Q.1. What are the characteristics of semiconductor?
Answer: The characteristics of semiconductor are as follows-
- Semiconductors have negative temperature coefficient of resistance.
- At very low temperatures, the resistivity of semiconductors matches with that of insulators.
- They don’t follow ohms law.
- The electrical conductivity is very much affected by even a very minute amount of other substances called impurities.
Q.2. What is the difference between direct and indirect band gap semiconductors?
Answer: In direct band gap semiconductor minimum of conduction band and maximum of valence band occurs for same K value (K is known as wave vector or propagation constant) while in indirect band gap semiconductors the minimum of conduction band and maximum of valence band occurs for different values of K. Direct band gap semiconductors are GaAs, GaSb, ZnS etc whereas Si and Ge are indirect band gap semiconductors.
Q.3. Define Fermi level.
Answer: Fermi level in a semiconductor can be defined as the maximum energy that an electron in an semiconductor have at zero degree Kelvin.
Q.4. What is a degenerate semiconductor?
Answer: A heavily doped semiconductor, in which the Fermi level lies in the conduction or valence band so that material behaves as a metal is known as degenerate semiconductor.
Q.5. What will be the direction of electrons and holes when an electric field is applied across the semiconductor?
Answer: when an electric field is applied across the semiconductor, direction of holes is same as the direction of electric field and direction of electrons is opposite to that of electric field.
Q.6. What is recombination and also tell what is used in semiconductor to increase the recombination rate?
Answer: Recombination is a process of merging a free electron and hole. Gold is used in semiconductor to increase the recombination rate.
Q.7. What will be the effect on the conductivity of metals, silicon and germanium semiconductor upon per degree rise in temperature?
Answer: Conductivity of metals is decreased by 0.4% per degree rise in temperature. Conductivity of silicon semiconductor is increased by 8% per degree rise in temperature. Conductivity of germanium semiconductor is increased by 6% per degree rise in temperature.
Q.8. What is mass action law?
Answer: Under thermal equilibrium, the product of concentration of free electrons (n) and the concentration of holes (p) is constant and is independent of the amount of doping by donor and acceptor impurities. This is known as mass action law. Thus, n.p = ni2. Where ni is the intrinsic concentration and is a function of temperature.
Q.9. How electrons and holes will move when an electric field is applied to an intrinsic semiconductor from left to right?
Answer: Electrons move to the left while holes drift to the left.
Q.10. What is Hall effect and its use?
Answer: When a specimen (metal or semiconductor) carrying a current ‘I’ is placed in a transverse magnetic field ‘B’, then an electric field ‘E’ is induced in the direction perpendicular to both ‘I’ and ‘B’. This phenomenon is called Hall effect. The Hall effect may be used for-
- Whether a semiconductor is N type or P type.
- Finding the carrier concentration.
- In calculating the mobility by measuring the conductivity.
Q.11. Which band gap materials are used for the fabrication of Laser?
Answer: Direct band gap materials having high carrier life time are used for fabrication of laser.
Q.12. Which type of semiconductor is not applicable to Einstein formula?
Answer: Degenerate semiconductors are not applicable to Einstein formula because in this type of semiconductor the Fermi level is with-in the valence band and conduction band.
Q.13. What are the properties of an ideal diode?
Answer: An ideal diode is a two terminal device which has the following properties-
- Conducts with zero resistance when forward biased.
- Offers an infinite resistance when reverse biased.
Q.14. What is the difference between static and dynamic resistance of a diode?
Answer: Static resistance is defined as the ratio of dc voltage across the diode to the dc current flowing through it. The dynamic or ac resistance of a diode at a particular dc voltage is defined as the reciprocal of the slope of the forward characteristics.
Q.15. What is the difference between drift current and diffusion current?
Answer: Drift current is the current which flows due to electric field applied whereas diffusion current flows due to concentration gradient.
Q.16. When consider together, what will be the combination when an ideal constant voltage source is connected in series with an ideal constant current source?
Answer: The combination will be a constant current source.
Q.17. What is the difference between transducer and sensor?
Answer: In transducer the input is some physical quantity like temperature, water level etc and output is either some electrical signal or may be some other physical quantity while in sensor the input may be a physical quantity or electrical signal while output is always electrical quantity.
Q.18. Which diode capacitance is higher in forward and reverse biased?
Answer: In forward biased diffusion capacitance is higher than transition capacitance whereas in reverse biased transition capacitance is higher than diffusion capacitance.
Q.19. What is zener breakdown?
Answer: When the PN junction diode is heavily doped the depletion layer is very thin, therefore a small reverse potential difference causes a high electric field at the junction. When field becomes sufficiently high, it may break the covalent bonds and reduce new electron hole pairs. Then the reverse current increases rapidly and the breakdown is known as zener breakdown.
Q.20. What is Avalanche breakdown?
Answer: The Avalanche breakdown occurs in those junctions which are lightly doped (means wide depletion layer). In this case, the minority carriers gain a large amount of kinetic energy from the applied reverse voltage to collide with the covalent bonds of the atom.
So due to head on collision between the covalent bonds and the minority carriers, bonds are broken and a new pairs of electrons and holes are generated. This process will continue and number of free electrons and holes are generated and therefore produces a rapid multiplication of reverse current. This phenomenon is known as Avalanche breakdown.
Q.21. What are the applications of zener diode?
Answer: The applications of zener diode are-
- As a voltage regulator.
- As a fixed reference voltage in transistor biasing.
- As a limiter in wave shaping circuits.
- For a meter protection.
Q.22. What is the temperature coefficient of resistance for zener and Avalanche diode?
Answer: Temperature coefficient of resistance of zener diode is negative whereas the temperature coefficient of resistance of Avalanche diode is positive.
Q.23. Whether diode is a linear or non-linear device?
Answer: A diode is a non-linear device.
Q.24. Which diode is used to generate harmonics?
Answer: Step recovery diode is used to generate harmonics.
Q.25. What Einstein’s equation relates?
Answer: Einstein’s equation relates diffusion constant with mobility.
Q.26. What is the difference between thermistor and sensistors?
Answer: Thermistor is a bulk semiconductor device which has negative temperature coefficient of resistivity i.e., the resistance decreases exponentially with increasing temperature whereas sensistors is a heavily doped bulk semiconductor which has positive temperature coefficient of resistance because heavily doped semiconductor acquires the properties of a metal.
Q.27. Does the thickness of the depletion region ever become zero as the applied forward biased to a p-n junction is increased?
Answer: No because as the applied voltage is increased, less and less voltage is dropped across it due to a progressive fall in its resistance.
Q.28. What is photo diode?
Answer: Photo diode is a two terminal device which operates on reverse bias. It has a small transparent window which allows light to strike the PN junction. When there is a dark current it means no radiation. In reverse bias condition in the absence of light the reverse current through diode is very small, but as soon as light is made to fall on the junction a large amount of current flows and the diode is forward biased.
Q.29. What are the applications of photo diode?
Answer: Some important applications are-
- Photo detection.
- Logic circuits.
- Optical communication system
Photo voltaic cell is a special application of photo diode.
Q.30. What is the difference between LED and PN junction diode?
Answer: PN junction which emits light when forward biased are called light emitting diode. Only difference between LED and PN junction diode is of the material used for manufacturing LEDs. Examples are-
- Gallium arsenide (GaAs) emits infrared radiation.
- Gallium arsenide phosphide (GaAsP) emits red or yellow light.
- Gallium phosphide (GaP) emits red or green light.
- Gallium nitride emits blue light.
Q.31. What is schottky diode?
Answer: It is formed by joining a doped semiconductor region with a metal such as gold, silver or platinum. Few important points about schottky diode are-
- Schottky diode is a metal to semiconductor junction.
- Schottky diode only operates with majority carriers.
- Semiconductor used is usually N type.
- It does not have any charge storage, therefore it is very fast.
- Semiconductor region is lightly doped.
Q.32. What is the use of Rectifier circuit?
Answer: Rectifier circuit is used to convert an ac voltage into dc voltage.
Q.33. What is the output frequency of full wave rectifier as compared to half wave rectifier?
Answer: The output frequency of the full wave rectifier will be doubled as compared to half wave rectifier.
Q.34. What is transistor?
Answer: A transistor is a three layer semiconductor device consisting of either two ‘n’ and one ‘p’ layer of material or two ‘p’ and one ‘n’ type layer of material to form npn and pnp transistor respectively. The word ‘TRANSISTOR’ means transfer + resistor. It means transfer of signal from low resistance circuit to a high resistance circuit i.e, from input to output. A transistor has a very important property that it can raise the strength of a weak signal, which is called amplification.
Q.35. Can any device acts as an amplifier?
Answer: Since no device can generate energy, so no one can act as an amplifier. Actually all the amplifying devices works as an energy converter.
Q.36. What is the difference between amplifier and transformer?
Answer: An amplifier changes the power level of a signal by changing either current or voltage or both whereas in a transformer if the voltage increases the current decreases and vice-versa. No change in the power level of the signal takes place. So we can say that transformer cannot be treated as an amplifier.
Q.37. What is Early effect or base width modulation effect in BJT?
Answer: In a BJT area of collector is largest while that of base is smallest so when reverse bias is applied to collector base junction, the depletion layer widens more in lightly doped base region than in collector region. A stage is reached when reverse bias voltage is made so large that effective base width becomes negligible and electrons flow directly from collector to emitter. This is known as Early effect or base width modulation effect in BJT.
Q.38. Which transistor configuration is most widely used and its application?
Answer: Common emitter configuration is most widely used configuration because it provides voltage as well as current gain. CE configuration is used in audio frequency applications.
Q.39. What is the application of common base configuration?
Answer: Common base (CB) configuration is used for high frequency applications.
Q.40. What is the application of common collector (CC) configuration?
Answer: Because of high input resistance and low output resistance, this circuit is used for impedance matching.
Q.41. What are the effects of fixed emitter bias configuration circuit?
Answer: When emitter resistance RE is introduced in fixed bias circuit is called emitter bias. It’s effects are-
- It improves stability by negative feedback.
- Input impedance increases due to resistance RE in the emitter.
- Emitter bias circuit is less sensitive for variation in β.
Q.42. What is thermal runaway?
Answer: Because of I2R losses, temperature of transistor increases which in turn further increases IC. This increase in IC will further increase the temperature and this process is cumulative which ultimately lead to very high value of IC at very high temperature. Because of this high temperature thermal breakdown in transistor is observed which is called thermal runaway. To avoid the thermal runaway, VCE < VCC / 2.
Q.43. Is it possible to measure the built in potential of a PN junction with the help of voltmeter?
Answer: Voltmeter cannot be used for measurement because the contact potential established at the points of connection of the voltmeter will give wrong measurements.
Q.44. What are the advantages of FET over conventional transistors?
Answer: The advantages of FET over conventional transistors are as follows-
- The FET is a unipolar device, depending only upon majority carriers. Vacuum tube is also an example of unipolar device. The conventional transistor is a bipolar device (operation depends on both the majority and minority carriers).
- FET has high input resistance. Thus , FET is a voltage controlled device like a vacuum tube and not current controlled like a conventional transistor.
- FET is less noisy than a vacuum tube or bipolar transistor because no junctions are present like BJT. so, the partition noise is absent.
- FET is relatively less affected by radiation.
- It has better thermal stability.
- In integrated form, the fabrication of FET is simpler and it occupies less space.
- FET has smaller size, longer life and high efficiency. FET has very high power gain.
Q.45. What are the disadvantages of FET over conventional transistor?
Answer: The disadvantages of FET over conventional transistor are as follows-
- Transconductance is low and hence the voltage gain is low.
- In case of transistor, Transconductance is high, so the voltage gain is high.
- They are more costly than junction transistors.
- FET has relatively small gain bandwidth product.
Q.46. Many FET devices can be connected in parallel increase the current and power capability. Is the same true for BJT’s?
Answer: No, it is not possible in BJT’s because the collector current in a BJT increases rapidly with temperature leading to second breakdown.
Q.47. Describe how an FET can be used as a voltage variable resistor (VVR)?
Answer: In most of the linear applications of FET the device is operated in the constant current portion of its output characteristics that is in saturation region. FET can also be used in the region before pinch-off VDS is small. FET when used in region before pinch-off, it works as variable resistance device i.e., the channel resistance is controlled by the gate bias voltage (VGS). In such an application the FET is referred as voltage variable or voltage dependent resistor.
Q.48. Why n-channel MOSFETS are preferred over p-channel MOSFETS in integrated circuits?
Answer: The n-channel MOSFETS are preferred over p-channel MOSFETS in integrated circuits due o following reasons-
- NMOS has faster operation than PMOS because the mobility of electrons is greater than mobility of holes.
- P-channel MOSFETS are very bulky and hence difficult to fabricate in integrated circuits.
- N-channel MOSFETs is equally suitable for both depletion and enhancement mode operation while P-channel MOSFET is suitable only for enhancement mode operation.
- N-channel MOSFETs have higher packing density than that of p-channel MOSFETs.
Q.49. What is channel length modulation?
Answer: Channel length modulation is present in MOSFET and it is similar to early effect in BJT. When the drain voltage is increased beyond the onset of saturation, the pinch-off point begins to shift towards the source. This displacement is called as channel length modulation.
Q.50. Which type of amplifier has feedback BJT or FE or both?
Answer: Due to component hre there is feedback from output to input in BJT while there is no feedback in FET.