Field Effect Transistors Questions and Answers

Q: Why does a junction field effect transistor have a negative temperature coefficient of drain current?

Falling mobility reduces the current at a given gate voltage, so a hot device carries less and thermal runaway does not occur - the opposite of a bipolar transistor's behaviour. Pinch-off voltage moves slightly the other way, gate leakage is negligible, and the channel geometry is fixed. That the dominant temperature effect opposes rather than reinforces the current is why these devices are so much easier to use in parallel.

Q: At what operating point is a junction field effect transistor's drain current nearly temperature independent?

The two temperature effects act oppositely, so at one particular gate voltage they cancel and the current barely drifts - a zero-temperature-coefficient point manufacturers sometimes specify. At the saturation current the mobility effect dominates. This is the same idea as the 5.6 volt Zener: where two mechanisms with opposite signs cross, a parameter passes through zero and is worth exploiting.

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.

111.

Why does a junction field effect transistor have a negative temperature coefficient of drain current?

A.The gate leakage current grows and reduces the bias
B.The channel becomes narrower as the device heats
C.Carrier mobility falls as the channel warms
D.The pinch-off voltage rises with temperature

Answer: C

Falling mobility reduces the current at a given gate voltage, so a hot device carries less and thermal runaway does not occur - the opposite of a bipolar transistor's behaviour. Pinch-off voltage moves slightly the other way, gate leakage is negligible, and the channel geometry is fixed. That the dominant temperature effect opposes rather than reinforces the current is why these devices are so much easier to use in parallel.

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112.

At what operating point is a junction field effect transistor's drain current nearly temperature independent?

A.At its saturation current with the gate at the source
B.Just above pinch-off, where the current is smallest
C.In the ohmic region at a low drain voltage
D.Where the mobility and pinch-off effects cancel

Answer: D

The two temperature effects act oppositely, so at one particular gate voltage they cancel and the current barely drifts - a zero-temperature-coefficient point manufacturers sometimes specify. At the saturation current the mobility effect dominates. This is the same idea as the 5.6 volt Zener: where two mechanisms with opposite signs cross, a parameter passes through zero and is worth exploiting.

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113.

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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114.

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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115.

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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116.

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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117.

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

118.

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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119.

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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120.

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

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