24.5 MOSFETs
733
(a)
S
G
D
n
+
n-AlGaAs
si-GaAs
undoped
AlGaAs
n
+
(b)
(c)
(d)
V >0
G
V >0
G
+ +
+ +
Fig. 24.44 a Scheme of a HEMT structure with n-AlGaAs/GaAs heterointerface on semi-insulating GaAs. The source
and drain contacts are n-doped such that this structure can be used as an n-channel (normally-on) HEMT. The horizontal
dashed line represents schematically the position of the 2DEG at the heterointerface on the GaAs side. b Band diagram
at zero gate voltage. c Band diagram at positive gate voltage, increase of channel carrier concentration. d Band diagram
at even larger positive gate voltage, formation of conducting channel in the AlGaAs layer
(a)
S
G
D
n
+
undoped AlGaAs
si-GaAs
dopant
sheet
n
+
(b)
+
Fig. 24.45 a Scheme of a δ-doped HEMT structure with AlGaAs/GaAs heterointerface on semi-insulating GaAs. The
source and drain contacts are n-doped such that this structure can be used as an n-channel HEMT. The horizontal dashed
line represents schematically the position of the 2DEG in the GaAs layer. b Band diagram at zero gate voltage
providing higher mobility than pure Si, can be fabricated using graded or stepped SiGe buffer layers
on Si substrate. With such Si-based MHEMTs frequencies up to 100 GHz can be achieved.
24.6 Thin-Film Transistors
Thin-film transistors (TFTs) are field-effect transistors with a channel formed as thin film on insulating
substrate. A detailed treatment is available in [2112]. TFTs are typically fabricated as large-area arrays
from thin layers of polycrystalline or amorphous silicon [2113] or organic semiconductors [2114–
2117] on cheap substrates such as glass. Their most prominent use is driving pixels in active-matrix
displays such as electroluminescence (EL) displays or twisted nematic liquid crystal displays (LCD)
[2118]. Various gates and gate geometries have been reported as depicted in Fig. 24.50.
733
(a)
S
G
D
n
+
n-AlGaAs
si-GaAs
undoped
AlGaAs
n
+
(b)
(c)
(d)
V >0
G
V >0
G
+ +
+ +
Fig. 24.44 a Scheme of a HEMT structure with n-AlGaAs/GaAs heterointerface on semi-insulating GaAs. The source
and drain contacts are n-doped such that this structure can be used as an n-channel (normally-on) HEMT. The horizontal
dashed line represents schematically the position of the 2DEG at the heterointerface on the GaAs side. b Band diagram
at zero gate voltage. c Band diagram at positive gate voltage, increase of channel carrier concentration. d Band diagram
at even larger positive gate voltage, formation of conducting channel in the AlGaAs layer
(a)
S
G
D
n
+
undoped AlGaAs
si-GaAs
dopant
sheet
n
+
(b)
+
Fig. 24.45 a Scheme of a δ-doped HEMT structure with AlGaAs/GaAs heterointerface on semi-insulating GaAs. The
source and drain contacts are n-doped such that this structure can be used as an n-channel HEMT. The horizontal dashed
line represents schematically the position of the 2DEG in the GaAs layer. b Band diagram at zero gate voltage
providing higher mobility than pure Si, can be fabricated using graded or stepped SiGe buffer layers
on Si substrate. With such Si-based MHEMTs frequencies up to 100 GHz can be achieved.
24.6 Thin-Film Transistors
Thin-film transistors (TFTs) are field-effect transistors with a channel formed as thin film on insulating
substrate. A detailed treatment is available in [2112]. TFTs are typically fabricated as large-area arrays
from thin layers of polycrystalline or amorphous silicon [2113] or organic semiconductors [2114–
2117] on cheap substrates such as glass. Their most prominent use is driving pixels in active-matrix
displays such as electroluminescence (EL) displays or twisted nematic liquid crystal displays (LCD)
[2118]. Various gates and gate geometries have been reported as depicted in Fig. 24.50.