Field Effect Transistors Questions and Answers

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.

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.

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.

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.

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.

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.

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.

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.

Answer: A

Integrated clamp diodes to the rails divert any excess charge before the oxide is stressed, and most MOSFETs are supplied with them built in. A series resistor limits the current after breakdown has begun rather than preventing it, and neither a capacitor nor a bead limits the voltage. Protecting against an overvoltage means providing a path at a safe voltage, which is what a clamp does and what the alternatives do not.

Answer: B

An enhancement device has no channel at zero gate voltage; the gate must exceed a threshold to induce one, which makes it a natural switch and the basis of all digital logic. A conducting channel at zero gate voltage describes a depletion device. Negative gate bias and falling current describe a p-channel or depletion device. Whether a device is normally on or normally off decides both its bias arrangement and the applications it suits.