How does DC input impedance at the gate of a field-effect transistor (FET) compare with that of a bipolar transistor?
Why An FET is a voltage-controlled device: the gate is either insulated from the channel by an oxide layer (MOSFET) or is a reverse-biased junction (JFET), so essentially no DC gate current flows and the DC input impedance is very high, often megohms or more. A bipolar transistor is current-controlled, with a forward-biased base-emitter junction that must draw base current, giving an input impedance of only hundreds of ohms to a few kilohms. So the FET gate presents the much higher DC input impedance.
Watch out The choice saying both are high is tempting because BJT amplifier stages can be designed for fairly high impedance with emitter degeneration, but the device's own forward-biased base junction is inherently low impedance.
FET gate = voltage control, almost no current in, so high Z. BJT base = current control, so low Z.