732
24 Transistors
Pseudomorphic HEMTs
Instead of increasing the height of the barrier, the depth of the well can be increased by using a lowbandgap material. On GaAs substrate, InGaAs is used (Fig. 24.46). However, strain is introduced in
this case and the InGaAs layer thickness is limited by the onset of dislocation formation (cf. Sect. 5.4.1)
(which reduces the channel mobility and the device reliability). For In 0.15 Ga 0.85 As (thickness about
10–20 nm), a total barrier height of about 400 meV can be obtained. A barrier height of 500 meV can
be reached with an InAlAs/InGaAs structure on InP (Fig. 24.47). The InAlAs does not suffer from
the problem related to DX centers. The channel indium concentration is typically 50%. The mobility
increases with increasing indium concentration. This InP-based HEMT structure is widely used in
satellite receivers for its excellent high-speed and low-noise properties in the 100–500 GHz range and
beyond.
However, the InP technology is economically less favorable than GaAs due to smaller available
substrate size and higher cost (2001: 4" InP substrate: $ 1000, 6" GaAs substrate: $ 450).
Metamorphic HEMTs
A unification of the InAlAs/InGaAs structure with the best figure of merit and the GaAs substrate
is achieved with the metamorphic HEMT (MHEMT). Here, a relaxed buffer is used to bring the inplane lattice constant from that of GaAs to about that of InP. It is key that the defects occurring are
confined to the relaxed buffer and do not enter the active device structure (see Fig. 24.48). The relaxed
buffer is typically about 1 µm thick. It can be grown, e.g., with a graded In x (Ga,Al) 1−x As layer with
x =0–42% or with a stepped structure with piecewise constant indium concentration in each layer. It
is important that a smooth interface of the channel is achieved in order to avoid additional scattering
mechanisms. For high-frequency operation, the fabrication of a small gate length is important, as shown
in Fig. 24.49 for a 70-nm gate of a f T =293 GHz, f max = 337 GHz transistor [2110]. SiGe channels,
(a)
S
G
D
n
+
undoped AlGaAs
si-GaAs
+
(b)
V >0
G
(c)
V <0
G
(d)
Fig. 24.43 a Scheme of a HIGFET structure with metal gate and undoped AlGaAs/GaAs heterointerface on semiinsulating GaAs. The source and drain contacts are n-doped such that this structure can be used as an n-HIGFET (see
part c). b Band diagram for zero gate voltage. c Band diagram for positive gate voltage and n-channel, d for negative
gate voltage and p-channel
24 Transistors
Pseudomorphic HEMTs
Instead of increasing the height of the barrier, the depth of the well can be increased by using a lowbandgap material. On GaAs substrate, InGaAs is used (Fig. 24.46). However, strain is introduced in
this case and the InGaAs layer thickness is limited by the onset of dislocation formation (cf. Sect. 5.4.1)
(which reduces the channel mobility and the device reliability). For In 0.15 Ga 0.85 As (thickness about
10–20 nm), a total barrier height of about 400 meV can be obtained. A barrier height of 500 meV can
be reached with an InAlAs/InGaAs structure on InP (Fig. 24.47). The InAlAs does not suffer from
the problem related to DX centers. The channel indium concentration is typically 50%. The mobility
increases with increasing indium concentration. This InP-based HEMT structure is widely used in
satellite receivers for its excellent high-speed and low-noise properties in the 100–500 GHz range and
beyond.
However, the InP technology is economically less favorable than GaAs due to smaller available
substrate size and higher cost (2001: 4" InP substrate: $ 1000, 6" GaAs substrate: $ 450).
Metamorphic HEMTs
A unification of the InAlAs/InGaAs structure with the best figure of merit and the GaAs substrate
is achieved with the metamorphic HEMT (MHEMT). Here, a relaxed buffer is used to bring the inplane lattice constant from that of GaAs to about that of InP. It is key that the defects occurring are
confined to the relaxed buffer and do not enter the active device structure (see Fig. 24.48). The relaxed
buffer is typically about 1 µm thick. It can be grown, e.g., with a graded In x (Ga,Al) 1−x As layer with
x =0–42% or with a stepped structure with piecewise constant indium concentration in each layer. It
is important that a smooth interface of the channel is achieved in order to avoid additional scattering
mechanisms. For high-frequency operation, the fabrication of a small gate length is important, as shown
in Fig. 24.49 for a 70-nm gate of a f T =293 GHz, f max = 337 GHz transistor [2110]. SiGe channels,
(a)
S
G
D
n
+
undoped AlGaAs
si-GaAs
+
(b)
V >0
G
(c)
V <0
G
(d)
Fig. 24.43 a Scheme of a HIGFET structure with metal gate and undoped AlGaAs/GaAs heterointerface on semiinsulating GaAs. The source and drain contacts are n-doped such that this structure can be used as an n-HIGFET (see
part c). b Band diagram for zero gate voltage. c Band diagram for positive gate voltage and n-channel, d for negative
gate voltage and p-channel