7.4 FIELD-EFFECT TRANSISTORS 367
iB=
19. 3
384
= 0 .05 mA
iC= 80 iB=4mA
Hence,
vCE= 20 − 4 × 81 iB= 3 .8V
which does satisfy the active-state condition:vCE>Vγ.
7.4 FIELD-EFFECT TRANSISTORS
Field-effect transistors (FETs) may be classified as JFETs (junction field-effect transistors),de-
pletionMOSFETs (metal-oxide-semiconductor field-effect transistors), andenhancementMOS-
FETs. Each of these classifications features a semiconductor channel of eithern-type orp-type,
whose conduction is controlled by a field effect. Consequently, all FETs behave in a similar
fashion. The FET classification is illustrated in Figure 7.4.1 along with the corresponding circuit
symbols. FETs have the useful property that very little current flows through their input (gate)
terminals.
Junction FETs (JFETs)
The JFET is a three-terminal, voltage-controlled current device, whereas the BJT is principally
a three-terminal, current-controlled current device. The advantages associated with JFETs are
much higher input resistance (on the order of 10^7 to 10^10 ), lower noise, easier fabrication, and
in some cases even the ability to handle higher currents and powers. The disadvantages, on the
other hand, are slower speeds in switching circuits and smaller bandwidth for a given gain in an
amplifier.
Figure 7.4.2 shows then-channel JFET and thep-channel JFET along with their circuit
symbols. The JFET, which is a three-terminal device, consists of a single junction embedded in a
semiconductor sample. When the base semiconductor forming the channel is of an-type material,
the device is known as ann-channel JFET; otherwise it is ap-channel JFET when the channel is
FET
G: gate; D: drain; S: source
Depletion
MOSFET
n
channel
p
channel
G
D
S
G
D
S
JFET
n
channel
p
channel
G
D
S
G
D
S
Enhancement
MOSFET
n
channel
p
channel
G
D
S
G
D
S
Figure 7.4.1FET classification.
