734
24 Transistors
(a)
S
G
D
n-AlGaAs
si-GaAs
undoped
AlGaAs
n
+
n
+
undoped
InGaAs
(b)
+ +
Fig. 24.46 a Scheme of a PHEMT structure with n-AlGaAs/InGaAs 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 InGaAs layer. b Band diagram at zero gate voltage
S
G
D
n-InAlAs
si-InP
undoped
InAlAs
n
+
n
+
undoped
InGaAs
n InGaAs
+
Fig. 24.47 Scheme of a PHEMT structure with n-AlInAs/InGaAs/InAlAs structure on semi-insulating InP. The source
and drain contacts (with a highly doped InGaAs contact layer) are an 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 InGaAs layer
24.6.1 Annealing of Amorphous Silicon
Since the mobility in polycrystalline silicon is much higher (up to several hundred cm
2 /Vs depending
on grain size, see Sect. 8.3.8) than in amorphous silicon (< 1 cm
2 /Vs), such material is much more
desirable as channel in TFTs. However, it requires high deposition temperatures. In order to achieve
polycrystalline silicon with large grain size from amorphous silicon films that can be deposited at low
temperature (down to room temperature) several schemes have been developed, the most important
being thermal annealing and (excimer) laser annealing (ELA). Crystallization occurs by thermally activated nucleation and growth processes [2119]. Polycrystalline layers with small grain size can be made
amorphous with implantation of Si (self-implantation) and a subsequent optimized (re-)crystallization
processes.
In laser annealing energy is locally introduced during short pulses (several 10 ns or even fs); subsequent material change occurs on a sub-µs time scale [2120]. Laser induced crystallization enables
the use of inexpensive low-temperature substrates, such as plastic or glass, since it involves the ultrafast melting and resolidification of the near-surface region of the sample, and minimal heating of the
substrate. Local processing is also possible using laser crystallization.
24 Transistors
(a)
S
G
D
n-AlGaAs
si-GaAs
undoped
AlGaAs
n
+
n
+
undoped
InGaAs
(b)
+ +
Fig. 24.46 a Scheme of a PHEMT structure with n-AlGaAs/InGaAs 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 InGaAs layer. b Band diagram at zero gate voltage
S
G
D
n-InAlAs
si-InP
undoped
InAlAs
n
+
n
+
undoped
InGaAs
n InGaAs
+
Fig. 24.47 Scheme of a PHEMT structure with n-AlInAs/InGaAs/InAlAs structure on semi-insulating InP. The source
and drain contacts (with a highly doped InGaAs contact layer) are an 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 InGaAs layer
24.6.1 Annealing of Amorphous Silicon
Since the mobility in polycrystalline silicon is much higher (up to several hundred cm
2 /Vs depending
on grain size, see Sect. 8.3.8) than in amorphous silicon (< 1 cm
2 /Vs), such material is much more
desirable as channel in TFTs. However, it requires high deposition temperatures. In order to achieve
polycrystalline silicon with large grain size from amorphous silicon films that can be deposited at low
temperature (down to room temperature) several schemes have been developed, the most important
being thermal annealing and (excimer) laser annealing (ELA). Crystallization occurs by thermally activated nucleation and growth processes [2119]. Polycrystalline layers with small grain size can be made
amorphous with implantation of Si (self-implantation) and a subsequent optimized (re-)crystallization
processes.
In laser annealing energy is locally introduced during short pulses (several 10 ns or even fs); subsequent material change occurs on a sub-µs time scale [2120]. Laser induced crystallization enables
the use of inexpensive low-temperature substrates, such as plastic or glass, since it involves the ultrafast melting and resolidification of the near-surface region of the sample, and minimal heating of the
substrate. Local processing is also possible using laser crystallization.