ELECTRONIC DEVICES INTERVIEW QUESTIONS AND ANSWERS
Q.1. What should be the input impedance of an ideal voltage source and current source?
Answer: For ideal voltage source input impedance should be zero so that there is no voltage drop across it for the same reason an ideal current source should have infinite impedance.
Q.2. Indicate whether β-value of the BJT increases or decreases with increase in the value of temperature?
Answer: With increase in temperature, minority carrier life time increases in base region and this tend to increase β while on the other hand due o increase in temperature, transit time increases which tend to decrease β, but the effect of increasing lifetime with temperature dominates so β increases with temperature.
Q.3. What are the limitations of an IC?
Answer: An IC has the following limitations-
- The inductors and transformers cannot be fabricated.
- Capacitors and resistors are limited in maximum value.
- It is not possible to produce high power ICs.
- Low noise and high voltage operation are not easily obtained.
- High frequency response is limited.
- If any component of the IC goes out of order, the whole IC has to be replaced by new one.
Q.4. In current shunt feedback amplifier which parameter is stabilized?
Answer: In current shunt feedback amplifier the current at the output is sampled and mixing at the input is done in shunt. So the parameter that is stabilized is current gain.
Q.5. Why RF input impedance of a MOSFET is low?
Answer: RF input impedance of a MOSFET is low because of the inter-electrode capacitances which provide a low reactance shunting path at the high frequencies.
Q.6. What is a snubber circuit?
Answer: Snubber circuit is a protection circuit, usually an R-C circuit connected across a thyristor to limit the dv/dt rise below the manufacturer’s specifications.
Q.7. Why a graded base BJTs exhibit high frequency response characteristic as compared to their uniform base counterparts?
Answer: The graded base BJTs exhibit high frequency response characteristic as compared to their uniform base counterparts because there exists a built-in electric field in the base region of the graded base BJTs which is absent in case of uniform base counterparts.
Q.8. Why X-rays are used for lithography in IC technology?
Answer: X-rays are used for lithography in IC technology because high resolution is achievable and scatting effects are small.
Q.9. How can you change an n-channel depletion mode MOSFET into an enhancement mode?
Answer: This change can be done by applying backgate bias that is by applying few volts to the p-substrate we can shift the threshold voltage for enhancement mode operation. This method requires an extra power supply.
Q.10. What are the disadvantages of ICs over discrete assembly?
Answer: The disadvantages of ICs over discrete assembly are as follows-
- A restricted range of values exists for resistors and capacitors.
- Poor tolerances are obtained in fabricating resistors and capacitors of specific magnitudes.
- The technology is very costly for small quantity production.
- No practical inductors or transformers can be integrated.
Q.11. In addition to avoiding damage from heat, MOSFET and MOS ICs has to be protected from which important factor?
Answer: MOSFET and MOS ICs have to be protected from electrostatic charge. This is the discharge of the static voltage stored in the capacitance of out body which occurs when we pick up a device. This voltage can cause a breakdown of the gate dielectric, damaging the device.
Q.12. Why are enhancement mode MOSFETs preferred over the depletion mode ones in the fabrication of integrated circuits?
Answer: The enhancement mode MOSFETs preferred over the depletion mode ones in the fabrication of integrated circuits due to some of the following reasons-
- The enhancement FET is very popular in MOS systems because it is much easier to produce than the depletion type device.
- Most of the contaminants in ICs fabrications are mobile ions which are positively charged and are trapped in the oxide layer (sio2-layer) between the gate and substrate.
- As we know in an enhancement mode MOSFETs the gate is normally positive with respect to the substrate and hence the positively charged contaminants collect along the interface between the sio2-layer and the Si The positive charge from this layer of ions attracts free electrons in the channel which tends to make the transistor turn ON prematurely.
Q.13. 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.14. 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.15. 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.16. 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.17. 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.18. 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.
Q19. 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.
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