Semiconductors — CBSE Class 12 board PYQs

A pure Si crystal having $5\times 10^{28}$ atoms ${\mathrm{m}}^{-3}$ is doped with 1 ppm concentration of antimony. If the concentration of holes in the doped crystal is found to be $4.5\times 10^9{\mathrm{m}}^{-3}$, the concentration (in ${\mathrm{m}}^{-3}$) of intrinsic charge carr …

In an extrinsic semiconductor, the number density of holes is $4\times 10^{20}{\mathrm{m}}^{-3}$. If the number density of intrinsic carriers is $1.2\times 10^{15}{\mathrm{m}}^{-3}$, the number density of electrons in it is

Pieces of copper and of silicon are initially at room temperature. Both are heated to temperature T. The conductivity of

The formation of depletion region in a p-n junction diode is due to

How does an increase in doping concentration affect the width of depletion layer of a p-n junction diode?

Why cannot we use Si and Ge in fabrication of visible LEDs?

Name the junction diode whose I-V characteristics are drawn in the given figure.

Explain how a depletion region is formed in a junction diode.

What happens to the width of depletion layer of a p-n junction when it is
(i) forward biased,
(ii) reverse biased?

A given logic gate inverts the input applied to it. Name this gate and give its symbol.

Give the logic symbol of a NAND gate.

Give the logic symbol of a NOR gate.

Identify the gate equivalent to the 'dotted box' shown in the figure. The input 'A' shown here is used with another unknown input 'B' in the same set-up. If the output 'Y' has the form shown, give the intervals over which the input 'B' is in its 'high' state.

State the reason why a photodiode is usually operated at reverse bias.

State the reason why GaAs is most commonly used in making of a solar cell.

What will be the values of input A and B for the Boolean expression $(\overline{A+B})\cdot (\overline{A\cdot B})=1$?

Draw energy band diagram of a p-type semiconductor.

Draw the voltage-current characteristic of a zener diode.

Draw the voltage-current characteristic of a p-n junction diode in forward bias.

Give the ratio of number of holes and number of conduction electrons in an intrinsic semiconductor.

In a semiconductor the concentration of electrons is $8\times 10^{13}\mathrm{c}{\mathrm{m}}^{-3}$ and that of holes is $5\times 10^{12}\mathrm{c}{\mathrm{m}}^{-3}$. Is it a p-type or n-type semiconductor?

How does the width of depletion region of a p-n junction vary if the reverse bias applied to it increases?

Name the logic gate shown in the figure and write its truth table.

Draw a logic circuit diagram showing how a NAND gate can be converted to a NOT gate.

Write the truth table for the combination of gates shown in the figure.

How does one understand the temperature dependence of resistivity of a semiconductor?

How does the conductance of a semi-conducting material change with rise of temperature?

Find the truth table of the gates shown in the figure.

In the given diagram, is the diode D forward or reverse biased?

Draw a p-n junction with reverse bias.

Find the truth table of the gate shown in the figure.

(a) Explain the characteristics of a p-n junction diode that makes it suitable for its use as a rectifier.
(b) With the help of a circuit diagram, explain the working of a full wave rectifier.

Explain the following, giving reasons.
(a) A doped semiconductor is electrically neutral.
(b) In a p-n junction under equilibrium, there is no net current.
(c) In a diode, the reverse current is practically independent of the applied voltage.

Answer the following giving reasons.
(i) A p-n junction diode is damaged by a strong current.
(ii) Impurities are added in intrinsic semiconductors.

Two crystals $C_1$ and $C_2$ made of pure silicon are doped with arsenic and aluminium respectively.
(i) Identify the extrinsic semiconductors so formed.
(ii) Why is doping of intrinsic semiconductors necessary?

Why is a photodiode operated in reverse bias whereas the current in forward bias is much larger than that in reverse bias? Explain. Mention its two uses.

(i) Draw the V-I characteristics of a p-n junction diode.
(ii) Differentiate between the threshold voltage and the breakdown voltage for a diode.
(iii) Write the property of a junction diode which makes it suitable for rectification of ac voltages.

Explain the terms 'depletion layer' and 'potential barrier' in a p-n junction diode. How are the
(a) width of depletion layer, and
(b) value of potential barrier affected when the p-n junction is forward biased?

(a) Draw a circuit diagram and explain the working of a zener diode as a dc voltage regulator with the help of its I-V characteristic.
(b) What is the purpose of heavy doping of the p- and n-sides of a zener diode?

(a) Three photodiodes $D_1$, $D_2$ and $D_3$ are made of semiconductors having band gaps of 2.5 eV, 2 eV and 3 eV respectively. Which of them will not be able to detect light of wavelength 600 nm?
(b) Why are photodiodes required to operate in reverse bias? Explain.

(a) A student wants to use two p-n junction diodes to convert alternating current into direct current. Draw the labelled circuit diagram she would use and explain how it works.
(b) Give the truth table and circuit symbol for a NAND gate.

Draw the typical input and output characteristics of an n-p-n transistor in CE configuration. Show how these characteristics can be used to determine
(a) the input resistance ($r_i$) and
(b) the current amplification factor ($\beta$).

Explain the two processes involved in the formation of a p-n junction diode. Hence define the term 'barrier potential'.

Using the waveforms of the inputs A and B, draw the output waveform of the given logic circuit. Identify the logic gate obtained. Write also its truth table.

For a CE-transistor amplifier, the audio signal voltage across the collector resistance of 2 k$\Omega$ is 2 V. Given that the current amplification factor of the transistor is 100, find the input signal voltage and the base current, if the base resistance is 1 k$\Omega$.

A zener diode is fabricated by heavily doping both the p- and n-sides of the junction. Explain why. Briefly explain the use of a zener diode as a dc voltage regulator with the help of a circuit diagram.

(i) Name two important processes that occur during the formation of a p-n junction.
(ii) Draw the circuit diagram of a full wave rectifier along with the input and output waveforms, and briefly explain how the output voltage/current is unidirectional.

(i) Distinguish between a conductor and a semiconductor on the basis of energy band diagram. (ii) The figure shows the input waveforms (A, B) and the output waveform (Y) of a gate. Identify the gate, write its truth table and draw its logic symbol.

(a) Write the functions of the three segments of a transistor.
(b) Draw the circuit diagram for studying the input and output characteristics of an n-p-n transistor in common emitter configuration, and explain how the input and output characteristics are obtained.

Distinguish between 'intrinsic' and 'extrinsic' semiconductors.

With what considerations in view is a photodiode fabricated? State its working with the help of a suitable diagram. Even though the current in forward bias is known to be more than in reverse bias, the photodiode works in reverse bias — what is the reason?

Draw a circuit diagram of a transistor amplifier in CE configuration. Define the terms
(i) input resistance and
(ii) current amplification factor. How are these determined using typical input and output characteristics?

Explain, with the help of a circuit diagram, the working of a p-n junction diode as a half-wave rectifier.

Draw a circuit diagram of an n-p-n transistor amplifier in CE configuration. Under what condition does the transistor act as an amplifier?

Write any two distinguishing features between conductors, semiconductors and insulators on the basis of energy band diagrams.

In the circuit shown in the figure, identify the equivalent gate of the circuit and make its truth table.

Draw the typical output characteristics of an n-p-n transistor in CE configuration. Show how these characteristics can be used to determine the output resistance.

Draw the V-I characteristics of a p-n junction diode. Answer the following, giving reasons:
(i) Why is the current under reverse bias almost independent of the applied potential up to a critical voltage?
(ii) Why does the reverse current show a sudden increase at the critical vol …

The current in forward bias is known to be more ($\sim \mathrm{mA}$) than the current in reverse bias ($\sim \mu \mathrm{A}$). What is the reason, then, to operate the photodiode in reverse bias?

Identify the gate equivalent to the 'dotted box' shown in the figure and give its symbol and truth table.

Draw the circuit diagram of an illuminated photodiode in reverse bias. How is a photodiode used to measure light intensity?

The output of a 2-input AND gate is fed to a NOT gate. Give the name of the combination and its logic symbol. Write down its truth table.

Give a circuit diagram of a common emitter amplifier using an n-p-n transistor. Draw the input and output waveforms of the signal. Write the expression for its voltage gain.

In only one of the two circuits shown, the lamp L glows. State, with reason, in which circuit the lamp lights up.

Draw the
(i) symbol and
(ii) the reverse I-V characteristic of a zener diode. Explain briefly which property of the characteristic enables us to use the zener diode as a voltage regulator.

The two input signals A and B shown in the figure are applied as input to
(i) AND,
(ii) NOR and
(iii) NAND gates. Draw the output waveform in each case.

State the principle of working of a p-n diode as a rectifier. Explain, with the help of a circuit diagram, the use of a p-n diode as a full wave rectifier. Draw a sketch of the input and output waveforms.

Draw a circuit diagram of a full wave rectifier. Explain the working principle. Draw the input/output waveforms indicating clearly the functions of the two diodes used.

The figure shows the V-I characteristics of a semiconductor diode.
(i) Identify the semiconductor diode used.
(ii) Draw the circuit diagram to obtain the given characteristic of this device.
(iii) Briefly explain how this diode can be used as a voltage regulator.

The output of an OR gate is connected to both the inputs of a NAND gate. Draw the logic circuit of this combination of gates and write its truth table.

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 a Light Emitting Diode over conventional incandescent lamps.

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

Draw and explain the output waveform across the load resistor $R$ if the input waveform is as shown in the given figure.

Explain
(i) forward biasing and
(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.

What is an intrinsic semiconductor? How can this material be converted into
(i) n-type and
(ii) p-type extrinsic semiconductor? Explain with the help of energy band diagrams.

Draw a circuit diagram for the use of an NPN transistor as an amplifier in common emitter configuration. The input resistance of a transistor is $1000\Omega$. On changing its base current by $10\mu \mathrm{A}$, the collector current increases by $2\mathrm{mA}$. If a load res …

Explain, with the help of a circuit diagram, how the thickness of the depletion layer in a p-n junction diode changes when it is forward biased. In the following circuits, state which one of the two diodes is forward biased and which is reverse biased.

With the help of a diagram, show the biasing of a light emitting diode (LED). Give its two advantages over conventional incandescent lamps.

In the diagrams shown, state which of the diodes are forward biased and which are reverse biased.

In the given circuit a voltmeter V is connected across the lamp L. What changes would occur at lamp L and the voltmeter V if the resistance R is reduced in value? Give reason for your answer.

A semiconductor has equal electron and hole concentration of $2\times 10^8{\mathrm{m}}^{-3}$. On doping with a certain impurity, the hole concentration increases to $4\times 10^{10}{\mathrm{m}}^{-3}$.
(i) What type of semiconductor is obtained on doping?
(ii) Calculate the new el …

Distinguish between p-type and n-type semiconductors on the basis of energy band diagram.

The following table gives the output of a two-input logic gate: $A=0,B=0,Y=1$; $A=1,B=0,Y=1$; $A=0,B=1,Y=1$; $A=1,B=1,Y=0$.
(i) Identify the logic gate and draw its logic symbol.
(ii) If the output of this gate is fed as input to a NOT gate, name the new logic gate so formed.

Define the terms 'potential barrier' and 'depletion region' for a p-n junction diode.

Draw a circuit for a p-n junction diode in forward bias. Sketch the voltage versus current graph for the same.

Draw the energy band diagram for an n-type and a p-type semiconductor at $T>0\mathrm{K}$.

Draw a simple circuit of a CE transistor amplifier. Explain its working. Show that the voltage gain $A_V$ of the amplifier is given by $A_V=-\frac{{\beta }_{ac}{R}_L}{r_i}$, where ${\beta }_{ac}$ is the current gain, $R_L$ is the load resistance and $r_i$ is the input resistance. What i …

(a) Draw the circuit diagram of a full wave rectifier using a p-n junction diode. Explain its working and show the input and output waveforms. (b) Show the output waveforms (Y) for the following inputs A and B of (i) an OR gate and (ii) a NAND gate.

(i) Draw a circuit diagram to study the input and output characteristics of an n-p-n transistor in its common emitter configuration, and draw the typical input and output characteristics.
(ii) Explain, with the help of a circuit diagram, the working of an n-p-n transistor as a co …

How is a zener diode fabricated so as to make it a special purpose diode? Draw the I-V characteristics for a zener diode and explain the significance of the breakdown voltage. Explain briefly, with the help of a circuit diagram, how a p-n junction diode works as a half wave recti …

Draw the symbolic representation of a
(i) p-n-p and
(ii) n-p-n transistor. Why is the base region of a transistor thin and lightly doped? With a proper circuit diagram, show the biasing of a p-n-p transistor in common base configuration. Explain the movement of charge carriers th …

What are energy bands? How are these formed? Distinguish between a conductor, an insulator and a semiconductor on the basis of energy band diagram.

Explain the function of the 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 an n-p-n transistor in common emitter (CE) configuration. Show these characteristics graphically. Expla …

(a) Distinguish between metals, insulators and semiconductors on the basis of their energy bands.
(b) Why are photodiodes used preferably in reverse bias condition? A photodiode is fabricated from a semiconductor with band gap of $2.8\mathrm{eV}$. Can it detect a wavelength of …