7.6. BIPOLAR TRANSISTORS: A TECHNOLOGY ROADMAP 345
Zo
CBC
1
ωTCBC
1
ωTCBC
RL=
1
ω^2 CBC
ZL=Zo*
Figure 7.21: Conjugately matched loadZL(right) for the output impedanceZoshown on the
left.
7.6 BIPOLAR TRANSISTORS: A TECHNOLOGY
ROADMAP
In this section we will discuss some of the important design considerations in the performance
of bipolar devices. Bipolar devices must compete with the field effect transistor (FETs) and in
many respects the two classes of families carry out similar functions. This puts a tremendous
pressure on the BJT and HBT device designers to design the best devices in a given material
system.
Bipolar devices are exploiting both fabrication techniques and new material systems to pro-
duce superior devices. A survey of the development of advanced devices was given in figure 7.22.
We will now give a brief overview of these developments.
7.6.1 Si Bipolar Technology ...........................
In spite of the superior performance of HBTs, the Si bipolars continue to be the workhorse
devices for both digital and some microwave applications. The advances in Si technology have
come from two directions. The first direction relates to advanced fabrication technology and the
second one relates to the use of polysilicon as a contact for the emitters.
The fabrication-technology-related advances in Si bipolars have resulted from: (i) self-aligned
emitter and base contacts, which allow extremely precise placement of the base contact next to
the emitter contact and thus reduce parasitic resistances; (ii) trench isolation, which allows very
dense packing of the transistors without cross-talk. This involves etching narrow grooves around
the transistor down to the substrate, lining them with SiO 2 , and filling them with polysilicon.
This greatly reduces the isolation capacitance; (iii) sidewall contact process, which dramatically
reduces the extrinsic base collector capacitance. In this process polysilicon is used to contact the
base and is isolated from the collector by a thick oxide. The device becomes essentially one-
dimensional as a result and also becomes quite symmetric between the emitter and the collector.
The second source of improvements in Si bipolar devices is the use of polysilicon to con-
tact the emitter. The advantages of polysilicon over metal contacts arise from the boundary