Electronic Devices and Circuits Questions and Answers – The Junction Field-Effect Transistor – 1

This set of Electronic Devices and Circuits Multiple Choice Questions & Answers (MCQs) focuses on “The Junction Field-Effect Transistor – 1”.

1. FET is a voltage controlled device.
a) True
b) False

Explanation: Field Effect Transistors are voltage controlled devices, by applying some voltage between the gate and source, the drain current can be controlled. In order to control the operation of FET the gate to drain voltage is varied to operate the FET in different regions of operation.

2. Which of the following statement is true about FET?
a) It has high output impedance
b) It has high input impedance
c) It has low input impedance
d) It does not offer any resistance

Explanation: Because of the Sio2 insulator, doped between drain and source at the top, the resistance offered by this is very high. The insulator will stop the flow of electron from one part to another which acts as an open circuit.

3. Comparing the size of BJT and FET, choose the correct statement?
a) BJT is larger than the FET
b) BJT is smaller than the FET
c) Both are of same size
d) Depends on application

Explanation: BJT usually are built with a thickness of up to 1cm whereas the FET uses a fabrication technique which makes its size in mm.

4. What is the main advantage of FET which makes it more useful in industrial applications?
a) Voltage controlled operation
b) Less cost
c) Small size
d) Semiconductor device

Explanation: Because of its small size, the IC chips can be made even smaller which reduces the wear and tear. The process technology used with process technology constant a which is the ratio of Width and Length, the FET is made more advantageous.

5. For a FET when will maximum current flows?
a) Vgs = 0V
b) Vgs = 0v and Vds >= |Vp|
c) VDS >= |Vp|
d) Vp = 0

Explanation: For a FET the current reaches maximum that is IDSS occurs when Vgs = 0V and VDS >= |Vp|
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6. What is the value of current when the gate to source voltage is less than the pinch off voltage?
a) 1A
b) 5A
c) 100A
d) 0

Explanation: When the gate to source voltage is less than pinch off, both of the junctions will be reverse biased and hence no current flows.

7. The Shockley equation is __________________
a) ID = (1 – Vgs/Vp)2
b) ID = IDSS (1 – Vgs/Vp)2
c) ID = IDSS (1 – Vgs/Vp)1
d) ID = IDSS (1 + Vgs/Vp)2

Explanation: By using the diode equation Shockley derived the equation between the drain current and voltage Vgs.

8. What is the value of drain current when Vgs=pinch off voltage?
a) 0A
b) 1A
c) 2A
d) Cannot be determined

Explanation: ID = IDSS (1-Vgs/Vp)2
If Vgs = Vp,then
ID = IDSS (1-1)=0.

9. To use FET as a voltage controlled resistor, in which region it should operate?
a) Ohmic region
b) cut off
c) Saturation
d) cut off and saturation

Explanation: By varying the gate to source voltage, Resistance can be varied as follows rd = ro/(1-Vgs/Vp)2

10. For an n-channel FET, What is the direction of current flow?
a) Source to drain
b) Drain to source
c) Gate to source
d) Gate to drain

Explanation: When a voltage greater than pinch off is applied, the current starts flowing from Drain to source.

11. For a p-channel FET, What is the direction of current flow?
a) Source to drain
b) Drain to source
c) Gate to source
d) Gate to drain

Explanation: When the voltage is lesser than pinch off, the current flows from Source to Drain.
The forward bias drain and gate is the reason for the flow of electron from Drain to source, as the conventional current flows opposite to the electron flow, the current will flow from Source to Drain.

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