Field Effect Transistors: Two Marks Important Questions and Answers

Electron Devices

Chapter 5: Field Effect Transistors

Two Marks Questions with Answers

1. Why is FET called a unipolar device?

We know that in BJT, the current is carried by both electrons and holes, and hence the name "bipolar" junction transistor. However in FET, current is carried by only one type of charge particles, either electrons or holes. Hence FET is called unipolar device.

2. How can a FET be used as a voltage controlled resistor?

As shown in Fig. 5.1.1, in BJT the output current, IC is controlled by the base current IB. Hence BJT is a current controlled device. On the other hand, in FET, the voltage applied between gate and source (VGS) controls the drain current ID. Therefore, FET is a voltage controlled device.

3. What are the advantages of FETS?

• Like BJT, the parameters of FET are also temperature dependent. In FET, as temperature increases drain resistance also increases, reducing the drain current. Thus unlike BJT, thermal runaway does not occur with FET. Thus we can say that FET is more temperature stable as compared to the BJT.

• FET has very high input impedance. Typically, it is in the range of one to several megaohms. Because FETs have higher input impedance than BJT they are preferred in amplifiers where high input impedance is required.

• FETs require less space than that for BJTs, hence they are preferred in integrated circuits.

4. Define Amplification factor in JFET.

The amplification factor, denoted by μ is defined as,

Amplification factor μ = 

 μ = rd ×gm

5. Mention the operating modes of MOSFET.

1. Depletion mode 2. Enhancement mode.

6. Calculate the amplification factor, μ of FET, if r_d = 4 kΩ and g_m = 4 mA/V.

  μ = g_m r_d = 4 × 10^‒3 × 4 × 10³ = 16

7. Give any two differences between E‒MOSFET and D‒MOSFET.

8. Compare p‒channel and n‒channel JFET.

9. Mention the three regions that are present in the drain‒source characteristics of JFET.

• Self pinch‒off at no bias (VGS = 0): At VGS = 0, in response to a small applied voltage VDS, the n‒type bar acts as a simple semiconductor resistor, and the current ID increases linearly with VDS.

• Breakdown region: We can observe from Fig. 5.5.1 that if we increase value of VDS beyond pinch‒off voltage, Vp, the drain current ID remains constant, up to certain value of VDS. 

• Ohmic and saturation regions:

It is seen that the drain characteristics of JFET is divided into two regions ohmic region and saturation region. In the ohmic region, the drain current ID varies with VDS and the JFET is said to behave as voltage variable resistance.

• Cut‒off: As we know, for an n‒channel JFET, the more negative VGS causes drain current to reduce and pinch‒off voltage to reach at a lower drain current. 

10. What is pinch‒off voltage in FET ?

At some value of VDS, drain current ID cannot be increased further, due to reduction in channel width. Any further increase in VDS does not increase the drain current ID. ID approaches the constant saturation value. The voltage VDS at which the current ID reaches to its constant saturation level is called 'Pinch‒Off Voltage', VP.

11. List the characteristics of JFET.

The important characteristics parameters of JFET are as follows:

• Transconductance (gm)

• Input resistance and capacitance

• Drain to source resistance (rd)

• Amplification factor (μ)

• Power Dissipation (PD).

12. Distinguish between BJT and FET.

13. Give the drain current equation of JFET

In a transfer characteristics we have seen that the relationship between the drain current ID and gate to source voltage VGS is non‒linear. This relationship is defined by Shockley's equation

14. List the JFET parameters.

The important characteristics parameters of JFET are as follows:

• Transconductance (gm)

• Input resistance and capacitance

• Drain to source resistance (rd)

• Amplification factor (μ)

• Power Dissipation (PD).

15. List the special features of FET.

• Voltage Controlled Device: As shown in Fig. 5.1.1, in BJT the output current, IC is controlled by the base current IB. Hence BJT is a current controlled device. On the other hand, in FET, the voltage applied between gate and source (VGS) controls the drain current ID. Therefore, FET is a voltage controlled device.

• The name field effect is derived from the fact that the output current flow is controlled by an electric field set up in the device by an externally applied voltage between gate and source terminals.

• Unipolar Device: We know that in BJT, the current is carried by both electrons and holes, and hence the name "bipolar" junction transistor. However in FET, current is carried by only one type of charge particles, either electrons or holes. Hence FET is called unipolar device.

• Like BJT, the parameters of FET are also temperature dependent. In FET, as temperature increases drain resistance also increases, reducing the drain current. Thus unlike BJT, thermal runaway does not occur with FET. Thus we can say that FET is more temperature stable as compared to the BJT.

• FET has very high input impedance. Typically, it is in the range of one to several megaohms. Because FETs have higher input impedance than BJT they are preferred in amplifiers where high input impedance is required.

• FETs require less space than that for BJTs, hence they are preferred in integrated circuits.

16. In which region JFET acts as a resistor and why?

In the ohmic region JFET acts as a resistor as the drain current I_D varies with V_DS.

17. Differentiate between JFET and BJT.

18. Give the current voltage relationship of the D‒MOSFET and E‒MOSFET.

For D‒MOSFET,

For E‒MOSFET,

I_D = K[V_GS ‒ V_T]²

19. What is pinch off voltage ?

At some value of VDS, drain current ID cannot be increased further, due to reduction in channel width. Any further increase in VDS does not increase the drain current ID. ID approaches the constant saturation value. The voltage VDS at which the current ID reaches to its constant saturation level is called 'Pinch‒Off Voltage', VP.

20. State the application and difference between BJT and FET.

21. What is a MOS capacitor ?

A MOS capacitor is a structure consisting of a Metal gate, Oxide layer (insulator), and Semiconductor substrate. The capacitance of the MOS structure varies with the applied gate voltage due to the formation of accumulation, depletion, or inversion layers in the semiconductor.

22. What is nMOS?

In nMOS technology, electrons are the charge carriers. The transistor conducts when a positive voltage is applied to the gate terminal, forming a conductive n‒channel between the source and drain.

23. State two advantages of nMOS.

1. Faster operation because electron mobility is high.

2. Requires smaller chip area for the same current drive.

24. State two disadvantages of nMOS.

1. High static power consumption during logic '1'.

2. Poor noise margin compared to CMOS.

25. What is pMOS ?

In PMOS technology, conduction occurs through holes. The transistor conducts when a negative voltage is applied to the gate terminal, forming a p‒channel.

26. State two advantages of pMOS.

1. Simple to design and fabricate.

2. Lower leakage current compared to nMOS.

27. State two disadvantages of pMOS.

1. Slower operation due to lower hole mobility.

2. Requires larger area and higher (negative) voltage.

28. What is CMOS technology?

CMOS uses both nMOS and pMOS transistors on the same chip. When one is ON, the other is OFF, resulting in very low power consumption.

29. State two advantages of CMOS.

1. Very low power consumption.

2. High noise immunity and stable operation.

30. State two disadvantages of CMOS.

1. Fabrication is more complex.

2. May suffer from latch‒up if not designed properly.