This set of Electronic Devices and Circuits Multiple Choice Questions & Answers (MCQs) focuses on “The Pinch off Voltage Vp”.
1. What is pinch off voltage?
a) The minimum voltage required to turn on the FET
b) The maximum voltage a FET can withstand
c) Current amplification factor/voltage gain
d) The value of voltage at which the current gets pinched to zero
View Answer
Explanation: Once the voltage difference between the gate and source goes near to the pinch off voltage, the channel will get pinched off resulting in off state of the FET, which makes no conventional current flow.
2. A p-channel Ge JFET has max-half channel width 5µm and channel conductivity of 2/Ωcm, if Er = 2000cm2/Vsec, What is the value of pinch off voltage?
a) 8.21V
b) 82.1V
c) 88.21V
d) 5.2V
View Answer
Explanation: Vp=qNA a2/2E
= (5×10-4) (5×10-4)×2/(2×16×8.854×2000×10-14)
= 88.21V.
3. What will happen if gate voltage applied is positive to pinch off voltage?
a) Device burns
b) More current flows
c) Nothing happens
d) Current remains the same
View Answer
Explanation: When the voltage applied across the gate terminal becomes more positive, all of the current will start flowing from drain to gate terminal. This results in breaking of insulator layer, resulting in device destruction.
4. How does a FET behave when the v-I characteristics are to the left of pinch off for an n channel FET?
a) Voltage controlled resistor
b) Amplifier
c) Switch
d) Diode
View Answer
Explanation: When the voltage Vgs is less than pinch off voltage, there will be no current flow, resulting in Ohmic region, on controlling the Vgs and VDs values, the FET acts as Voltage controlled resistor.
5. What is the relation between the drain current and source current once the voltage crosses pinch off?
a) ID = IS
b) ID = IS + 1.5
c) ID = 1/IS
d) IS – 2ID = 0
View Answer
Explanation: For a FET, once the voltage exceeds the pinch off voltage, the electrons will start flowing from source to drain or vice versa, since no current flows through the gate terminal, the current transfer takes place only between the drain and source. Hence ID = IS.
6. If a FET has a pinch off voltage =-1V and ID=1mA, If Vgs=0V, What is the value of IDSS?
a) 1A
b) 1mA
c) 0A
d) 100mA
View Answer
Explanation: ID=IDSS (1-Vgs/Vp)2
Vgs=0V => : ID=IDSS=1mA.
7. For what value of Vgs, the drain current will be 1/4th of its maximum current?
a) 0
b) 1
c) Vp
d) Vp/2
View Answer
Explanation: ID=IDSS (1-Vgs/Vp)2
When ID=IDSS/4 => (1-Vgs/Vp)2 = 1/4
=>(1-Vgs/Vp)2=1/2
=>Vgs=Vp/2.
8. For a n-channel FET, what is the condition of Vgs for which the current becomes zero?
a) 0
b) 100V
c) Vp
d) Infinite
View Answer
Explanation: ID=IDSS (1-Vgs/Vp)2
When ID=0, (1-Vgs/Vp)2=0
1=Vgs/Vp => Vgs=Vp.
9. For a FET having IDSS=2mA Vgs=2V and Vp=-1V, What is the value of source current?
a) 9mA
b) 18mA
c) 3mA
d) 1mA
View Answer
Explanation: ID=IDSS (1-Vgs/Vp)2
ID=IS=2×(1+2)(1+2)=32×2=18mA
IS=18mA.
10. Find the current through gate if the FET was given with gate to source voltage =10V and drain to source voltage =20V, the pinch off voltage was -2V and ID=2mA.
a) 10mA
b) 20mA
c) 0mA
d) 2mA
View Answer
Explanation: We know that for a FET, the gate terminal is heavily doped with an insulator, resulting in infinite resistance. Hence no current flows through the gate terminal. It has no relationship with the other current parameters. It is constant for all FETs.
Sanfoundry Global Education & Learning Series – Electronic Devices and Circuits.
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