706
24 Transistors
FETs come in ‘n’ and ‘p’ flavors, depending on the conductivity type of the channel. For highfrequency applications, typically an n-channel is used due to the higher mobility or drift velocity. In
CMOS (complementary MOS) technology, both n-FETs and p-FETs are integrated in high density,
allowing the effective realization of logic gates with minimized power consumption.
24.4 JFET and MESFET
24.4.1 General Principle
The principal characteristic of a JFET is shown in Fig. 24.15. At V D = 0 and V G = 0, the transistor
is in thermodynamic equilibrium and there are no net currents. Underneath the gate diode, a depletion
layer is present. If for zero gate voltage the source–drain voltage is applied to the channel, the current
increases linearly. The positive voltage at the drain contact causes the expansion of the depletion layer
of the (reversely biased) gate–drain pn-diode. When the two (the upper and the lower) depletion regions
meet (pinch-off), the current saturates. The respective source–drain voltage is denoted as V D,sat . For
high gate–drain (reverse) voltage V D , breakdown occurs with a strong increase of the source–drain
current. A variation of the gate voltage V G leads to a variation of the source–drain current. A reverse
voltage leads to a reduction of the saturation current and saturation at lower source–drain voltage. For
a certain gate voltage V P , the pinch-off voltage, no current can flow in the channel any longer since
pinch-off exists even for V D = 0.
24.4.2 Static Characteristics
Here, we will calculate the general static behavior outlined in the previous section. We assume a long
channel (L a), the abrupt approximation for the depletion layer, the gradual channel approximation,
i.e. the depletion layer depth changes slowly along x, and a field-independent, constant mobility. In
(a)
D
D
linear
region
saturation
region
breakdown
V
I
V =0
G
V =|V |/4
G
P
V =|V |/2
G
P
(b)
1.0
0.8
0.6
0.4
0.2
0
I
/ I
D,sat
P
0
0.2
0.4
0.6
0.8
1.0
(V +V ) / V
G
b i
P
Fig. 24.15 a Principal characteristics of a JFET. The channel current I D is shown as a function of the source–drain
voltage V D for three different values of the (absolute value of the) gate voltage V G . The saturation values V D,sat and I D,sat
are indicated for one curve. The intersections with the dash-dotted line yield the saturation voltage. Adapted from [574].
b Transfer behavior of a JFET for two different carrier distributions, homogeneous (solid line) and δ-like (dashed line).
The blue, dash-dotted line is
√
I D /I P vs. the gate voltage. After [2058, 2059]
24 Transistors
FETs come in ‘n’ and ‘p’ flavors, depending on the conductivity type of the channel. For highfrequency applications, typically an n-channel is used due to the higher mobility or drift velocity. In
CMOS (complementary MOS) technology, both n-FETs and p-FETs are integrated in high density,
allowing the effective realization of logic gates with minimized power consumption.
24.4 JFET and MESFET
24.4.1 General Principle
The principal characteristic of a JFET is shown in Fig. 24.15. At V D = 0 and V G = 0, the transistor
is in thermodynamic equilibrium and there are no net currents. Underneath the gate diode, a depletion
layer is present. If for zero gate voltage the source–drain voltage is applied to the channel, the current
increases linearly. The positive voltage at the drain contact causes the expansion of the depletion layer
of the (reversely biased) gate–drain pn-diode. When the two (the upper and the lower) depletion regions
meet (pinch-off), the current saturates. The respective source–drain voltage is denoted as V D,sat . For
high gate–drain (reverse) voltage V D , breakdown occurs with a strong increase of the source–drain
current. A variation of the gate voltage V G leads to a variation of the source–drain current. A reverse
voltage leads to a reduction of the saturation current and saturation at lower source–drain voltage. For
a certain gate voltage V P , the pinch-off voltage, no current can flow in the channel any longer since
pinch-off exists even for V D = 0.
24.4.2 Static Characteristics
Here, we will calculate the general static behavior outlined in the previous section. We assume a long
channel (L a), the abrupt approximation for the depletion layer, the gradual channel approximation,
i.e. the depletion layer depth changes slowly along x, and a field-independent, constant mobility. In
(a)
D
D
linear
region
saturation
region
breakdown
V
I
V =0
G
V =|V |/4
G
P
V =|V |/2
G
P
(b)
1.0
0.8
0.6
0.4
0.2
0
I
/ I
D,sat
P
0
0.2
0.4
0.6
0.8
1.0
(V +V ) / V
G
b i
P
Fig. 24.15 a Principal characteristics of a JFET. The channel current I D is shown as a function of the source–drain
voltage V D for three different values of the (absolute value of the) gate voltage V G . The saturation values V D,sat and I D,sat
are indicated for one curve. The intersections with the dash-dotted line yield the saturation voltage. Adapted from [574].
b Transfer behavior of a JFET for two different carrier distributions, homogeneous (solid line) and δ-like (dashed line).
The blue, dash-dotted line is
√
I D /I P vs. the gate voltage. After [2058, 2059]