544
21 Diodes
Fig. 21.27 Measured
Fermi energies E F (labeled
data in eV) and the work
functions W m of various
metals contacting (a)
pentacene, (b) Alq 3 and (c)
MEH-PPV. E C (E V )
denotes the energy position
of the electron (hole)
polaron. Measured data for
E F for MEH-PPV from
[1753], other from [1754].
Data for W m from
Table 21.2
(a)
vacuum
level
W m
Ca
Al
Au
Pt
3.0
4.3
4.8
2.4
4.9
Pentacene
E F
2.9
4.3
4.8
5.3
Ca
Al
Au,Pt
E C
E V
(b)
vacuum
level
W m
Ca
Al
Cu
Sm
Au
Pt
3.6
4.0
4.6
5.2
5.6
3.0
6.0
Alq3
E F
2.9
4.3
4.6
2.7
4.8
5.3
Sm,Ca,Mg
Al
Cu
Au
Pt
Mg 3.7
E C
E V
(c)
vacuum
level
W m
Ca
Al,Ag
Cu
Sm
Au
Pt
4.3
4.6
3.0
5.1
5.2
2.90
5.30
MEH-PPV
E C
2.9
4.3
4.6
2.7
4.8
5.3
Al,Ag
Cu
Sm,Ca
Au
Pt
E F
E V
barrier (see Fig. 21.5 for inorganic semiconductors). An electron trap, such as C 60 in MEH-PPV, can
pin the Fermi level of the n-contact metal and leads to a change of the built-in potential [1752].
Space-charge limited currents (see Sect. 21.2.6) are often observed in organic semiconductors. An
analysis including the effects of traps has been reported in [1755].
21.3 Metal-Insulator-Semiconductor Diodes
In a metal-insulator-semiconductor (MIS) diode an insulator is sandwiched between the metal and
the semiconductor. Subsequently, a MIS contact has zero dc conductance. The semiconductor typically has an ohmic back contact. As insulator, often the oxide of the respective semiconductor, is
used. In particular SiO 2 on Si has been technologically advanced (Fig. 21.29). In the latter case, the
diode is called a MOS (metal-oxide-semiconductor) diode. This structure has great importance for the
investigation of semiconductor surfaces and overwhelming importance for semiconductor technology
(planar integration of electronic circuits, CMOS technology). Also, CCDs (Sect. 22.3.8) are based on
MIS diodes.