21.3 Metal-Insulator-Semiconductor Diodes
545
(a)
met
glass
(b)
3
2
1
0
-1
-2
-3
2
.
2
4
.
2
2.6
2.0
T/T (10 )
-3
0.4
0.3
0.2
0.1
0.0
(c)
-1.0
0.0
-2.0
1.0
2.0
|
-3
1.0
0.5
0.0
Au-Al
Al-Al
Sm-Al
Ca-Al
(d)
Al
Au
Ca
Sm
W (eV)
m
0.0
-1.0
1.0
E (eV)
F
1.0
0.0
-1.0
(e)
Pt
W (eV)
m
-2.0
-3.0
0.0
E (eV)
F
0.0
-1.0
-2.0
-1.0
Au
Cu
Al
Ag
Fig. 21.28 a Schematic MSM structure with organic semiconductor (sc.) on transparent glass substrate. Metal 1 is thin
and semitransparent. Thickness of organic semiconductor (polymer or small molecules) is about 50 nm. b Electroabsorption spectra of Al/MEH-PPV/Al structure at four dc bias voltages (solid lines) and optical density spectrum (dashed
line). c Magnitude of the electroabsorption response at 2.1 eV as a function of bias for metal/MEH-PPV/Al structures.
d, e Calculated (solid lines) and experimental (points) potential difference across (d) metal/MEH-PPV/Al structures and
(e) metal/MEH-PPV/Ca structures as a function of the work-function difference of the contacts. Parts (b)–(e) adapted
from [1753]
21.3.1 Band Diagram for Ideal MIS Diode
An ideal MIS diode has to fulfill the following three conditions:
(i) (as shown in Fig. 21.30) without external bias the energy difference W ms between the work function
of the metal and the semiconductor
W ms = (−e) φ ms = W m −
χ sc +
E g
2
± e B
(21.77)
is zero (φ ms = 0). The ‘+’ (‘−’) sign in (21.77) applies to a p-type, Fig. 21.30b (n-type, Fig. 21.30a)
semiconductor. B is the potential difference between the intrinsic and actual Fermi level, B =
|E i − E F |/e > 0.
(ii) The only charges present are those in the semiconductor and the opposite charge is on the metal
surface close to the insulator.
(iii) There is no dc current between the metal and the semiconductor, i.e. the conductivity of the insulator
is zero.
The quantity B (Fig. 21.32) is given as (for a p-type material, using (7.18), the high temperature
approximation (7.32) and (7.15))
545
(a)
met
glass
(b)
3
2
1
0
-1
-2
-3
2
.
2
4
.
2
2.6
2.0
T/T (10 )
-3
0.4
0.3
0.2
0.1
0.0
(c)
-1.0
0.0
-2.0
1.0
2.0
|
-3
1.0
0.5
0.0
Au-Al
Al-Al
Sm-Al
Ca-Al
(d)
Al
Au
Ca
Sm
W (eV)
m
0.0
-1.0
1.0
E (eV)
F
1.0
0.0
-1.0
(e)
Pt
W (eV)
m
-2.0
-3.0
0.0
E (eV)
F
0.0
-1.0
-2.0
-1.0
Au
Cu
Al
Ag
Fig. 21.28 a Schematic MSM structure with organic semiconductor (sc.) on transparent glass substrate. Metal 1 is thin
and semitransparent. Thickness of organic semiconductor (polymer or small molecules) is about 50 nm. b Electroabsorption spectra of Al/MEH-PPV/Al structure at four dc bias voltages (solid lines) and optical density spectrum (dashed
line). c Magnitude of the electroabsorption response at 2.1 eV as a function of bias for metal/MEH-PPV/Al structures.
d, e Calculated (solid lines) and experimental (points) potential difference across (d) metal/MEH-PPV/Al structures and
(e) metal/MEH-PPV/Ca structures as a function of the work-function difference of the contacts. Parts (b)–(e) adapted
from [1753]
21.3.1 Band Diagram for Ideal MIS Diode
An ideal MIS diode has to fulfill the following three conditions:
(i) (as shown in Fig. 21.30) without external bias the energy difference W ms between the work function
of the metal and the semiconductor
W ms = (−e) φ ms = W m −
χ sc +
E g
2
± e B
(21.77)
is zero (φ ms = 0). The ‘+’ (‘−’) sign in (21.77) applies to a p-type, Fig. 21.30b (n-type, Fig. 21.30a)
semiconductor. B is the potential difference between the intrinsic and actual Fermi level, B =
|E i − E F |/e > 0.
(ii) The only charges present are those in the semiconductor and the opposite charge is on the metal
surface close to the insulator.
(iii) There is no dc current between the metal and the semiconductor, i.e. the conductivity of the insulator
is zero.
The quantity B (Fig. 21.32) is given as (for a p-type material, using (7.18), the high temperature
approximation (7.32) and (7.15))