300 Chapter 9
rather worse noise fi gure than an equivalent JFET, but, on the plus side, they have rather
more closely controlled operating characteristics. The range of types available covers
the very small-signal, low-working voltage components used for VHF amplifi cation in
TVs and FM tuners (for which applications a depletion-mode dual-gate device has been
introduced that has very similar characteristics to those of an RF pentode valve) to high-
power, high-working voltage devices for use in the output stages of audio amplifi ers, as
well as many other high-power and industrial applications. They are made in both
depletion- and enhancement-mode forms (the former having gate characteristics similar
to that of the JFET, whereas the latter description refers to the style of device in which
there is normally no drain current in the absence of any forward gate bias), in N-channel
and P-channel versions, and, at the present time, in voltage and dissipation ratings of up
to 1000 V and 600 W, respectively.
All MOSFETs operate in the same manner, in which a conducting electrode (the gate)
situated in proximity to an undoped layer of very high purity single-crystal silicon (the
channel), but separated from it by a very thin insulating layer, is caused to induce an
electrostatic charge in the channel, which will take the form of a layer of mobile electrons
or holes. In small-signal devices this channel is formed on the surface of the chip between
two relatively heavily doped regions, which will act, respectively, as the source and the
drain of the FET, while the conducting electrode will act as the current controlling gate.
Although modern photolithographic techniques are capable of generating exceedingly
precise diffusion patterns, the length of the channel formed by surface-masking
VABQ 1RV1Figure 9.17 : Current source layout.