Important short Questions and Answers: Field Effect Transistors(FET)

FIELD EFFECT TRANSISTORS

1.  Define FET?

A field effect transistor (FET) is a three terminal semiconductor device which can be used as an amplifier or switch. The three terminals are Drain (D), Source (S), and Gate (G).

2.  Define channel?

It is a bar like structure which determines the type of FET. Different types of N channel are FET and P channel FET.

3.  Draw the transfer characteristic for n-channel depletion type MOSFET?

Transfer characteristic:

4.  What do you understand by pinch off voltage and out of voltage?

As the reverse bias is further increased, the effective width of the channel decreases, the depletion region or the space charge region widens, reaching further into the channel and restricting the passage of electrons from the source to drain. Finally at a certain gate to source voltage V_GS = V_P.

5.  Why FET is called as “voltage operated device”?

In FET the output current, I _D is controlled by the voltage applied between gate and source (V_GS). Therefore FET is said to be voltage controlled device.

6.     Which MOSFET is called as Normally ON MOSFET and NORMALLY OFF MOSFET? Why?

This is just one type of MOSFET, called 'normally -off' because it is only the application of a positive gate voltage above the critical voltage which allows it to pass current between source and drain.Another type of MOSFET is the 'normally-on', which has a conductive channel of less heavily doped n-type material between the source and drain electrodes.

7.  Compare BJT and MOSFET

BJT

1        CB,CE,CC configurations

2        Less input resistance compared to JFET

3        Input output relation is linear

4        Gain bandwidth product is high

5        Thermal noise is more

6        Thermal stability is less

7        Bigger size than MOSFET

MOSFET

1 CS,CG,CD configurations

2 Very high input resistance

3 Input output relation is non-linear

4 Gain bandwidth product is low

5 Thermal noise is less

6 Thermal stability is more

7 Smaller size

8. Sketch the ohmic region in drain characteristics of JFET? Drain characteristics:

9. Define Amplification factor in JFET?

10.What are the advantages of FET over BJT?

·        In FET input resistance is high compared to BJT Construction is smaller than BJT.

·     Less sensitive to changes in applied voltage

·     Thermal stability is more and Thermal noise is much lower Thermal runaway does not exist in JFET

11. Comparison between JFET and BJT.

BJT

1       Low input impedance

2       High output impedance

3       Bipolar device

4       Noise is more

5       Cheaper

6       Gain is more

7       Current controlled device

JFET

1       High input impedance

2       Low output impedance

3       Unipolar device

4       Less noise

5       Costlier

6       Less gain

7       Voltage controlled device

12.                        What are the important features of FET?

·        The parameters of FET are 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 FET is more temperature stable.

·        FET has very high input impedance. Hence FET is preferred in amplifiers. It is less noisy.

·        Requires less space.

·        It exhibits no offset voltage at zero drain current.

13.                        Comparison between JFET and MOSFET.

JFET

1        Operated in depletion mode

2        High input impedance(>10MΩ)

3        Gate is not insulated from channel

4        Channel exists permanently

5        Difficult to fabricate than MOSFET

6        Drain resistance is high

7        Gate is formed as a diode

MOSFET

1 Operated in depletion mode and enhancement mode

2 Very  High input impedance(>10000MΩ)

3 Gate is insulated from channel by a layer of  Sio2

4 Channel exists permanently in depletion type but not in enhancement type.

5 Easier to fabricate

6 Drain resistance is less

7 Gate is formed as a capacitor

14. Explain the biasing of JFET?

Input is always reverse biased and output is forward biased. (Note: In transistor input is forward biased and output is reverse biased).

15. Define Drain resistance.

It is the ratio of change in Drain – source voltage (∆V_DS) to the change in Drain current (∆I_D) at constant gate source voltage (V_GS).

16. Define Tran’s conductance?

It is the ratio of change in drain current (∆I_D) to the change in Gate – Source Voltage (∆V_GS) at constant Drain – Source voltage (V_DS).

17. Write the advantages of JFET?

Input impedance of JFET is very high.

This allows high degree of Isolation between the Input and Output circuit. Current carriers are not crossing the junction hence noise is reduced drastically

18.                        List the JFET parameters?

A.C drain resistance (r_d) Trans conductance (g_m) Amplification factor (µ)

19.                        Explain the depletion node of operation in MOSFET?

When the gate is at negative bias, the thickness of the depletion layer further increases owing to the further increase of the induced positive charge. Thus the drain current decreases, as the gate is made more negative. This is called depletion mode of operation.

20. Explain the term Drain in FET?

The drain is the terminal through which the current leaves the bar. Convention current entering the bar is designated as I_D.

21. Define the term Gate in FET?

The gate consists of either P⁺ or N⁺ impurity regions, heavily doped and diffused to the bar. This region is always reverse biased and in fact, controls the drain current I_D.

22.                        Write the relative disadvantages of an FET over that of a BJT?

1. The gain bandwidth product in case of a FET is low as compared with a BJT.

2.    The category, called MOSFET, is extremely sensitive to handling therefore additional precautions have to be considered while handling.

23.                        Mention the methods used for biasing circuits in FET?

Self-bias and Potential divider bias.

24.                        Explain the term MOSFET?

In the insulated gate FET, conductivity is controlled by the potential on the insulated metal plate lying on the top of the channel the insulated gate field effect transistor is often called metallic oxide semiconductor FET.

25.                        Mention the three regions that are present in the drain source characteristics of JFET?

·        Saturation region

·        Break down region

·        Ohmic region

26.                        List the characteristics of JFET.

·        Drain characteristics

·        Transfer characteristics.

28. Why MOSFET is called IGFET?

MOSFET is constructed with gate terminal insulated from the channel. So it is also called as insulated gate FET or IGFET.

29. Comparison between JFET and MOSFET

JFET

Gate is not insulated from channel

There are two types – N-channel and P-Channel

Cannot be operated in depletion and enhancement modes

There is a continuous channel

MOSFET

Gate is insulated from channel by a thin layer of SiO2

Four types - P-channel enhancement, P-channel depletion, N-channel enhancement, N-channel Depletion

Can be operated in depletion and enhancement Modes

There is a continuous channel only in depletion type, but not in enhancement type

30. Compare P channel and N channel JFET.

N- channel JFET

1        Current carriers are electrons

2        Mobility of electrons is almost twice that of holes in p-channel

3        Low input noise  

4        Transconductance is very high

P-channel JFET

1 Current carriers are holes

2 Mobility of holes is poor

3 Large input noise

4 Transconductance is very small