Field Effect Transistors Questions and Answers

Q: Why is a junction field effect transistor called a depletion-mode device?

With zero gate voltage the channel already conducts, and reverse-biasing the gate widens the depletion region until the channel is pinched off. Forward biasing the gate would conduct through the junction and destroy the control. The doping is normal, and the drain current actually saturates rather than falling. Whether a device conducts or blocks at zero gate voltage is the single distinction between depletion and enhancement modes, and it decides the whole bias arrangement.

Q: What is pinch-off in a junction field effect transistor?

Pinch-off is the point at which the depletion regions meet at the drain end and the channel closes there, after which the current saturates rather than stopping. That the current continues to flow past pinch-off surprises people, but the carriers are swept across the narrow depleted region by the field. Gate breakdown, current rating and minimum resistance are separate matters entirely.

Q: What happens to the drain current beyond pinch-off?

Past pinch-off the current levels off, controlled by the gate voltage rather than by the drain, which is exactly what makes the device useful as an amplifier and as a current source. Below pinch-off it does rise with drain voltage, in the ohmic region where the device behaves as a resistor. Two regions with quite different behaviour, and knowing which one a circuit operates in settles what the device is doing.

Q: What is the ohmic region of a field effect transistor used for?

Below pinch-off the channel behaves as a resistance the gate voltage controls, which is the basis of voltage-controlled attenuators, automatic gain control and analogue switches. Amplification and constant-current behaviour both need the saturation region. Switching speed depends on capacitances rather than region. The same device serving as an amplifier in one region and a resistor in another is unusual enough to be worth remembering explicitly.

Q: Why must a junction field effect transistor's gate never be forward biased?

Forward bias makes the gate junction conduct, so the input impedance collapses and the device no longer behaves as a voltage-controlled one. The channel is not damaged, breakdown is unaffected, and the transconductance would actually be high. The device's defining virtue is its non-conducting gate, so any condition that makes the gate conduct removes the reason for choosing it. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision. The shared properties explain why these belong to one family, and the differences explain why more than one exists. Framing the question as which quantity the change acts upon makes the answer follow without calculation. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted. Checking the units of each quantity catches a wrong formula before the arithmetic begins. Checking the units of each quantity catches a wrong formula before the arithmetic begins. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Q: What separates the gate from the channel in a MOSFET?

A MOSFET's gate sits on an insulating oxide, so no gate current flows at all, in either direction and at any voltage - unlike a junction field effect transistor, whose gate is a reverse-biased junction and does leak a little. That insulator is why the input resistance is measured in gigohms, and also why the device is destroyed by static discharge. One structural difference accounts for both the family's greatest strength and its greatest fragility.

Q: Why is a MOSFET vulnerable to static discharge?

The oxide is only tens of nanometres thick, so a few tens of volts across it punctures it irreversibly, and since the gate draws no current there is nothing to bleed the charge away either. The third option states a contributing fact rather than the mechanism, and the threshold and capacitance are beside the point. A structure that provides perfect isolation also provides no path for accumulated charge, which is the whole handling problem.

31.

List - I (Structure/ Characteristics)	List - II (Reasons)
a. n-channel JFET is better than p-channel JFET	i. Reverse bias increases along the channel
b. Channel is wedge shaped	ii. High electric field near the drain and directed towards source
c. Channel is not completely closed at pinch-off	iii. Low leakage current at the gate terminal
d. Input impedance is high	iv. Better frequency performance since \(\mu_{n} \gg \mu_{p}\)

A.a-iv, b-i, c-ii, d-iii
B.a-iv, b-ii, c-iii, d-i
C.a-iii, b-i, c-ii, d-iv
D.a-ii, b-iv, c-iii, d-i

Exam Year(s)

UGC NET - 2014,

Answer: A

The correct matching is a-iv, b-i, c-ii, d-iii. An n-channel JFET is preferred because electron mobility is much higher than hole mobility, which gives better frequency performance (iv). The channel is wedge shaped because the reverse bias between gate and channel increases towards the drain (i). The channel is not completely closed at pinch-off, because a high electric field near the drain, directed toward the source, sweeps carriers through (ii). The input impedance is high because the gate junction is reverse biased, so the leakage current is low (iii).

The remaining options pair these properties with the wrong reasons.

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32.

In an FET, following characteristics are given :

1. JFET is called as variable current resistor.
2. JFET is called Voltage variable resistor.
3. \(g_{d} = g_{d0}\left[1 - \sqrt{\frac{V_{GS}}{V_{P}}}\right]\), where, \(g_{d0}\) is value of drain to source conductance for zero bias.
4. \(g_{d} = g_{d0}\left[1 + \left(\frac{V_{GS}}{V_{P}}\right)^{2}\right]\)
Which one of the following is true ?

A.1 and 4
B.3 and 4
C.1 and 3
D.2 and 3

Exam Year(s)

UGC NET - 2014,

Answer: D

Statements 2 and 3 are true. In the ohmic region a JFET behaves as a resistor whose value is set by the gate voltage, so it is called a voltage variable resistor, and the drain conductance follows \[ g_{d} = g_{d0}\left[1 - \sqrt{V_{GS}/V_{P}}\right] \] where \(g_{d0}\) is the conductance at zero bias.

Statement 1 is false because the control is by voltage, not current. Statement 4 is false: the channel conductance falls as the gate bias approaches pinch-off, so the expression with a plus sign and a square does not describe it. That leaves the pair 2 and 3.

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33.

What controls the current in a field effect transistor?

A.The voltage on the gate
B.The current supplied to the gate terminal
C.The voltage between drain and source alone
D.The doping level chosen for the channel

Answer: A

A field effect device is controlled by gate voltage, drawing essentially no gate current at all, which is the fundamental difference from a bipolar transistor. Drain-source voltage matters only in deciding which region the device is in, and the doping is fixed at manufacture. That control is by voltage rather than by current explains the enormous input impedance and most of the family's other properties. Most errors on this topic come from carrying a result across from a neighbouring scheme where it does not hold. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour. Framing the question as which quantity the change acts upon makes the answer follow without calculation. Checking the units of each quantity catches a wrong formula before the arithmetic begins. The shared properties explain why these belong to one family, and the differences explain why more than one exists. Most errors here come from carrying a result across from a neighbouring case where it does not hold. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted. Framing the question as which quantity the change acts upon makes the answer follow without calculation. Most errors here come from carrying a result across from a neighbouring case where it does not hold. Framing the question as which quantity the change acts upon makes the answer follow without calculation. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb. Most errors here come from carrying a result across from a neighbouring case where it does not hold. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb. Most errors here come from carrying a result across from a neighbouring case where it does not hold. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb. Most errors here come from carrying a result across from a neighbouring case where it does not hold. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

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34.

Why is a junction field effect transistor called a depletion-mode device?

A.Its drain current falls as the drain voltage rises
B.Its channel conducts until the gate depletes it
C.Its gate must be forward biased to create a channel
D.Its channel is depleted of dopant during manufacture

Answer: B

With zero gate voltage the channel already conducts, and reverse-biasing the gate widens the depletion region until the channel is pinched off. Forward biasing the gate would conduct through the junction and destroy the control. The doping is normal, and the drain current actually saturates rather than falling. Whether a device conducts or blocks at zero gate voltage is the single distinction between depletion and enhancement modes, and it decides the whole bias arrangement.

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35.

What is pinch-off in a junction field effect transistor?

A.The drain current reaching its rated maximum value
B.The channel resistance falling to its lowest value
C.The channel closing at the drain end
D.The gate junction breaking down under reverse bias

Answer: C

Pinch-off is the point at which the depletion regions meet at the drain end and the channel closes there, after which the current saturates rather than stopping. That the current continues to flow past pinch-off surprises people, but the carriers are swept across the narrow depleted region by the field. Gate breakdown, current rating and minimum resistance are separate matters entirely.

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36.

What happens to the drain current beyond pinch-off?

A.It falls steadily as the drain voltage is increased further
B.It rises linearly with the drain voltage applied to the device
C.It becomes proportional to the square of the drain voltage
D.It saturates and is nearly independent of drain voltage

Answer: D

Past pinch-off the current levels off, controlled by the gate voltage rather than by the drain, which is exactly what makes the device useful as an amplifier and as a current source. Below pinch-off it does rise with drain voltage, in the ohmic region where the device behaves as a resistor. Two regions with quite different behaviour, and knowing which one a circuit operates in settles what the device is doing.

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37.

What is the ohmic region of a field effect transistor used for?

A.Making a voltage-controlled resistor
B.Amplifying a small signal at high gain
C.Providing a constant current independent of load
D.Switching at the highest possible speed

Answer: A

Below pinch-off the channel behaves as a resistance the gate voltage controls, which is the basis of voltage-controlled attenuators, automatic gain control and analogue switches. Amplification and constant-current behaviour both need the saturation region. Switching speed depends on capacitances rather than region. The same device serving as an amplifier in one region and a resistor in another is unusual enough to be worth remembering explicitly.

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38.

Why must a junction field effect transistor's gate never be forward biased?

A.The transconductance would fall to zero immediately
B.Gate current would flow and control would be lost
C.The channel would be pinched off permanently
D.The drain-source breakdown voltage would collapse

Answer: B

Forward bias makes the gate junction conduct, so the input impedance collapses and the device no longer behaves as a voltage-controlled one. The channel is not damaged, breakdown is unaffected, and the transconductance would actually be high. The device's defining virtue is its non-conducting gate, so any condition that makes the gate conduct removes the reason for choosing it. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision. The shared properties explain why these belong to one family, and the differences explain why more than one exists. Framing the question as which quantity the change acts upon makes the answer follow without calculation. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted. Checking the units of each quantity catches a wrong formula before the arithmetic begins. Checking the units of each quantity catches a wrong formula before the arithmetic begins. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

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39.

What separates the gate from the channel in a MOSFET?

A.A lightly doped drift region within the channel
B.A depletion region maintained by the drain voltage
C.A thin insulating oxide layer
D.A reverse-biased junction of the opposite type

Answer: C

A MOSFET's gate sits on an insulating oxide, so no gate current flows at all, in either direction and at any voltage - unlike a junction field effect transistor, whose gate is a reverse-biased junction and does leak a little. That insulator is why the input resistance is measured in gigohms, and also why the device is destroyed by static discharge. One structural difference accounts for both the family's greatest strength and its greatest fragility.

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40.

Why is a MOSFET vulnerable to static discharge?

A.Its channel carries no current until a threshold is reached
B.Its gate draws no current and so cannot dissipate charge
C.Its input capacitance is too small to absorb any energy
D.Its gate oxide is thin enough to be punctured

Answer: D

The oxide is only tens of nanometres thick, so a few tens of volts across it punctures it irreversibly, and since the gate draws no current there is nothing to bleed the charge away either. The third option states a contributing fact rather than the mechanism, and the threshold and capacitance are beside the point. A structure that provides perfect isolation also provides no path for accumulated charge, which is the whole handling problem.

Discuss in Forum

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