Field Effect Transistors Questions and Answers

Answer: A

A dynamic microphone is a low-impedance source, and a bipolar device's low equivalent input noise voltage suits a low source impedance best. Field effect devices excel with high-impedance sources, where their negligible input current noise dominates the comparison. Which family is quieter depends entirely on the source impedance, which is why low-noise design begins by measuring the source rather than by choosing a device.

Answer: B

The saturation current is the drain current with the gate at the source potential, operating beyond pinch-off - the top of the transfer characteristic. It is a characteristic value rather than a rating, and the device's continuous rating is a separate and usually larger figure. Distinguishing a point on a characteristic from a limit the manufacturer imposes matters, because the two are quoted in the same units and often confused.

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.

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.

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.

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.

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.

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.

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.

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.