CBSE Important Questions Class 12 Physics Chapter 14

Important Questions for CBSE class 12 physics chapter 14 – Semiconductor Electronics: Material, devices and simple circuits.

With Class 12 Physics chapter 14 important questions, students will get detailed and authentic solutions to their doubts regarding the topic of semiconductor electronics. These Physics class 12 chapter 14 important questions will prepare students for their exams keeping in mind the CBSE syllabus. After studying these chapter 14 class 12 physics important questions, students will be able to solve CBSE sample papers and CBSE Past year question papers.

In addition, these important question notes also contain important formulas and CBSE extra questions that can help students test their understanding and also go through CBSE revision notes, which are mentioned in this pdf.

CBSE class 12 physics chapter 14- Semiconductor Electronics: Materials, Devices, and simple circuits important questions

Study important Questions for class 12 physics chapter 14 – Semiconductor Electronics: Material, devices and simple circuits.  

In this document of Important Questions, class 12 physics chapter 14 article students will get questions from past year papers, and sample papers so that they can study very well along with their standard NCERT books.

Introduction to Semiconductor Electronics: Materials, Devices and Simple Circuits

Devices in which a controlled flow of electrons can be obtained are the basic building blocks of all electronic circuits. Before the discovery of the transistor in 1948, such devices were mostly vacuum tubes (also called valves) like the vacuum diode, which has two electrodes, viz., anode (often called a plate) and cathode; triode, which has three electrodes – cathode, plate and grid; tetrode and pentode (respectively with 4 and 5 electrodes).

The seed of the development of modern solid-state semiconductor electronics goes back to the 1930s when it was realized that some solid-state semiconductors and their junctions offer the possibility of controlling the number and the direction of flow of charge carriers through them.

In the following sections of the question, we will introduce the various types of questions based on semiconductor physics, and we will discuss some semiconductor devices like junction diodes (a 2-electrode device) and bipolar junction transistors (a 3-electrode device).

1 MARK QUESTIONS

  1. How does the width of the depletion layer of the p-n-junction diode change with a decrease in reverse bias?

ANS. The width of the depletion layer of p-n junction diodes decreases with a decrease in reverse bias.

  1. What does a photodiode do?

ANS. A photodiode is a special-purpose p-n junction diode fabricated with a transparent window to allow light to fall on the diode. It is operated under reverse bias.

  1. Define valence band and conductor band.

ANS. Valence band: The valence band is the energy band consisting of valence (tightly bound) electrons. 

Conduction band: The conduction band is the energy band consisting of conduction (loosely bound) electrons.

  1. What do you mean by depletion region and potential barrier in junction diode?

Ans: Depletion region: A layer around the intersection between p and n sections of a junction diode where charge carriers, electrons, and holes are less in number is called the depletion region. 

Potential barrier: The potential difference created due to the diffusion of charge carriers across the junction is called the potential barrier.

  1. What is a semiconductor diode? How can we symbolize this?

ANS. A semiconductor diode is basically a p-n junction with metallic contacts provided at the ends for the application of an external voltage. It is a two-terminal device.                                                    

A p-n junction diode is symbolically represented as shown in fig. a and fig b.

  1. What is reverse saturation current?

ANS. For the diode in reverse bias, the current is very small (~µA) and almost remains constant with change in bias. It is called reverse saturation current.

2 MARK QUESTIONS

  1. A transistor has a current gain of 30. If the collector resistance is 6kΩ and the input resistance is 1kΩ, calculate its voltage gain.

Ans: 

Given that: in =1kΩ out = 6kΩ ∴ꭆgain = ꭆout in = 61 = 6

We also know that voltage gain = current gain × ꭆgain

Clearly, voltage gain=30 × 6=180

  1. Why is the base of the transistor lightly doped?

Ans: In a transistor, most carriers from the emitter region move toward the collector region through the base.

If the base is made thick and highly doped, most carriers will combine with the other carriers within the base, and only a few are collected by the collector, leading to a small output collector current.

Thus, to have a large output collector current, the base is made thin and lightly doped.

  1. Sn, C Si, and Ge are all group XIV elements. Yet, Sn is a conductor, C is an insulator and Si and Ge are semiconductors. Explain why.

ANS. Concept- The property of the conduction level of any element depends on the energy gap between its conduction band and valence band.

Explanation – a material is a conductor if, in its energy band diagram, there is no energy gap between the conduction band and the valence band. for insulators, the energy cap is large, and for semiconductors the energy gap is moderate.

The energy gap for Sn is 0 eV, for C is 5.4 eV, for Si is 1.1 EV and for Ge is 0.7 eV, related to their atomic size. Therefore Sn is a conductor, C is an insulator, and Ge and Si are semiconductors.

  1. What is meant by Zener diodes? How is it symbolically represented? With the help of a circuit diagram, explain the use of Zener diodes as a voltage stabilizer.

Ans. Zener diode: The specially designed junction diodes, which can operate in the reverse breakdown voltage region continuously without getting damaged, are called Zener diodes.

The unregulated dc voltage (filtered output of a rectifier) is connected to the Zener diode through a series resistance R such that the Zener diode is reverse biased. If the input voltage increases, the current through R and Zener diodes also increases. This increases the voltage drop across R without any change in the voltage across the Zener diode. This is because, in the breakdown region, Zener voltage remains constant even though the current through the Zener diode changes. Thus any increase or decrease in input voltage results in an increase or decrease of the voltage drop across R without any change in voltage across the Zener diode. Thus, the Zener diode acts as a voltage regulator.

  1. A transistor is a temperature-sensitive device. Explain.

Ans. In a transistor, free electrons and holes are the charge carriers and are responsible for the current through the transistor as well as in the external circuit. If the temperature rises, more covalent bonds are broken in the semiconducting material of the transistor giving rise to additional free electrons and holes, resulting in a larger current in the transistor and in the external circuit. The effect may be cumulative, and due to excessive heat, the structure of the transistor can be permanently damaged.

3 MARKS QUESTIONS 

  1. What is Fermi level in semiconductors?

Ans. Fermi level (denoted by EF) is present between the valence and conduction bands. It is the highest occupied molecular orbital at absolute zero. The charge carriers in this state have their own quantum states and generally do not interact with each other. When the temperature rises above absolute zero, these charge carriers will begin to occupy states above the Fermi level.

In a p-type semiconductor, there is an increase in the density of unfilled states. Thus, accommodating more electrons at the lower energy levels. However, in an n-type semiconductor, the density of states increases, therefore, accommodating more electrons at higher energy levels.

  1. Why does the resistivity of semiconductors go down with temperature?

Ans. The difference in resistivity between conductors and semiconductors is due to their difference in charge carrier density.

The resistivity of semiconductors decreases with temperature because the number of charge carriers increases rapidly with an increase in temperature, making the fractional change i.e., the temperature, it works as a conductor.

  1. What are some important properties of semiconductors? 

Ans. The important properties of the semiconductors are as follows – 

  1. Semiconductor acts like an insulator at Zero Kelvin. On increasing the temperature, it works as a conductor.
  2. Due to their exceptional electrical properties, semiconductors can be modified by doping to make semiconductor devices suitable for energy conversion, switches, and amplifiers.
  3. Lesser power losses.
  4. Semiconductors are smaller in size and possess less weight.
  5. Their resistivity is higher than conductors but lesser than insulators.
  6. The resistance of semiconductor materials decreases with the increase in temperature and vice-versa.
  7. What are the three characteristics of transistors? How can we design any transistor?

Ans. Characteristics of transistors are as follows-

Any two-port network which is analogous to transistor configuration circuits can be analyzed using three types of characteristic curves. They are –

  1. Input characteristics: The curve describes the changes in the values of input current with respect to the values of input voltage, keeping the output voltage constant.
  2. Output characteristics: The curve is obtained by plotting the output current against the output voltage, keeping the input current constant.
  3. Current transfer characteristics: This characteristic curve describes the variation of output current in accordance with the input current, keeping the output voltage constant.

Designing of a transistor –

Any transistor circuit can be designed using three types of configuration. Three configurations of the transistor are based on the connection of the transistor terminal. The three types of transistor circuit configurations are

  1. common emitter transistor,
  2. common base transistor,
  3. common collector transistor (emitter follower).

Each of these three circuit configurations has its own characteristics curve. Based on the requirement, the type will be chosen for the circuit.

  1. What are the applications of transistors as a switch?

Ans. 

The transistor as a switch has the following uses:

  1. The LED feature is the most widely employed practical application that is used as a switch for the transistor.
  2. The relay operation can be managed by making the necessary circuit changes in order to connect and control some external devices with respect to the relay.
  3. With this idea of transistors, the dc motors can be controlled and monitored. This software is used to turn the engine on and off. The motor speed can be modified by changing the transistor frequency values.
  4. A light-bulb is an example of one of these switches. It can switch the light on if the setting is bright and off, depending on the dark surroundings. A light-dependent resistor (LDR) is used to do this.
  5. An element called a thermistor can be controlled using this switching method, which detects the ambient temperature. The thermistor is called a resistor. This resistance increases when the temperature sensed is low and the resistance decreases when the sensed temperature is high. 

 

  1. What is the positive feedback in an oscillator working principle?

Ans. The positive feedback adopted in the working of an oscillator stimulates an output frequency without the implementation of any input. Positive feedback boosts the output signal by charging a quicker and higher signal in the direction of the input. It functions in a loop permitting continuous and undamped oscillations. Following the principle ‘more produces more’, it is utilized in procedures like fruit ripening and contractions in childbirth. Amplification in an oscillator only administers positive feedback as it feeds the output signal back to the input to model it to be in phase. Further, the feedback and input enhance the amplifier.

5-MARKS QUESTIONS

  1. Explain the working principle of the solar cell. Mention its application.

Ans. A solar cell is basically a p-n junction that generates electro-motive force in the battery when solar radiation falls on the p-n junction.

It works on the same principle (photovoltaic effect) as the photodiode, except that no external bias is applied, and the junction area is kept much larger for solar radiation to be incident because we are interested in more power. A solar cell is of two types – namely, p-type and n-type. Both types use a combination of p-type and n-type silicon, which together form the p-n junction.

MECHANISM- In solar cells, electron-hole pairs are generated due to the absorption of light near the junction. As electrons move towards n-type silicon, holes move toward the p-type silicon layer. Electrons reaching the n-side are collected by front contact, and holes reaching the p-side is collected by back electrical contact. Thus, the potential difference is developed across solar cells; hence, when an external load is connected, a photocurrent flows through it.

Many solar cells are connected in series or parallel to form solar panels or modules.

A simple p-n junction solar cell is shown in the figure below-

The application of solar cells are as follows –

  1. It is widely used in calculators, watches, toys, portable power supplies, etc. 
  2. Solar cells are used in satellites and space vehicles.
  3. Solar panels are used to generate electricity, which is very useful for human needs.
  4. What is an integrated circuit (IC)? What are the commonly used integrated circuits (ICs)?

Ans. Integrated circuits are made up of several components such as R, C, L, diodes, and transistors. They are built on a small single block or chip of a semiconductor known as an integrated circuit (IC). All of them work together to perform a particular task. The IC is easily breakable, so to be attached to a circuit board, it is often housed in a plastic package with metal pins.

Commonly used integrated circuits –

  1. Logic Gate ICs – The combinational circuit generates logical outputs based on various input signals. It may only have two to three inputs but one output.
  2. Timer ICs – A timer IC is produced with accurate timing cycles with a 100 % or 50 % duty cycle.
  3. Operational Amplifiers – An OpAmp or an Operational Amplifier is a high-gain voltage amplifier with a differential input and a single-ended output.
  1. Voltage Regulators – A voltage regulator IC provides a constant DC output irrespective of the changes in DC input.

The development of integrated circuits led to the development of numerous household products, CD players, computers, and televisions. Additionally, the proliferation of chips contributed to the globalization of cutting-edge electronic equipment.

  1. What are logic gates? Define five common logic gates that are of significant importance.

Ans. A gate is a digital circuit that follows a certain logical relationship between the input and output voltages. Therefore, they are generally known as logic gates— gates because they control the flow of information.

The five common logic gates used are NOT, AND, OR, NAND, NOR.

Each logic gate is indicated by a symbol, and its function is defined by a truth table that shows all the possible input logic level combinations with their respective output logic levels. Truth tables help understand the behaviour of logic gates. These logic gates can be realized using semiconductor devices.

  1. NOT GATE – This is the most basic gate, with one input and one output. It produces a ‘1’ output if the input is ‘0’ and vice-versa. That is, it produces an inverted version of the input at its output. This is why it is also known as an inverter. 

The commonly used symbol together with the truth table for this gate is given in figure A and B.

 Figure A and B

  1. OR GATE – An OR Gate has two or more inputs with one output. The output Y is 1 when either input A or input B or both are 1s, that is, if any of the input is high, the output is high.

The commonly used symbol together with the truth table for this gate is given in figure C and D.

  • AND GATE – An AND gate has two or more inputs and one output. The output Y of the AND gate is 1 only when input A and input B are both 1.

The commonly used symbol together with the truth table for this gate is given in figure E and F.                            

  • NAND GATE – This is an AND gate followed by a NOT gate. If inputs A and B are both ‘1’, the output Y is not ‘1’. The gate gets its name from this NOT AND behavior. NAND gates are also called universal gates since, by using these gates you can realize other basic gates like OR, AND, and NOT.

The commonly used symbol, together with the truth table for this gate, is given in figure G and H.

  • NOR GATE – It has two or more inputs and one output. A NOT-operation applied after OR gate gives a NOT-OR gate (or simply NOR gate). Its output Y is ‘1’ only when both inputs A and B are ‘0’, i.e., neither one input nor the other is ‘1’. NOR gates are considered universal gates because you can obtain all the gates like AND, OR, NOT by using only NOR gates.

The commonly used symbol, together with the truth table for this gate, is given in figure I and J. 

  1. What are LIGHT EMITTING DIODES (L.E.D.s) ? Explain how an L.E.D. works.

Ans. When the diode is forward-biased, the minority electrons are sent from p → n while the minority holes are sent from n → p. At the junction boundary, the concentration of minority carriers increases. The excess minority carriers at the junction recombine with the majority charges carriers

Working – When the diode is forward biased, the minority electrons are sent from p → n while the minority holes are sent from n → p. At the junction boundary, the concentration of minority carriers increases. The excess minority carriers at the junction recombine with the majority charge carriers.

The energy is released in the form of photons on recombination. In standard diodes, the energy is released in the form of heat. But in light-emitting diodes, the energy is released in the form of photons. We call this phenomenon electroluminescence. Electroluminescence is an optical phenomenon, and electrical phenomenon where a material emits light in response to an electric current passed through it. As the forward voltage increases, the intensity of the light increases and reaches a maximum.

CBSE CLASS 12 PHYSICS CHAPTER – 14 – FREE PDF DOWNLOAD 

Semiconductor Class 12 Important questions

  1. Write two advantages of L.E.D. over incandescent lamps.

Ans. The two advantages of L.E.D. over incandescent lamps are mentioned below –

  1. Light-emitting Diodes are utilized in numerical displays.
  2. Light-emitting Diodes operate at low voltage and have a longer life than glowing bulbs.
  3. How do LEDs work?

Ans. LEDs work on the principle of electroluminescence. On passing a current through the diode, minority charge carriers and majority charge carriers recombine at the junction. On recombination, energy is released in the form of photons. As the forward voltage increases, the intensity of the light increases and reaches a maximum.

  1. What are some devices that use semiconductors?

Ans. A list of common devices that use semiconductors is as follows-

  1. DIAC
  2. Diode (rectifier diode)
  3. Gunn diode
  4. IMPATT diode
  5. Laser diode
  6. Light-emitting diode (LED)
  7. Photocell
  8. Phototransistor
  1. Write the two processes that take place in the formation of a p-n junction.

Ans. Drift and diffusion are the two processes that take place in the formation of a p-n junction.

Chapter 14 consists of a few key points, as listed below –

IMPORTANT POINTS –

  1. In transistors, the base region is both narrow and lightly doped, otherwise, the electrons or holes coming from the input side (say, emitter in CE configuration) will not be able to reach the collector.
  2. Lattice structure and the atomic structure of constituent elements decide whether a particular material will be an insulator, metal or semiconductor.
  3. Semiconductors are elemental (Si, Ge) as well as compound (GaAs, CdS, etc.).
  4. The number of charge carriers can be changed by the ‘doping’ of a suitable impurity in pure semiconductors. Such semiconductors are known as extrinsic semiconductors. These are of two types (n-type and p-type).
  5. The “light-emitting diode,” or LED, is a p-n junction diode that generates spontaneous radiation when forward-biased
  6. When the transistor is used in the cut-off or saturation state, it acts as a switch.
  7. In modern-day circuits, many logical gates or circuits are integrated into one single ‘Chip’. These are known as Integrated circuits (IC).
  8. A bipolar junction transistor is a solid-state device in which the current flow between two terminals (collector and emitter) is controlled by the amount of current that flows through a third terminal (base).
  9. Doping of silicon with indium produces a p-type semiconductor as Indium is a trivalent impurity.
  10. Reverse biasing gives a semiconductor diode very high resistance.

Conclusion 

Semiconductor is a class of crystalline solids with electrical conductivity between that of a conductor and an insulator. Such materials can be treated chemically to allow transmission and control of an electric current. Semiconductors are used in the manufacture of electronic devices such as diodes, transistors, and integrated circuits. Intrinsic semiconductors have a high degree of chemical purity, but their conductivity is poor. Extrinsic semiconductors contain impurities that produce much greater conductivity. Some common intrinsic semiconductors are single crystals of silicon, germanium, and gallium arsenide; such materials can be converted into the technologically more important extrinsic semiconductors by the addition of small amounts of impurities, a process called doping.

Advances in semiconductor technology in recent years have gone hand in hand with the increased operational speed of computers.

Q1-A full-wave rectifier is given an input frequency of 40Hz. The output frequency will be

opt-

100 Hz

80 Hz

60 Hz

40 Hz

Ans-In full-wave rectification, each input wave is broken into two positive half waves. So, output frequency is 2 x 40 Hz = 80 Hz.

Q2-An n-p-n transistor is preferred over a p-n-p transistor because in n-p-n transistor, the current carriers are

opt-

a-mainly electrons

b-mainly holes

c-both electrons and holes

d-positive ions

Ans-mainly electrons

Q3-The current in the arrangement shown below is(assuming junction diode as ideal)

opt-

10 mA

20 mA

20 A

30 A

Ans-The junction diode is in forward bias.

So, I = [(4 – 2) V/100?]A

= 20 mA

For viewing question paper please click here

FAQs (Frequently Asked Questions)

1. State the reason why GaAs are most commonly used in making a solar cell.

solar radiation is nearly 1.5 eV. In order to have photoexcitation, the energy of radiation (hv) must be greater than the energy band gap (Eg). Therefore, the semiconductor with an energy band gap of about 1.5 eV or lower than it and with a higher absorption coefficient is likely to give better solar conversion efficiency. The energy band gap for Si is about 1.1 eV, while for GaAs, it is about 1.53 eV. The GaAs are better in spite of their higher band gap than Si because it absorbs relatively more energy from the incident solar radiation being of a relatively higher absorption coefficient.

2. At what temperature would an intrinsic semiconductor behave like a perfect insulator?

At O° K temperature, an intrinsic semiconductor behaves like a perfect insulator.

3. Why should a photodiode be operated at reverse bias?

 The photodiodes are used to detect optical signals. The fraction change in minority carriers in the case of the reverse-biased diode is more easily measurable than that of the forward-biased. That is why photodiodes are operated at reverse bias.

4. What determines the colour of an LED?

The colour of an LED is determined by the material used in the semiconducting element. The two primary materials used in LEDs are aluminium gallium indium phosphide alloys and indium gallium nitride alloys. Aluminium alloys are used to obtain red, orange and yellow light, and indium alloys are used to get green, blue and white light. Slight changes in the composition of these alloys change the colour of the emitted light.

5. What is electroluminescence?

Electroluminescence is an optical phenomenon, and electrical phenomenon where a material emits light in response to an electric current passed through it.

6. Which type of semiconductor is used in manufacturing diodes?

Semiconductor materials like silicon and germanium are commonly used in diodes.

7. What are universal gates?

A universal gate is a logic gate that can implement any Boolean function without using another logic gate. The universal gates are the NOR and NAND gates.

8. What distinguishes a semiconductor from an integrated circuit?

Integrated circuits are small silicon devices with at least two interconnected semiconductor components. A semiconductor, on the other hand, is a substance having electrical properties that fall somewhere between those of a good conductor and a good insulator.

9. What is meant by a monolithic integrated circuit?

An integrated circuit (IC) made from a single piece of silicon is known as a monolithic integrated circuit (MIC).

10. What are analogue signals?

Analog signals were used in many systems to produce signals to carry information. These signals are continuous in both values and time. The use of analogue signals has declined with the arrival of digital signals. In short, to understand analogue signals – all signals that are natural or come naturally are analog signals.

11. What are digital signals?

Unlike analog signals, digital signals are not continuous, but signals are discrete in value and time. These signals are represented by binary numbers and consist of different voltage values.

12. How can we convert analog signals into digital signals?

For the conversion of an analogue signal into a digital signal, there are two steps that need to be followed. 

The first step is sampling. In this step, continuous electrical signals with varying times are considered. Both the x-axis and y-axis are considered. Sampling is usually done along the x-axis and is classified into two categories, and they are sampling and downsampling. 

The second step is known as quantization. Quantization is done along the y-axis, and it processes the image in which the continuous signals are divided into overlapping and non-overlapping signals.