Field Effect Transistors Questions and Answers

Answer: C

Pinch-off is found by increasing the reverse gate bias until the drain current falls almost to zero, and reading the gate voltage there. The drain voltage at which saturation begins is a related but distinct quantity, numerically equal to the pinch-off voltage's magnitude at zero gate bias. Gate conduction and breakdown are limits rather than parameters. Two quantities of the same magnitude measured on different axes are easy to conflate, and this pair is the standard example.

Answer: D

The channel pinches off when the gate-to-drain voltage reaches the pinch-off value, so at zero gate bias the drain voltage alone must supply it. Symmetry and the square law are real properties but neither produces the equality. Recognising that pinch-off is a condition on a voltage difference rather than on either terminal separately explains why the same number appears on two different axes.

Answer: A

The first is true and the second is false: the gate is a capacitance, so switching it means moving real charge, and at high frequency that charge dominates the drive power. Zero steady current and zero switching charge are quite different claims, and confusing them leads to underestimating gate drive requirements badly. A static property and a dynamic one have to be quoted separately.

Answer: B

The first is true and the second overstates: MOSFETs are made in both enhancement and depletion modes, though enhancement types dominate because they suit logic and switching. A junction device, by contrast, can only be depletion mode, since its gate cannot be forward biased to create a channel. A statement containing always is worth checking against the less common member of the family.

Answer: C

The highest voltage gain per stage is a bipolar strength, since its exponential characteristic gives about twenty times the transconductance at the same current. The other three are genuine field effect advantages. Choosing a device family means naming which of its strengths the application needs, and gain is the one strength this family does not offer.

Answer: D

An electrometer measures currents of femtoamps, so the input device must draw less than that, which only an insulated gate achieves. Even a low-current bipolar base draws picoamps, and a junction gate leaks nanoamps, worse still at high current. A Darlington's input current is lower than a single device's but nowhere near enough. When a specification is orders of magnitude beyond the alternatives, only one structure will do. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

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.