Radio Frequency Integrated Circuit Design - Webs

3
A Brief Review of Technology
3.1 Introduction
At the heart of RF integrated circuits are the transistors used to build them.
The basic function of a transistor is to provide gain. Unfortunately, transistors
are never ideal, because along with gain comes nonlinearity and noise. The
nonlinearity is used to good effect in mixers and in the limiting function in
oscillators. Transistors also have a maximum operating frequency beyond which
they cannot produce gain.
Metal oxide semiconductor (MOS) transistors and bipolar transistors will
be discussed in this chapter. CMOS is the technology of choice in any digital
application because of its very low quiescent power dissipation and ease of
device isolation. However, traditionally, MOS field-effect transistors (MOSFETs)
have had inferior speed and noise compared to bipolar transistors. Also, CMOS
devices have proved challenging to model for RF circuit simulation, and without
good models, RFIC design can be a very frustrating experience. In order to
design RFICs, it is necessary to have a good understanding of the high-speed
operation of the transistors in the technology that is being used. Thus, in this
chapter a basic introduction to some of the more important properties will be
provided. For more detail on transistors, the interested reader should consult
[1–10].
3.2 Bipolar Transistor Description
Figure 3.1 shows a cross section of a basic npn bipolar transistor. The collector
is formed by epitaxial growth in a p− substrate (the n− region). A p region
inside the collector region forms the base region; then an n+ emitter region is
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