Field Effect Transistors Questions and Answers

Q: Why is a junction field effect transistor's gate leakage worse at high temperature?

The gate is a reverse-biased junction, so its leakage is a saturation current and roughly doubles every ten degrees - nanoamps at room temperature but microamps in a hot enclosure. There is no oxide in a junction device, which is the difference from a MOSFET. Knowing which structure a leakage path belongs to predicts how it will behave with temperature, and here it behaves like every other reverse-biased junction.

Q: A device's gate leaks \( 2 \ \mathrm{nA} \) at \( 25 \ \mathrm{^\circ C} \). At \( 85 \ \mathrm{^\circ C} \) it leaks about

Six doublings over sixty degrees:\[ 2 \times 2^{6} = 128 \ \mathrm{nA} \]giving about 128 nanoamps. Across a ten megohm gate resistor that is over a volt of unwanted bias, which would shift the operating point substantially - the reason high-impedance gate circuits are limited in hot equipment. An exponential growth turns a negligible current into a design constraint over quite a modest temperature range.

Q: What limits how large a gate bias resistor can be made?

Gate leakage through a very large resistor develops a voltage that shifts the operating point, and the shift grows exponentially with temperature. Capacitance, noise and settling time are all real secondary considerations. That the limit is set by a current the device is supposed not to draw is a good illustration of why negligible quantities have to be checked rather than assumed at the extremes.

Q: Why does a MOSFET have no such limit on its gate resistor?

With an insulator rather than a junction, the gate current is smaller than any leakage path on the circuit board, so the resistor may be as large as desired - gigohms if wanted. The threshold does not compensate anything, the capacitance stores charge rather than regulating voltage, and the channel supplies nothing to the gate. The same structural difference that makes MOSFETs static-sensitive also removes this constraint.

Q: What is the body effect in a MOSFET?

When the source is not tied to the substrate, the reverse bias between them widens the depletion region and raises the threshold voltage, which matters in stacked switches and in integrated circuits where a common substrate is unavoidable. The other options describe other effects entirely. An unwanted dependence on a terminal often ignored in the simple model is worth knowing about, since it is what makes series MOSFET switches behave worse than a single one suggests.

Q: Consider the following statements about field effect transistor temperature behaviour. 1. Drain current falls as the device warms, at a fixed gate voltage 2. Gate leakage in a junction device doubles every ten degrees 3. A MOSFET's on-resistance falls as it warms Which of the statements given above is or are correct?

The first two are correct and the third is the reverse: on-resistance rises with temperature, which is precisely what makes current sharing between paralleled devices self-correcting. Getting that direction right matters, because it is the whole reason MOSFETs parallel without ballast resistors while bipolar devices do not.

Q: What is channel-length modulation?

Channel-length modulation is the depletion region at the drain end growing with drain voltage, shortening the effective channel and raising the current slightly - which gives the output characteristics their finite slope, exactly as the Early effect does in a bipolar device. The other options describe the ohmic region, the body effect and gate capacitance. Recognising an analogous mechanism across the two families saves learning each family's output resistance from scratch.

Q: The MOSFET parameter describing the gate voltage at which an inversion layer forms is the ______ voltage.

This is the threshold voltage. Pinch-off belongs to junction devices and has the opposite sense, breakdown is a destructive limit, and the body effect describes how the threshold shifts with substrate bias rather than being a voltage itself. Since the threshold is what separates the on and off states of every logic gate ever made, it is the most consequential single number in the family.

Q: Why did MOSFETs displace bipolar transistors in digital logic?

A complementary pair has one device off in each stable state, so a gate consumes power only while it is switching - which is what allowed circuit density to grow by orders of magnitude without the heat becoming impossible. Bipolar logic families draw current continuously through their resistors and their conducting transistors, which is why the fastest of them were also the hottest. Raw switching speed favoured bipolar for years, and supply tolerance and fabrication cost both cut in more than one direction. The decisive property was static power, and it is the reason essentially every processor made today is built from these devices.

81.

Why is a junction field effect transistor's gate leakage worse at high temperature?

A.It is a reverse-biased junction's saturation current
B.The oxide becomes more conductive as it warms
C.The channel injects carriers into the gate region
D.The pinch-off voltage falls and forward biases the gate

Answer: A

The gate is a reverse-biased junction, so its leakage is a saturation current and roughly doubles every ten degrees - nanoamps at room temperature but microamps in a hot enclosure. There is no oxide in a junction device, which is the difference from a MOSFET. Knowing which structure a leakage path belongs to predicts how it will behave with temperature, and here it behaves like every other reverse-biased junction.

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82.

A device's gate leaks \( 2 \ \mathrm{nA} \) at \( 25 \ \mathrm{^\circ C} \). At \( 85 \ \mathrm{^\circ C} \) it leaks about

A.2 microamps
B.128 nanoamps
C.12 nanoamps
D.4 nanoamps

Answer: B

Six doublings over sixty degrees:

\[ 2 \times 2^{6} = 128 \ \mathrm{nA} \]

giving about 128 nanoamps. Across a ten megohm gate resistor that is over a volt of unwanted bias, which would shift the operating point substantially - the reason high-impedance gate circuits are limited in hot equipment. An exponential growth turns a negligible current into a design constraint over quite a modest temperature range.

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83.

What limits how large a gate bias resistor can be made?

A.The noise the resistor contributes to the signal
B.The time the circuit takes to settle at switch-on
C.The voltage the gate leakage develops across it
D.The input capacitance it forms with the gate

Answer: C

Gate leakage through a very large resistor develops a voltage that shifts the operating point, and the shift grows exponentially with temperature. Capacitance, noise and settling time are all real secondary considerations. That the limit is set by a current the device is supposed not to draw is a good illustration of why negligible quantities have to be checked rather than assumed at the extremes.

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84.

Why does a MOSFET have no such limit on its gate resistor?

A.Its threshold voltage compensates for any leakage drift
B.Its oxide capacitance holds the gate voltage steady
C.Its channel supplies the leakage current internally
D.Its gate current is immeasurably small at any temperature

Answer: D

With an insulator rather than a junction, the gate current is smaller than any leakage path on the circuit board, so the resistor may be as large as desired - gigohms if wanted. The threshold does not compensate anything, the capacitance stores charge rather than regulating voltage, and the channel supplies nothing to the gate. The same structural difference that makes MOSFETs static-sensitive also removes this constraint.

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85.

What is the body effect in a MOSFET?

A.The threshold voltage rising with source-to-substrate bias
B.The channel resistance falling as the substrate warms
C.The gate capacitance changing with drain voltage
D.The drain current saturating beyond the pinch-off point

Answer: A

When the source is not tied to the substrate, the reverse bias between them widens the depletion region and raises the threshold voltage, which matters in stacked switches and in integrated circuits where a common substrate is unavoidable. The other options describe other effects entirely. An unwanted dependence on a terminal often ignored in the simple model is worth knowing about, since it is what makes series MOSFET switches behave worse than a single one suggests.

Discuss in Forum

86.

Consider the following statements about field effect transistor temperature behaviour.

1. Drain current falls as the device warms, at a fixed gate voltage

2. Gate leakage in a junction device doubles every ten degrees

3. A MOSFET's on-resistance falls as it warms

Which of the statements given above is or are correct?

A.1, 2 and 3
B.1 and 2 only
C.1 and 3 only
D.2 and 3 only

Answer: B

The first two are correct and the third is the reverse: on-resistance rises with temperature, which is precisely what makes current sharing between paralleled devices self-correcting. Getting that direction right matters, because it is the whole reason MOSFETs parallel without ballast resistors while bipolar devices do not.

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87.

Consider the following pairs of term and its device.

Pinch-off voltage - Junction field effect transistor

Body effect - MOSFET

On-resistance - MOSFET switch

Early voltage - Junction field effect transistor

How many of the pairs given above are correctly matched?

A.Only two
B.All four
C.Only three
D.Only one

Answer: C

The first three are correctly paired and the fourth is not, so three match. The Early voltage is a bipolar parameter, describing the base-narrowing that tilts its output characteristics; the field effect equivalent is channel-length modulation, which produces a similar tilt by a different mechanism. Analogous effects with different names in the two families are worth pairing up deliberately.

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88.

What is channel-length modulation?

A.The channel resistance varying with the gate voltage applied
B.The threshold voltage shifting with substrate bias
C.The gate capacitance changing as the channel forms
D.The effective channel shortening as drain voltage rises

Answer: D

Channel-length modulation is the depletion region at the drain end growing with drain voltage, shortening the effective channel and raising the current slightly - which gives the output characteristics their finite slope, exactly as the Early effect does in a bipolar device. The other options describe the ohmic region, the body effect and gate capacitance. Recognising an analogous mechanism across the two families saves learning each family's output resistance from scratch.

Discuss in Forum

89.

The MOSFET parameter describing the gate voltage at which an inversion layer forms is the ______ voltage.

A.threshold
B.pinch-off
C.breakdown
D.body

Answer: A

This is the threshold voltage. Pinch-off belongs to junction devices and has the opposite sense, breakdown is a destructive limit, and the body effect describes how the threshold shifts with substrate bias rather than being a voltage itself. Since the threshold is what separates the on and off states of every logic gate ever made, it is the most consequential single number in the family.

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90.

Why did MOSFETs displace bipolar transistors in digital logic?

A.They are cheaper to fabricate than bipolar devices of the same size
B.They draw no steady current in either logic state
C.They switch faster than bipolar devices at every current
D.They tolerate a wider supply voltage range than bipolar logic

Answer: B

A complementary pair has one device off in each stable state, so a gate consumes power only while it is switching - which is what allowed circuit density to grow by orders of magnitude without the heat becoming impossible. Bipolar logic families draw current continuously through their resistors and their conducting transistors, which is why the fastest of them were also the hottest. Raw switching speed favoured bipolar for years, and supply tolerance and fabrication cost both cut in more than one direction. The decisive property was static power, and it is the reason essentially every processor made today is built from these devices.

Discuss in Forum

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