Electronics Devices & Circuits

Field Effect Transistor (FET) Viva Questions

Field Effect Transistor (FET) Viva Questions

Field Effect Transistor (FET) Viva Questions, Viva Questions on Field Effect Transistor (FET), junction field-effect transistor (JFET) Viva Questions, MOSFET Viva Questions, Engineering Viva Questions, Electronics Devices Viva Questions

Multistage Amplifiers Viva Questions

Short Questions with Answers

Q.1. What is a junction field-effect transistor (JFET)? Explain.

Ans. The junction field-effect transistor (JFET) may be divided, depending on its structure, into the following two categories:

  • N-channel JFET
  • P-channel JFET.

Q.2. What is the depletion region in JFET? Explain.

Ans. Whenever a PN-junction is reverse-biased, the electrons and holes diffuse across the junction and leave behind the positive ions on the N-side and negative ions on the P-side. The region, containing these immobile ions, is called the depletion region. As the reverse bias voltage across the junction is increased, the thickness of the depletion region is also increased. If both the P-side and N-side of the Junction are equally doped, the depletion region will extend equally in both regions.

Q.3. What is the pinch-off region? Explain.

Ans. This region has been shown in the figure below by the curve BC. It is also known as the saturation region or constant current region. In this region, the drain current remains constant at its maximum value (i.e., IDSS). The drain current in the pinch-off region depends on on-the gate-to-source voltage and is given by the following relation:

I_{D}=I_{DSS}\left ( 1-\frac{V_{GS}}{V_{P}} \right )^{2}

This relation is known as Shockley’s equation. The pinch-off region is the normal operating region of JFET when used as an amplifier.

Field Effect Transistor (FET) Viva Questions

Q.4. Define breakdown region for JFET.

Ans. This region has been shown in the above figure by the curve CD. In this region, the drain current increases rapidly as the drain-to-source voltage is also increased. It happens because of the breakdown of the gate-to-source junction due to the avalanche effect. The drain-to-source voltage corresponding to point C is known as the breakdown voltage.

Q.5. How MOSFET works like a resistor?

Ans. The MOSFETs have an important property that they can be used as resistor-capacitor amplifiers and a switch. This makes the design of electronic circuits very simple since the entire circuit consists of only MOSFETs and no other component. Examples of such circuits are microprocessors and memory circuits.

Q.6. What is FET biasing? Explain.

Ans. We know the meaning and the need to biasing a device. As a matter of fact, the biasing deals with setting a fixed level of current which should flow through the FET with a desired fixed voltage drop across the FET junctions.

Q.7. What are the FET biasing circuits?

Ans. The commonly used biasing circuits for FET are as under:

  • Fixed-bias circuit.
  • Self-bias circuit.
  • Voltage divider bias circuit.

Q.8. What do you mean by field-effect transistor amplifier?

Ans. The field-effect transistor (FET) has the capability to amplify A.C. signals like a bipolar junction transistor (BJT). Depending upon the types of configuration, the FET amplifiers may be classified under the following three heads:

  • Common source amplifier
  • Common drain amplifier
  • Common gate amplifier.

Q.9. Why are field-effect transistors called unipolar transistors?

Ans. In field-effect transistors, current conduction is by only one type of majority carriers (either by electrons or by holes) and, therefore are called unipolar transistors.

Q.10. Why the channel of a JFET is never completely closed at the drain end?

Ans. If the channel is completely closed in JFET, then there will be no drain current, so there will be no voltage drop along the channel length and the amount of reverse bias will become uniform and the wedge-shaped depletion region will become a rectangular one.

Q.11. How is drain current controlled in a JFET?

Ans. In a JFET, drain current is controlled by controlling the reverse bias given to its gate (i.e. VGS).

Q.12. What is meant by drain characteristics of FETs?

Ans. The curve is drawn between drain current and drain-source voltage with gate-to-source voltage as the parameter is called the drain characteristic.

Q.13. What is meant by the transfer characteristic of FETs?

Ans. The curve drawn between drain current and gate-source voltage for a given value of drain-source voltage is called the transfer characteristic.

Q.14. What is pinch-off voltage in a JFET?

Ans. The value of the drain-source voltage at which the channel is pinched-off (i.e., all the free charges from the channel get removed) is called the pinch-off voltage.

Q.15. What is meant by saturation region?

Ans. The region of drain characteristic of a FET in which drain current remains fairly constant is called the saturation or pinch-off region.

Q.16. Why is the input impedance of the FET very high?

Ans. FET has a very high input impedance because its input circuit (gate-to-source) is reverse biased and the input gate current is very small (of the order of a few nano-amperes).

Q.17. What is the significant difference between the construction of an enhancement type MOSFET and a depletion type MOSFET?

Ans. In the depletion type MOSFET, a channel is physically constructed and a current between drain and source is due to the voltage applied across the drain-source terminals while in enhancement type construction, no channel is formed during its construction. Voltage is applied to the gate in case of enhancement type MOSFET, to develop a channel of charge carriers so that a current results when a voltage is applied across the drain-source terminals.

Q.18. Why E-MOSFET is called sometimes normally-off MOSFET?

Ans. E-MOSFET operates with large positive gate voltages only and does not conduct when gate-source voltage VGS = 0, so it is called normally-off MOSFET.

Q.19. Why MOSFETs are never connected or disconnected in the circuit when power is ON?

Ans. If a MOSFET is connected or disconnected in the circuit while the power is ON, transient voltages caused by inductive kickback and other effects may exceed VGS(max) and thus wipe out the MOSFET.

Q.20. Name the factors which make the JFET superior to BJT?

Ans. The high input impedance, low output impedance, and low noise level make JFET far superior to the BJT.

Q.21. Explain FET is a voltage-controlled device or current-controlled device.

Ans. The voltage applied between gate and source (VGS) controls the drain current ID. Therefore, FET is a voltage-controlled device. The transistor on the contrary is a current-controlled device because the output current IC is controlled by varying the base current IB. The name field effect is derived from the fact that the current flow in FET is controlled by an electric field set by an external voltage.

Q.22. Explain FET is a unipolar device or a bipolar device.

Ans. FET is a unipolar device. This means that the current flowing through it is only due to one type of charge particles, holes, or electrons. Transistor, on the other hand, is a bipolar device as holes and electrons both contribute to the flow of current. Just as there are NPN and PNP transistors, there are n-channel and p-channel field-effect transistors. One of the most important characteristics of the FET is its very high input impedance. Typically, it is in the range of 1 to several megaohms, which is much higher than the input resistance of a BJT. FETs are more temperature stable as compared to the BJT and it requires less space than that for a BJT. Therefore, FETs are preferred in integrated circuits.

Q.23. Write the Advantages of JFET Over BJT.

Ans. The FET enjoys several advantages over the conventional bipolar junction transistor (BJT). Some of them may be listed as under:

  1. It is a unipolar device so its operation depends on the flow of majority carriers only.
  2. It is relatively immune to radiation.
  3. FET has a very high input resistance (typically few megaohms).
  4. FET is less noisy.
  5. It does not exhibit any offset voltage at zero drain current hence it can be used as an excellent signal chopper.
  6. FET has a better thermal stability.

Q.24. Write Disadvantage of JFET.

Ans. The main disadvantage of the JFET is that it has a relatively small gain-bandwidth product as compared with that of a conventional transistor (i.e. BJT).

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