Discussion

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.