor, hence its resistance grows as temperature decreases.
Ques 48. Carbon, Silicon, and Germanium atoms have four valence electrons each. Their valence and conduction bands are separated by energy band gaps represented by (Eg)C, (Eg)Si, and (Eg)Ge respectively. Which one of the following relationships is true in their case?
- (Eg)C > (Eg)Si
- (Eg)C < (Eg)Si
- (Eg)C = (Eg)Si
- (Eg)C < (Eg)Ge
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Answer: (a)
Explanation: Each atom of carbon, silicon, and germanium has five valence electrons. Their energy band gaps, denoted by (Eg)c, (Eg)si, and (Eg)Ge, respectively, divide their valence and conduction bands.
Ques 49. A semiconductor device is connected in a series circuit with a battery and a resistance. A current is found to pass through the circuit. If the polarity of the battery is reversed, the current drops to almost zero. The device may be a/an
- intrinsic semiconductor
- p-type semiconductor
- n-type semiconductor
- p-n junction diode
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Answer: (d)
Explanation: A Zener diode is a particular kind of diode made to consistently permit current to flow "backward" when a specific reverse voltage, called the Zener voltage, is attained. There is a wide range of Zener voltages used in the fabrication of Zener diodes, some of which are even changeable.
Ques 50. If the two ends of a p-n junction are joined by a wire
- there will not be a steady current in the circuit
- there will be a steady current from the n-side to the p side
- there will be a steady current from the p-side to the n side
- there may or may not be a current depending upon the resistance of the connecting wire
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Answer: (a)
Explanation: There won't be a constant current in the circuit if a wire connects the p and n ends of a p-n diode junction. because the drift current and diffusion current cancel one another out.
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