Question Set D

CBSE 12th Standard Physics Semiconductor Electronics: Materials, Devices and Simple Circuits HOT Questions Study Material - QB365

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Questions:

Generic 3

2 x 3 = 6

To reduce the ripples in a rectifier circuit with capacitor filter
(a) \(R_{ L }\) should be increased
(b) input frequency should be decreased.
(c) input frequency should be increased.
(d) Capacitors with high capacitance should be used.

Consider a \(npn\) transistor with its base-junction forward biased and collector base junction reverse biased. Which of the following statements are true?
(a) Electrons crossover from emitter to collector.
(b) Holes move from base to collector.
(c) Electrons move from emitter to base.
(d) Electrons from emitter move out of base without going to the collector.

Generic 2

11 x 2 = 22

Write the functions of three segments of a transistor.

What is the function of base region of a transistor? Why is this region made thin and slightly doped?

A \(p-n\) junction is fabricated from a semiconductor width band gap of 2.8 eV. Can it detect a wavelength of 6000 nm?

How does the conductivity and resistivity change with rise of temperature in case of intrinsic semiconductors?

The energy gap of silicon is 1.14 eV. Find the maximum wavelength at which silicon starts energy absorption.

Write the full forms of the terms
(i) MSI and
(ii) LSI used for different types of integrated circuits.

Why diamond behaves like an insulator?

10.

Why doping is done in semiconductor?

11.

State the rule used in the operation of NOT gate.

12.

Can we take one slab of p-type semiconductor and physically join it to another n-type semiconductor to get p-n junction?

13.

Suppose a pure Si crystal has \(5\times 10^{ 28 }\) atmos \(m^{ -3 }\). It is doped by ppm concentration of pentavalent As. Calculate the number of electrons and holes. Given that \({ n }_{ i }=1.5\times { 10 }^{ 16 }m^{ 3 }\)

Multiple Choice Question

5 x 1 = 5

14.

The dominant mechanisms for motion of charge carriers in forward and reverse biased silicon p-n junction are

(a)

drift in forward biased,diffusion in reverse bias

(b)

diffusion in forward biased,drift in reverse bias

(c)

diffusion in both forward and reverse bias

(d)

drift in both forward and reverse bias

15.

Which type of semiconductor is obtained by mixing arsenic with silicon?

(a)

\(n-type\)

(b)

\(p-type\)

(c)

Both

(d)

None

16.

In a p-type semiconductor, the acceptor level is situated 60m eV above the valence band. The maximum wavelength of light required to produce a hole will be [use hc = 12400eV ]

(a)

\(0.207\times10^{ -5 }m\)

(b)

\(2.07\times10^{ -5 }m\)

(c)

\(20.7\times10^{ -5 }m\)

(d)

\(207\times10^{ -5 }m\)

17.

A p-type semiconductor is obtained by doping silicon with

(a)

germanium

(b)

gallium

(c)

bismuth

(d)

Phosphorus

18.

The conduction band in a solid is partially filled at 0 k. The solid sample is

(a)

Conductor

(b)

Semiconductor

(c)

Insulator

(d)

none of these

Generic 5

3 x 5 = 15

19.

Manoj wanted to do social work during vacations.He visited a remote village where there was no electricity. He made up his mind to help the villagers for getting the solar panels. For this he educated the villagers about techonology and uses of solar panels.he requested the villagers to Villagers agreed and applied for the solar panels. They got the same from government at reduced price. When the solar panels started working,the villagers were happy.
Read the passage and answer the following questions:
(i) What is solar panel?
(ii) What is the basic principle of working of a solar cell?
(iii) What are the basic values you assess in Manoj?

20.

Rohan was studying in a science college and was staying with his grandfather. One day, the old torch which was being used by Rohan's grand father stopped working.He asked Rohan to purchase a new torch for him.Rohan himself made a torch using LED with a small recharge battery and gave it to his grand father as a gift.Rohan explained the advantages of LED over a bulb. Rohan's grand father was very happy.
Read the above passage and answer the following questions:
(i) What is LED? Name the two materials used in making LED, whose light falls in the visible region.
(ii) Why LED is a better choice than a bulb in torch?
(iii) What do you think about the attitude of Rohan towards his grand father ?

21.

\(p-n\) junction is a semiconductor diode.It is obtained by bringing \(p-\) type semiconductor include contact with \(n-\) type semiconductor. A thin layer is developed at the \(p-\) junction which is devoid of any charge carrier but has immobile ions. It is called depletion layer. At the junction a potential barrier appears, which does not allow the movement of majority charge carriers across the junction in the absence of any biasing of the junction.
\(p-\)n junction offers low resistance when forward biased and high resistance when reverse biased. Read the above paragraph and answer the following questions:
(i) Can we measure the potential barrier of \(p-n\) junction by putting a sensitive voltmeter across its terminals?
(ii) What practical lesson do you draw from the above study?

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Answers Key:

Generic 3

2 x 3 = 6

(a), (b), (d)
Since ripple factor with capacitor filter is:
\(\gamma =\frac { 1 }{ \sqrt [ 4 ]{ 3R_{ L } } { C }v } ,\)
So \(\gamma \propto \frac { 1 }{ R_{ L } } ,\ \gamma \propto \frac { 1 }{ C } ,\gamma \propto \frac { 1 }{ v } \)
So to reduce \(\gamma \) , Increase \(R_{ L }\) , C and V.

(a), (c) Knowledge based question.

Generic 2

11 x 2 = 22

The function of emitter is to emit the majority carriers. Function of collector is to collect the majority carriers coming from emitter through base. Base provides the proper interaction between the emitter and the collector.

In a transistor, the base region brings an interaction between emitter and collector. The base is kept thin and slightly doped so that, when emitter of transistor is forward biased, the less number of electron-hole combination takes place in base region and majority of charge carriers going from emitter towards base, pass over to collector. As a result of it, the collector current becomes quite good in comparison to base current.

Energy of wavelength to be detected
\(E=\frac { hc }{ \lambda } =\frac { \left( 6.6\times 10^{ -34 } \right) \times \left( 3\times 10^{ 8 } \right) }{ 6000\times 10^{ -9 }\times 1.6\times 10^{ -9 } } eV\)
\(=0.2 \ eV\)
As \(E<{ E }_{ g }\) , so \(p-n\) junction cannot detect the radiation of given wavelength.

When the temperature of the semiconductor is raised, more covalent bonds are broken in the given semiconductor. Due to it, there is a large increase in the charge carrier concentration in semiconductor. The fraction \((f)\) of the number of electrons raised from valence band to conduction band at temperature TK in intrinsic semiconductor is given by
\(f\propto e^{ -E_{ g }/2kT }\)
where \(E_{ g }\) is the value of forbidden energy gap. If means, if T increases, \(f\) also increases, i.e., with the increase in temperature, the number of electrons in conduction band increases. This leads to increase in conductivity of intrinsic semiconductor. As resistivity of pure semiconductor decreases with increase in temperature.

Here,\(E=1.14 \ eV=1.14\times 1.6\times 10^{ -19 } \ J\)
\(\\ \lambda =\frac { hc }{ E } =\frac { 6.62\times 10^{ -34 }\times 3\times 10^{ 8 } }{ 1.6\times 10^{ -19 }\times 1.14 } =10,888 \ A°\)

(i) MSI stands for Medium Scale Integration circuits.
(ii) LSI stands for Large Scale Integration circuits.

In the energy band diagram of diamond, there is large energy gap =5.54 eV. Due to it , no electron can go from valence band to conduction band

10.

The doping is done in semiconductor to increase the number of mobile electrons /holes and hence to increase the conductivity of semiconductor

11.

The output of a NOT gate is 1 (in level) if input is 0 (in level) and vice versa.

12.

No. Any slab, howsoever flat, will have roughness much larger than interatomic crystal spacing \((2\ to\ 3\overset { \circ }{ A } )\), so continuous contact at the atomic level is not possible.

13.

Note that thermally generated electrons (ni ~10¹⁶m^–3) are negligibly small as compared to those produced by doping.
Therefore, ne \(\approx\) ND
Since n_e n_h = \(n_{i}^{2}\) , The number of holes
n_h = (2.25 x 10³²) / (5 x10²²)
= ~ 4.5 x 10⁹ m^–3

Multiple Choice Question

5 x 1 = 5

14.

(b)
diffusion in forward biased,drift in reverse bias

15.

16.

(b)
\(2.07\times10^{ -5 }m\)

17.

(b)
gallium

18.

Conductor

Generic 5

3 x 5 = 15

19.

(i) Solar panel is a panel having large number of solar cells fitted side by side on it.
(ii) Solar cell is basically a solar energy concerter.It is a \(p-n\)junction device which converts solar energy into electrical energy .If this energy is stored in a capacitor,it can be used for glowing bulbs.
(iii) Manoj displayed his will to do something for society.Trough his shear in a capacitor,it can be used for glowing bulbs.
(iii) Manoj displayed his will to do something for soceity.Through his shear efforts,he got electricity within the reach of villagers, who were suffering otherwise.?The work of Manoj was highly appreciated by villagers.

20.

LED stands for light emitting diode . The following materials are used in making LEDs whose light falls in the visible region : (a) Gallium-Phosphide(Gap)-giving red or green light (b) Gallium-arsenide-phosphide (GaAsp)-giving red or yellow light.
(ii) LED is better than conventional bulb as the torch with LED is handy and light.It gives intense light and consumes less power.
(iii)Rohan had a great love and respect for his grand father.He proved it by making a torch for him using new technology . By this act,Rohan earned more love and affection from his grand father

21.

(i) We cannot measure the potential barrier of \(p-n\) junction by putting a sensitive voltmeter across its terminals because there are no free electrons or holes in the depletion layer. And in the absence of forward biasing, the depletion layer offers infinite resistance.
(ii) From the above study,we find that when some move supports the tendency of majority of the people, the resistance/(opposition) is lowered and output current (desired result) is large (appreciable) However, when some move is against the tendency of majority of people, it suffers rough weather.