24.4 JFET and MESFET
709
In more detail, the mobility used in (24.45), (24.46) and (24.47) must be distinguished as saturation,
field-effect and effective mobility (cmp. more details given for MOSFETs on p. 780).
The drain transconductance for V D → 0 (linear regime, dashed straight lines in Fig. 24.15a) is
given by
g D0 = g max
1 −
V bi + V G
V P
= g m,sat ,
(24.49)
which is equal
6 to the forward transconductance in the saturation regime g m,sat = ∂ I D,sat /∂V G .
24.4.3 Normally On and Normally Off FETs
The JFET discussed so far had an n-conductive channel and was conductive at V G = 0. It is termed
an ‘n-type, normally on’ (or depletion) FET. If the channel is p-conductive, the FET is called ‘p-type’.
A FET that has a nonconductive channel at V G = 0 is called ‘normally off’ (or accumulation) FET.
In this case, the built-in voltage must be large enough to cause pinch-off. For a positive gate voltage
(in the forward direction of the gate–drain diode), current begins to flow. The I –V characteristics of
the four FET-types are depicted in Fig. 24.16. The circuit symbols for the four different FET types are
shown in Fig. 24.17.
Fig. 24.16 Scheme (top),
I D vs. V D (center) and I
1/2
D
vs. V G (bottom) I –V
characteristics for a
normally on (depletion)
and b normally off
(accumulation) n-type
JFET. Adapted from [574]
(a)
V S
V D
n
V G
V (V)
G
V D
I D
0.0
-0.1
-0.2
-0.3
-0.4
0
0
V G
I D
1/2
0
V <0
T
(b)
V S
V G
V D
n
V (V)
G
V D
I D
+0.4
0.3
0.2
0.1
0
0
V G
I D
1/2
0 V >0
T
6 Technically, here g D0 = −g m,sat , however, we had counted V G positive for the reverse direction.