## Semiconductor Devices :: Some Questions for Practice

SEMICONDUCTORS

• Draw a circuit diagram of OR gate.
• What is a logic gate?
• What types of charge-carriers are there in a n-type semi conductor?
• Give the logical symbol of an AND gate. Mark the inputs and outputs.
• Draw the logic symbol for a NAND gate.
• Which biasing will make the resistance of p-n junction high?
• Write the truth table for OR gate.
• What is the change in the collector current, in a transistor of a.c. current gain 150, for a 100µA change in its base current?
• When the voltage drop across a p-n junction diode is increased from 0.65V to 0.70V, the change in the diode current is 5mA. What is the dynamic resistance of the diode?
• How does conductivity of a semiconductor change with rise in its temperature?
• Draw a p-n junction with reverse bias.
• What is the order of energy gap in a semiconductor?
• Give the ratio of the number of holes and the number of conduction electrons in an intrinsic semiconductor.
• In the given diagram, is the diode D forward or reversed biased?

• Convert the number 39 into binary system.
• Write the truth table for the combination of gates shown here.
• How does the conductance of a semi conducting material change with rise in temperature?
• Draw energy band diagram for a                                                                              (i) p- type extrinsic semiconductor                                                                                (ii) n-type extrinsic semiconductor                                                                         (iii) intrinsic semiconductors
• Write the truth table for the following combination of gates:
• How does the collector current change in a junction transistor, if the base region has larger width?
• How does the thickness of the depletion layer in a p-n junction vary with increase in reverse bias?
• How does the energy gap in an intrinsic semiconductor vary, when doped with a pentavalent impurity?
• Name the logic gate realized using p-n junction diodes in the given diagram. Give its logic symbol.
• How does the energy gap in an intrinsic semiconductor vary, when doped with a trivalent impurity?
• How does the width of the depletion region of a p-n junction vary, if the reverse bias applied to it increases?
• What is  a solar cell? How does it work? Give its one use.
• Frequency of input voltage to a half –wave rectifier is 50 Hz. What will be the frequency of the output voltage?
• Draw the graph showing the variation of current with voltage for a p-n junction diode.
• The output of an AND gate is connected to both the inputs of NAND gate. Draw the logic circuit of this combination of gates and write its truth table.
• Derive a relationship between current gain of common base amplifier and current emitter amplifier.
• Draw a circuit for p-n junction diode in forward bias. Sketch the voltage versus current graph for the same.
• In the following diagrams, indicate which of the diodes are forward biased and which are reverse biased.   (i)                      (ii)                           (iii)                             (iv)
• If the emitter and base of n-p-n transistor have same doping concentration, explain how will the collector and base-currents be affected?
• What is an intrinsic semiconductor? How can you convert an intrinsic semiconductor into N-type extrinsic semiconductor?
• Distinguish between n-type and p-type semiconductors on the basis of energy band diagram.
• What is an ideal diode? Draw the output waveform across the load resistor R, if the input waveform is as shown in the figure.

• Write the function of base region of a transistor. Why is this region made thin and slightly doped?
• The output of a two input NAND gate is fed to a NOT gate. Write down the truth table for the final output of the combination.
•  A B Output 0011 0101 1110
• The input resistance of a CE amplifier is 2K? and a current gain is 20. If the load resistance is 5 k?, Calculate: (i) the voltage gain of the amplifier and (ii) the trans-conductance of transistor used.
• Draw the logic symbol of a 2-input NAND gate. Write down its truth table.
• Draw the logic symbol of a 2-input NOR gate. Write down its truth table.
• The following truth table gives the output of a                   2-input logic gate.
• (i) Identify the logic gate used and draw its logic symbol.
• (ii) If the output of  this gate is fed as input to aNOT gate, name the new logic gate so formed.
• Give the logic symbol of AND gate. Explain with the help of circuit diagram, how this gate is realized in practice.
• Give the logic symbol of OR gate. Explain with the help of circuit diagram, how this gate is realized in practice.
• What is meant by the term, dopping of an intrinsic semiconductor? How does it affect the conductivity of a semiconductor?
• If the output of a 2-input NAND gate is fed a the input to a NOT gate (i) name the new logic gate obtained and (ii) write down its truth table.
• Explain briefly why the output and input signals of a common-emitter amplifier differ in phase by 1800.
• In only one of the circuits given below the lamp L lights. Which circuit is it? Give reason for your answer.

(a)                                                                              (b)

• In the circuit diagram given, a voltmeter V is connected across a lamp L. What changes would occur at lamp L and voltmeter V, if the resistor R is reduced in value? Give reason for your answer.
• Draw the energy band diagram of an N-type semiconductor. How does the forbidden energy gap of an intrinsic semiconductor vary with increase in temperature?
• If the emitter and base region of a transistor have same doping concentration, state how (i) collector current and (ii) d.c. current gain of the transistor will change.
• Determine the currents through the resistance ‘R’ of the circuits (i) and (ii), when similar diodes D1 and D2 are connected as shown below.
• Pure silicon at 300 K has equal electron and hole concentrations of 1.5 x 1016/m3. Doping by Indium    increases the hole concentration to 4.5 x 1022/m3. Calculate the new electron concentration in the doped silicon.
• Distinguish between n-type and p-type semiconductors on the basis of energy band diagram.
• 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.
• Draw a circuit diagram to show the biasing of a n-p-n transistor. Explain the transistor action.
• Define the terms ‘potential barrier’ and ‘depletion region’ for a p-n junction diode. State how the thickness of depletion region will change when the p.n junction diode is (i) forward biased. (ii) reverse biased.
• With the help of labelled circuit diagram, explain the rectification action of a full wave rectifier.
• If the base region of a transistor is made large, as compared to a usual transistor, how does it affect (i) the collector current, and (ii)current gain of this transistor? What is the phase difference between the input and output signals of a common emitter amplifier?
• Explain how the depletion layer and barrier potential are formed in a p-n junction diode.
• 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.
• Symbolically represent a pnp transistor. Show the biasing of a pnp transistor and explain the transistor action.
• Distinguish between conductors, semiconductors and insulators on the basis of band theory of solids.
• Draw the circuit diagram of a common-emitter amplifier, with appropriate baising. What is the phase difference between the input and output signals ?State two reasons why a common-emitter-amplifier is preferred to a common base amplifier.,
• Draw the energy band diagram of a p-type semiconductor. Deduce an expression for the conductivity of a p-type semiconductor.
• Draw a circuit diagram to obtain the characteristics of a npn transistor in emitter configuration. Describe    how you will obtain input and output characteristics. Give shape of the curves.
• Draw the circuit diagram of a full-wave rectifier and briefly explain its working principle.
• Identify the logic gates marked X and Y in the figure given. Write the truth-table to find the. output at Z for all values of A and B.
• Explain how the depletion region and barrier potential are formed in a p-n junction diode.
• Identify the logic gates marked X and Y in the figure given. Write the truth-table to find the. output at Z for all values of A and B.
• With a circuit diagram, briefly explain how a zener diode can be used as a voltage regulator.
• In the figure given below is (i) the emitter, and (ii) the collector forward or reverse biased? With the help of a circuit diagram, explain the action of a n-p-n transistor.

• Explain how an intrinsic semiconductor can be converted into (i)n-type and             (ii) p-type semiconductor. Give one example of each and their energy band diagrams.
• Draw the circuit diagram to show the use of a transistor as an oscillator. State how the positive feedback is provided in the circuit.
• In the figure below, circuit symbol of a logic gate and two input waveforms ‘A’ and ‘B’ are shown.