7.6. BIPOLAR TRANSISTORS: A TECHNOLOGY ROADMAP 347
are: (i) amorphous Si, which has a large “effective bandgap” (∼1.5 eV). The problems include
poor-quality contacts to amorphous Si; (ii)β-SiC with bandgap of 2.2 eV. The material has a
strong lattice mismatch with Si and it is not clear how reliable the technology will be; (iii) semi-
insulating polycrystalline Si, which has a gap of 1.5 eV. High current gains have been reported
for this system; (iv) use of III-V compounds like GaP. The main problem here is the cross-doping
issue since Si dopes GaP while Ga and P dope Si.
A material system that appears to have a tremendous advantage and is still compatible with
Si technology is the Si-SiGe system. The Si 1 −xGexis an alloy with lattice constant that is
mismatched from Si by 4x%. However, for very thin base regionsn-Si/p-SiGe/n-Si HBTs can
be fabricated with very high performance. The smaller gap of SiGe suppresses hole injection
into the emitter. Devices operating up to 350 GHz have been reported in this material system.
7.6.3 GaAs/AlGaAs HBTs
In chapter 3 we discussed the bandstructure of GaAs and AlAs systems. The two semiconduc-
tors have excellent lattice matching (∼0.14%) and high-quality GaAs/AlGaAs heterostructures
can be grown. The bandgap of the alloy AlxGa 1 −xAs up to compositions ofx∼0.45 is given
by
Eg(x)=1.42 + 1. 247 x
Abovex∼0.45, the material becomes indirect and is usually not used for most device applica-
tions because of poor transport and optical properties.
GaAs material has a high bandgap and thus the intrinsic carrier concentration is quite low
(∼ 2. 2 × 106 cm−^3 ) at room temperature. Thus the semi-insulating GaAs can have a very
high resistivity (∼ 5 × 108 Ω-cm), with the result that there is essentially negligible capacitance
between the substrate and the interconnects or the collector. This is a serious problem for Si at
high frequencies.
An important advantage of GaAs technology is that the electronic devices can be monolith-
ically integrated with optoelectronic devices, leading to optoelectronic integrated circuits (OE-
ICs), which are certainly not possible for Si technology (so far).
Another important advantage of GaAs technology is the ability to fabricate millimeter mi-
crowave integrated circuits (MMICs) in which the active and passive elements of the circuit are
all made on the same chip. MMIC technology is quite advanced in GaAs while it is still primitive
in Si.
In the GaAs/AlGaAs system, HBTs withfτvalues around 200 GHz have been achieved,
making this material system an important player in microwave technology.
7.6.4 InGaAs/InAlAsandInGaAs/InPHBTs ..................
An important consideration in the development of any material technology is the substrate
availability. One must have a high-quality substrate that is lattice-matched to the material and has
very few defects. There are three main substrates that have reached a very high quality level: Si,
GaAs, and InP. The material systems In 0. 53 Ga 0. 47 As(Eg∼ 0. 75 eV) and In 0. 52 Al 0. 48 As(Eg∼=
- 4 eV) are lattice-matched to InP. Thus the In 0. 53 Ga 0. 47 As/In 0. 52 Al 0. 48 As and InGaAs/InP both