21.4 Bipolar Diodes
573
Fig. 21.58 Microscopic
(a) forward bias
electroluminescence and
(b) microscopic reverse
bias electroluminescence
( µ-ReBEL) image of an
alkaline texturized solar
cell (cmp. Fig. 22.61) in a
region without specific
surface features at
U = −17 V. The bulk
defects at these positions
are revealed
topographically in the
neighboring acidic
texturized solar cell in the
microscopic (c) image.
Their ReBEL pattern
appears similarly in the
corresponding EL image
(d). Adapted from [1771]
21.4.6 Heterostructure Diodes
In a heterostructure diode, the n- and p-regions are made of different semiconductors. Such structures
are discussed in detail in [1780].
Type-I Heterostructure Diodes
In Fig. 21.59, the band diagram is shown for a type-I heterostructure with the n (p) region having
the larger (smaller) band gap. Additionally to the built-in voltage, the barrier in the valence band
is increased. Such a diode finds application in particular as an injection (emitter-base) diode in heterostructure bipolar transistors (Sect. 24.2.7). In such diode the (mostly undesired) hole current from
the p- to the n-side is reduced. The peak in the conduction band poses potentially a greater barrier for
electron transport across the interface than the diffusion potential itself. The spike can be reduced by
grading the materials across the heterojunction and creating a smooth transition of E g between the
materials. The effect of grading on the properties of the heterojunction is discussed in detail in [1781].
Type-II Heterostructure Diodes
Many semiconductors can only be doped n-type or p-type (cmp. Sect. 7.4.2). Bipolar heterostructures
can be formed and, depending on the position of the bands relativ to the vacuum level, can form a
type-II heterostructure. Examples are p-NiO/n-ZnO or p-CuI/n-ZnO; the band diagram of the latter is
depicted in Fig. 21.60a for a typical situation (p
+ -n diode) [568, 1782]. Rectification for such device
can be very high (> 10
10 , Fig. 21.60b). Minority carrier injection is excluded due to the high barriers.
The current across the interface is a recombination current, presumably via interface defects, which
exhibits an ideality factor around 2 [1783].
21.4.7 Organic Semiconductor Diodes
A bipolar diode from organic semiconductors consists of the p-conductive hole transport layer (HTL)
and the n-conductive electron transport layer (ETL). The low conductivity of organic semiconductors
573
Fig. 21.58 Microscopic
(a) forward bias
electroluminescence and
(b) microscopic reverse
bias electroluminescence
( µ-ReBEL) image of an
alkaline texturized solar
cell (cmp. Fig. 22.61) in a
region without specific
surface features at
U = −17 V. The bulk
defects at these positions
are revealed
topographically in the
neighboring acidic
texturized solar cell in the
microscopic (c) image.
Their ReBEL pattern
appears similarly in the
corresponding EL image
(d). Adapted from [1771]
21.4.6 Heterostructure Diodes
In a heterostructure diode, the n- and p-regions are made of different semiconductors. Such structures
are discussed in detail in [1780].
Type-I Heterostructure Diodes
In Fig. 21.59, the band diagram is shown for a type-I heterostructure with the n (p) region having
the larger (smaller) band gap. Additionally to the built-in voltage, the barrier in the valence band
is increased. Such a diode finds application in particular as an injection (emitter-base) diode in heterostructure bipolar transistors (Sect. 24.2.7). In such diode the (mostly undesired) hole current from
the p- to the n-side is reduced. The peak in the conduction band poses potentially a greater barrier for
electron transport across the interface than the diffusion potential itself. The spike can be reduced by
grading the materials across the heterojunction and creating a smooth transition of E g between the
materials. The effect of grading on the properties of the heterojunction is discussed in detail in [1781].
Type-II Heterostructure Diodes
Many semiconductors can only be doped n-type or p-type (cmp. Sect. 7.4.2). Bipolar heterostructures
can be formed and, depending on the position of the bands relativ to the vacuum level, can form a
type-II heterostructure. Examples are p-NiO/n-ZnO or p-CuI/n-ZnO; the band diagram of the latter is
depicted in Fig. 21.60a for a typical situation (p
+ -n diode) [568, 1782]. Rectification for such device
can be very high (> 10
10 , Fig. 21.60b). Minority carrier injection is excluded due to the high barriers.
The current across the interface is a recombination current, presumably via interface defects, which
exhibits an ideality factor around 2 [1783].
21.4.7 Organic Semiconductor Diodes
A bipolar diode from organic semiconductors consists of the p-conductive hole transport layer (HTL)
and the n-conductive electron transport layer (ETL). The low conductivity of organic semiconductors