How does DC input impedance at the gate of a field-effect transistor (FET) compare with that of a bipolar transistor?

A field-effect transistor (FET) operates by using an electric field applied to its gate terminal to control the flow of current through its channel. Because the gate is either electrically insulated (as in a MOSFET) or forms a reverse-biased PN junction (as in a JFET), practically no DC current flows into the gate. This gives the FET an extremely high DC input impedance, often in the megohm or gigohm range. In contrast, a bipolar junction transistor (BJT) is a current-controlled device. It requires a continuous flow of current into its base terminal to allow current to flow from the collector to the emitter. Because the BJT base-emitter junction is forward-biased during normal operation, it draws current and therefore has a comparatively much lower input impedance than an FET.