584
21 Diodes
(a)
+
n GaAs
+
n Al Ga As
+
0.3
0.7
p Al Ga As
+
0.3
0.7
Al Ga As
x
1-x
p GaAs
+
x=0-0.15
800nm
200nm
200nm
400nm
300nm
(b)
~
V S
R S
V L
R L
I d
I f
SRD
(c)
0
T
0
8
0
0
1
0
6
0
4
0
2
0
2
1
0
4
1
0
0
2
0
6
1
0
8
1
output
input
Fig. 21.71 a Schematic layer sequence for fast GaAs/AlGaAs step-recovery diode. b Circuit with input and output
pulse. c Input (dashed line) and output (solid line) waveforms. Vertical division is 2 V. Adapted from [1797]
Via the Poisson equation, the charge in the intrinsic layer is related to the electric field. If no dopants
are present, there is a constant (maximum) field in the i-region at zero bias. If there is low doping, a
field gradient exists.
The capacitance for reverse bias is s A/w and is constant starting at fairly small reverse bias (10 V).
The series resistance is given by R s = R i + R c . The contact resistance R c dominates the series resistance
for large forward bias (Fig. 21.72).
21.5.9 Tunneling Diodes
For the invention of the tunneling diode and the explanation of its mechanism the 1973 Nobel Prize in
Physics was awarded to Esaki [1798]. Eventually, the tunneling diode did not make the commercial
breakthrough due to its high basis capacitance. It is used for special microwave applications with
low power consumption, for frequency stabilization and possibly in tunneling field-effect transistors
(Sect. 24.5.6). Reviews can be found in [1799, 1800].
First, the tunneling diode is a pn diode. While the tunnel effect [1801] has already been discussed
for Schottky diodes, it has not yet been considered by us for pn diodes. We expect the tunnel effect to
be important if the depletion-layer is thin, i.e. when the doping of both sides is high.
The doping is so high that the quasi-Fermi levels lie within the respective bands (Fig. 21.73). The
degeneracy is typically several kT and the depletion layer width is in the 10 nm range.
In the forward direction, the I –V characteristic of the tunneling diode exhibits a maximum followed
by a minimum and subsequently an exponential increase (Fig. 21.74a). As shown in Fig. 21.74b, the
total current consists of three currents, the band-to-band tunneling current, the excess current and the
thermal (normal thermionic diode) current.
The V = 0 situation is again shown in Fig. 21.75b. Upon application of a small forward bias,
electrons can tunnel from populated conduction-band states on the n-doped side into empty valance-
21 Diodes
(a)
+
n GaAs
+
n Al Ga As
+
0.3
0.7
p Al Ga As
+
0.3
0.7
Al Ga As
x
1-x
p GaAs
+
x=0-0.15
800nm
200nm
200nm
400nm
300nm
(b)
~
V S
R S
V L
R L
I d
I f
SRD
(c)
0
T
0
8
0
0
1
0
6
0
4
0
2
0
2
1
0
4
1
0
0
2
0
6
1
0
8
1
output
input
Fig. 21.71 a Schematic layer sequence for fast GaAs/AlGaAs step-recovery diode. b Circuit with input and output
pulse. c Input (dashed line) and output (solid line) waveforms. Vertical division is 2 V. Adapted from [1797]
Via the Poisson equation, the charge in the intrinsic layer is related to the electric field. If no dopants
are present, there is a constant (maximum) field in the i-region at zero bias. If there is low doping, a
field gradient exists.
The capacitance for reverse bias is s A/w and is constant starting at fairly small reverse bias (10 V).
The series resistance is given by R s = R i + R c . The contact resistance R c dominates the series resistance
for large forward bias (Fig. 21.72).
21.5.9 Tunneling Diodes
For the invention of the tunneling diode and the explanation of its mechanism the 1973 Nobel Prize in
Physics was awarded to Esaki [1798]. Eventually, the tunneling diode did not make the commercial
breakthrough due to its high basis capacitance. It is used for special microwave applications with
low power consumption, for frequency stabilization and possibly in tunneling field-effect transistors
(Sect. 24.5.6). Reviews can be found in [1799, 1800].
First, the tunneling diode is a pn diode. While the tunnel effect [1801] has already been discussed
for Schottky diodes, it has not yet been considered by us for pn diodes. We expect the tunnel effect to
be important if the depletion-layer is thin, i.e. when the doping of both sides is high.
The doping is so high that the quasi-Fermi levels lie within the respective bands (Fig. 21.73). The
degeneracy is typically several kT and the depletion layer width is in the 10 nm range.
In the forward direction, the I –V characteristic of the tunneling diode exhibits a maximum followed
by a minimum and subsequently an exponential increase (Fig. 21.74a). As shown in Fig. 21.74b, the
total current consists of three currents, the band-to-band tunneling current, the excess current and the
thermal (normal thermionic diode) current.
The V = 0 situation is again shown in Fig. 21.75b. Upon application of a small forward bias,
electrons can tunnel from populated conduction-band states on the n-doped side into empty valance-