Audio Engineering

194 Chapter 7

7.10.2.1 Metal–Oxide–Semiconductor Field-Effect Transistors

Metal–oxide–semiconductor fi eld-effect transistors, in which the gate electrode is isolated
from the source/drain channel, have very similar transfer characteristics to that of junction
FETs. They have an advantage that, since the gate is isolated from the drain/source
channel by a layer of insulation, usually silicon oxide or nitride, no maximum forward
gate voltage can be applied—within the voltage breakdown limits of the insulating layer.
In a junction FET the gate, which is simply a reverse biased PN diode junction, will
conduct if a forward voltage somewhat in excess of 0.6 V is applied.

The chip constructions and circuit symbols employed for small signal lateral MOSFETs
and junction FETs (known simply as FETs) are shown in Figures 7.26 and 7.27.

Figure 7.25 : Gate voltage versus drain current characteristics of fi eld-effect devices.

0

Id

Vg 0.6 VVg

Junction FET

(a)

0

Id

Vg

“Enhancement” type MOSFET

(b)

0

Id

Vg Vg

“Depletion” type MOSFET

(c)