564
21 Diodes
(a)
6
5
4
3
2
1
0
-0.1
-0.2
-0.3
-0.4
-0.5
-0.6
2
1
0
0
2
-
0
6
-
0
4
-
0
8
-
0
0
1
-
0
2
1
-
Ge
i
S
e
G
Si
(b)
GaN
0
1
2
3
4
5
V
2
10 4
2
0
-2
-4
-6
-8
-10
10
10
10
10
10
10
10
n=1.5
n=2
Fig. 21.47 Comparison of the characteristics of a Ge and Si and b GaN pn diodes at room temperature. Note the
different scales in the forward and reverse regime in (a). The lines in (b) are related to ideality factors 2 and 1.5 as
labeled. Adapted from [1764]
Fig. 21.48 Characteristics
of a Si power diode at two
temperatures, 25 ◦ C and
100 ◦ C
increases at higher temperature (Fig. 21.48). A wide-gap pn diode (GaN) exhibits small saturation
currents (The n = 1.5 part of the GaN in Fig. 21.47b belongs to a saturation current density of
7 × 10
−27 A/cm
2 , the n = 1.5 part to ≈ 10
−34 A/cm
2 ) and displays sizable current (density) only for
larger voltages.
Real I –V Characteristics
Besides the ideal I –V characteristics due to diffusion several effects contribute to the characteristics
of real bipolar diodes:
– a generation-recombination (G–R) current is present due to traps in the depletion layer
– already for fairly small forward voltages, high injection conditions are present, i.e. p n n n is no
longer valid
– the series resistance R s of the diode is finite (ideally R s = 0)
– the diode has a finite parallel (shunt) resistance R p (ideally R p = ∞ )
– at high reverse voltage the junction breaks down; this phenomenon is treated in Sect. 21.4.5
First, we consider the generation-recombination current due to band-impurity (b–i) processes (see
Sect. 10.9). Such recombination is nonradiative or at least does not produce photons with an energy
close to the band gap. The impact on the I –V diode characteristic has first been considered in [1765].
The net b–i recombination rate is given by (10.52). For reverse voltage, the generation dominates
the G–R current. For n < n i and p < n i , the net recombination rate r is
21 Diodes
(a)
6
5
4
3
2
1
0
-0.1
-0.2
-0.3
-0.4
-0.5
-0.6
2
1
0
0
2
-
0
6
-
0
4
-
0
8
-
0
0
1
-
0
2
1
-
Ge
i
S
e
G
Si
(b)
GaN
0
1
2
3
4
5
V
2
10 4
2
0
-2
-4
-6
-8
-10
10
10
10
10
10
10
10
n=1.5
n=2
Fig. 21.47 Comparison of the characteristics of a Ge and Si and b GaN pn diodes at room temperature. Note the
different scales in the forward and reverse regime in (a). The lines in (b) are related to ideality factors 2 and 1.5 as
labeled. Adapted from [1764]
Fig. 21.48 Characteristics
of a Si power diode at two
temperatures, 25 ◦ C and
100 ◦ C
increases at higher temperature (Fig. 21.48). A wide-gap pn diode (GaN) exhibits small saturation
currents (The n = 1.5 part of the GaN in Fig. 21.47b belongs to a saturation current density of
7 × 10
−27 A/cm
2 , the n = 1.5 part to ≈ 10
−34 A/cm
2 ) and displays sizable current (density) only for
larger voltages.
Real I –V Characteristics
Besides the ideal I –V characteristics due to diffusion several effects contribute to the characteristics
of real bipolar diodes:
– a generation-recombination (G–R) current is present due to traps in the depletion layer
– already for fairly small forward voltages, high injection conditions are present, i.e. p n n n is no
longer valid
– the series resistance R s of the diode is finite (ideally R s = 0)
– the diode has a finite parallel (shunt) resistance R p (ideally R p = ∞ )
– at high reverse voltage the junction breaks down; this phenomenon is treated in Sect. 21.4.5
First, we consider the generation-recombination current due to band-impurity (b–i) processes (see
Sect. 10.9). Such recombination is nonradiative or at least does not produce photons with an energy
close to the band gap. The impact on the I –V diode characteristic has first been considered in [1765].
The net b–i recombination rate is given by (10.52). For reverse voltage, the generation dominates
the G–R current. For n < n i and p < n i , the net recombination rate r is