In many kinds of integrated circuits, the preferred element of amplification/switching is...

In many kinds of integrated circuits, the preferred element of amplification/switching is not the bipolar transistor discussed in the chapter, but the MOSFET (metal oxide semiconductor field effect transistor). The accompanying diagram shows one in its "normally off" state: Conduction electrons cannot flow from the n-type source, which is analogous to the emitter, "over the bump" in the p-type region to the n-type drain, analogous to the collector. (An npn arrangement is shown, but just as for the bipolar transistor, a pnp would yield the complementary device.) The important difference is that rather than a direct electrical contact to the p-type region, as in the base of the bipolar, the center lead, the gate, is a conductor bonded to the p-type region but separated by a thin insulating layer.

(a) Explain how applying a bias to the gate can cause this device to turn on. Should the gate bias voltage be positive or negative (relative to the source)? Why is the control mechanism referred to as "field effect"?

(b) The MOSFET is often said to be a "unipolar" device because valence holes (conduction electrons in the pnp device) do not play the important role that they do in the bipolar. Explain. Would you expect a significant current through the gate due to electron-hole recombination in the p-type region? Why or why not?

(c) A low-input-impedance device is one in which there are large oscillations in input current for small oscillations in the input voltage. Correspondingly, a high-input-impedance device has a small input current for a large input voltage. Bearing in mind that the voltage across the forward-biased base-emitter diode of a bipolar transistor is always about Egaple, while the input current is proportional to the output current, would you say that the bipolar transistor has low or high input impedance? What about the MOSFET?