Solids And Semiconductor Devices
CONCEPTUAL QUESTIONS
Q.1 The resistance of the pn junction is low when forward biased and is high when it is reverse biased. Explain.
A.1 The resistance of pn junction is low when it is forward biased because force acts on charge carriers to move them across the junction whereas the force in reverse biasing acts in opposite direction which opposes the motion of charge carriers across the junction.
Q.2 How will you test whether a transistor is spoiled or in working order?
A.2 a working transistor has low resistance when it is forward biased whereas it has high resistance when it is reverse biased. But if the transistor is spoiled the resistance is low for both forward and reverse biasing.
Q.3 Would you prefer to use a transistor as common base or common emitter configuration.
A.3 We prefer to use the common emitter configuration of the transistor because the current gain is more in that configuration
Q.4 What is the function of grid in a triode valve?
A.4 The grid in triode valves helps us to control the current in the triode valve more effectively because it lies closer to the cathode
Q.5 How will you compare a transistor with a triode?
A.5 In transistor the electron and hole move from emitter to collector whereas in cathode electrons emitted by the cathode are received by the plate. In transistor flow of current is controlled by base whereas in triode it is done by the grid.
Q.6 What is the difference between the transistor as amplifier and the step up transformer?
A.6 The difference between the transistor as an amplifier and the transformer is that in the transistor increase in voltage does not imply the decrease in current whereas in transformer if the magnitude of the voltage increases there is a corresponding decrease in current.
Q.7 Why is the base of transistor of made thin in comparison to the emitter or collector region.
A.7 The base of transistor is made thin in comparison to the collector and the emitter region because the recombination in the base region should be small.
Q.8 What is the effect of temperature on the conductivity of a semiconductor?
A.8 The conductivity of the semiconductor increases with the increase in temperature because as the temperature increases more and more covalent bonds break resulting in the release of charge particles.
Q.9 In a transistor base is very lightly doped, why?
A.9 In transistor the base is lightly doped so that the recombination in the base region should be small and the magnitude of output current is large
Q.10 In what type of semiconductor the electron and the hole concentrations are equal?
A.10 In intrinsic semiconductors the concentrations of the electrons and the holes are equal.
Q.11 Explain why the transistor starts working immediately on switching on whereas vacuum tube circuits take some time before they starts working?
A.11 Vacuum tube circuits are based on the heating effect of the current and thermionic emission takes place from cathode when it is heated. Thus, it takes some time to start as cathode can’t be heated instantly.
Q.12 Why is the depletion region formed at the pn junction?
A.12 The depletion region is formed because of the recombination of electrons and the holes at the junction
Q.13 What will happen if collector as well emitter are forward biased?
A.13 In this case transistor will work as two pn diodes and it can not work as amplifier or oscillator.
Q.14 What is Fermi level and what is Fermi energy?
A.14 Fermi energy level is the highest energy level occupied by the electrons at 00‑kelvin and the energy of that level is fermi energy.
Q.15 Can we use pn junction as amplifier?
A.15 No, a pn junction diode cannot be used as amplifier.
Q.16 Is the number of electrons and holes equal in extrinsic semiconductors. Are they charged?
A.16 No, the number of electrons and holes are not equal in extrinsic semiconductors. Yes, both n type and p type semiconductors are charged
Q.17 Is the junction diode linear or a non linear circuit element?
A.17 Junction diode is a non linear circuit element because the V-I curve is not a straight line
Q18 What will happen if both emitter and collector junction are reverse biased
A.18 In this situation no current will flow in the semiconductor because majority carriers cannot move across the emitter base or base collector junctions.
Q.19 How will you detect intensity of light using diode?
A.19 Light intensity can be measured using a photodiode in reverse biasing.
Q.20 Why gallium arsenide solar cells are preferred over silicon solar cells?
A.20 Gallium arsenide solar cells are used because they can operate with visible energy whereas the silicon diodes works with infrared energies.
Q.21 Define input and the output resistance of a transistor?
A.21 Input resistance of transistor is the ratio of change in base voltage to the change in base current at constant collector voltage .
Q.22 What is the phase relation between the input and the output signal in an amplifier?
A.22 Input and output current are in phase in common base transistor base in transistor as common emitter the input and the output are out of phase by 1800
Q.23 What is zener breakdown and what is the zener breakdown voltage?
A.23 Zener breakdown takes place when a strong reverse bias electric field is applied across the semiconductor. The covalent bonds in the structure break simultaneously resulting in large increase in the magnitude of the current. The voltage at which zener breakdown occurs is zener breakdown voltage
Q.24 What is the avalanche breakdown voltage of a junction diode?
A.24 When the reverse bias voltage is increased to a large value large number of covalent bonds break near the junction resulting in large reverse currents flowing in the transistor. The voltage at which breakdown occurs is avalanche breakdown voltage
Q.25 What will happen if the input circuit is reverse biased and the output circuit is forward biased?
A.25 In this case the collector starts acting as emitter and the emitter as collector with transistor functioning as usual.
Q.26 What type of charge carriers flow during reverse biasing of the diode?
A.26 In reverse biasing the charge carriers flowing in a semiconductor are minority charge carriers.
Q.27 Transistor is a temperature sensitive device. Explain.
A.26 Transistor is a temperature sensitive device because the number of charge carriers increases with the increase in current but if the current increases to a very large magnitude it may damage the transistor.
Q.28 Why an extrinsic semiconductor gets permanently damaged if temperature is increased beyond a certain limit?
A.28 If temperature is increased beyond a limit large number of covalent bonds break in the structure resulting in release of charge carriers and making it highly conducting
Q.29 Explain why input resistance of transistor is low and the output resistance is high.
A.29 Input resistance of the transistor is low because input circuit is forward biased whereas the output circuit is reverse biased.
Q.30 How can you increase the current gain in an amplifier circuit?
A.30 Current gain can be increased by decreasing the doping level in the base and keeping it thin.
Q.31 What is the potential barrier of pn junction in a silicon transistor?
A.31 Potential barrier for silicon transistor is of the order of 0.3V and for germanium transistor it is of the order of 0.7V.
Q.32 Why are transparent bodies insulators?
A.32 Energy of the photon of visible light is of the order of 1eV to 3eV. Such an energy can be absorbed by the valence shell electrons and these electrons still lacks the energy required to jump to the conduction band
Q.33 For what particular application the common base configuration is preferred over the common emitter configuration?
A.33 Common base is preferred over common emitter if we want the voltage amplification without the phase difference between input and the output.
Q.34 By increasing load resistance can we increase or not the gain of transistor indefinitely.
A.34 No, because increases RL will decrease the net output voltage. If the output voltage becomes less than input then it cannot act as amplifier.
Q.35 Why a transistor cannot be used as rectifier?
A.35 Transistor can not be used as rectifier because the two extreme layers are identical whereas for rectification two extreme layers should be of similar type.
Q.36 In the depletion region of pn junction what are the charge carriers in its unbiased state?
A.36 In depletion region there are no charge carriers present
Q.37 What type of feedback is required in transistor as an oscillator?
A.37 In transistor as an oscillator positive feedback is required i.e. emf is induced in such a way that if current in output increases emf is induced in input to support the forward bias and if the current in output decreases emf is induced in input to oppose the forward bias.
Q.38 Can we measure the potential barrier of a pn junction by putting voltmeter across is?
A.38 In the depletion layer, there are no free charges present, thus it offers infinite resistance to the flow of current through it. Therefore potential barrier across a p-n junction cannot be measured using voltmeter.
Q.39 Fill in the blanks
[1] When p-n junction if forward biased, then the motion of charges across the barrier is due to……..
and when it is reverse biased then the motion of charge carriers is due to……..
[2] An ideal pn junction diode conducts, when __________ and does not conduct when ______
A.39 [1] diffusion, drift [2] forward biased, reverse biased
Q.40 Can two p-n junction diodes back to back work as transistor?
A.40 In a transistor, the base region should be thin and lightly doped so that the recombination rate in the base should be small. But if the two diodes are joined back to back the base will be almost double the thickness as emitter or collector and it can’t work as transistor.
Q.41 Portable radio receiver sets are generally not made with electronic tube circuits?
A.41 A portable radio receiver should be small in size and should work on dc. A receiver set made of electronic vacuum tubes is large in size and works only on ac.
Q.42 State two disadvantages of semiconductor devices.
A.42 [1] The semiconductor devices can’t withstand high temperature and an get damaged
[2] the semiconductor devices gets damaged by power surge and can’t withstand high power.
Q.43 When semiconductor junction diode is formed electrons should flow from n to p region but all the electrons do not do so? Explain why?
A.43 When electrons are transferred from p type to n type, the n type semiconductor gets positively charged and p type gets negatively charged which creates potential difference across junction. This potential difference after some time prevents the flow of electron from n type to p type.
Q.44 Why does the thickness of depletion region in a pn junction diode increases with increase in reverse bias?
A.44 When a pn junction is formed, a small potential difference is set up across the depletion layer. But when it reverse biased the charges move away from the junction thus increasing the width of the depletion region.
Q.45 Name the p-n junction diode which emit spontaneous radiation when forward biased. How do we choose the semiconductor, to be used in these diodes, if emitted radiation lies in visible region.
A.45 The p-n junction that emits radiation when forward biased is called photodiode. In case of gallium arsenide the emitted radiations lies in the visible region.
Q.46 In a transistor forward bias is very small as compared to reverse bias. Explain why?
A.46 If the emitter voltage is large, then the number of charges drifting from emitter to collector through base becomes very large. It results in lot of heat generation which can damage the transistor. But if collector reverse bias is large charges may drift quickly to collector but there is small and no heating effect is produced.
Q.47 Why base region in the transistor is made thin?
A.47 Base is thin and lightly doped so that the recombination rate always remains small in the base region and the base current is never more than 5% of the emitter current.
Q.48 Why transistor can’t be used as rectifier?
A.48 Transistor cannot be used as rectifier because to work as transistor either base emitter or base collector region is to be used. But as base is thin and lightly doped neither of the two regions will work as diode or rectifier.
Q.49 Identify the logic gates marked X and Y. write down the value of y when A=1, B=1 and A=0, B=1
A.49 X is NAND gate and Y is Or gate. The output in both the cases is 1
Q.50 Identify the gates marked X and Y and write output when A=1, B=1 and when both are zero.
A.50 Gate X is NAND gate and gate Y is NOT gate. The out put is one when both inputs are one and output is zero if both the inputs are zero.
Q.51 NAND and NOR gate may be considered as digital building blocks. Why?
A.51 The repeated use of NAND or NOR can produce all 3 basic gates i.e. OR, AND and NOT. Thus they are called basic building blocks.
Q.52 The gain of common emitter amplifier is given by Av= -gmRL. does it mean that if we keep on increasing the value of RL indefinitely, the gain of amplifier will increase? Explain.
A.52 No because increasing the RL will decrease Vc and a stage is come where both input and output gets forward biased.
|
1 comments:
Management Courses in Bangalore AICTE approved colleges | Top b schools
Post a Comment