704
24 Transistors
(a)
(b)
(c)
(d)
Material
(In,Ga)As
(In,Al)As
(In,Al)As
(In,Al,Ga)As
(In,Ga)As
(In,Ga)As
(In,Ga)As
InP substrate
d (nm)
75
45
100
45
80
700
430
type
n
+
N
+
N
N
p
+
n
-
n
+
function
emitter cap
emitter cap
emitter
grade
base
collector
subcollector
(e)
25
20
15
10
5
0
0
0.5
1.0
1.5
2.0
I (mA)
C
V (V)
CE
0.5
0.4
0.3
0.2
0.1
I (mA)
B
Fig. 24.12 a Schematic layout of a high-frequency HBT and SEM images b without and c with contacts. d Epitaxial
layer sequence and e static performance data. Parts a, b from [2055], parts d, e from [2056]
surfaces, realized second. The principle is fairly simple: A current flows through a channel from source
to drain. The current is varied via the channel conductivity upon the change of the gate voltage. The
gate needs to make a nonohmic contact to the semiconductor. Since the conductivity in the channel is
a property related to the majority charge carriers, FETs are called unipolar transistors. FETs feature a
higher input impedance than bipolar transistors, a good linearity, and a negative temperature coefficient
and thus a more homogeneous temperature distribution. According to the structure of the gate diode
we distinguish JFETs, MESFETs and MOSFETs, as discussed in the following:
In the junction FET (JFET), the variation of channel conductivity is accomplished via the extension
of the depletion layer of the pn-junction formed by the gate and the channel material (Fig. 24.14a).
The JFET was analyzed by Schottky in 1952 [108] and realized by Dacey and Ross in 1953 [109]. The
JFET can be made with a heterostructure gate to improve the frequency response.