Page 101 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim
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82 STANDARD MICROELECTRONIC TECHNOLOGIES
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Aluminium
n epi
(a)
n epi
(b)
Figure 4.17 Two types of capacitors that are available from a bipolar process: (a) dielectric and
(b) junction
4.3.2 Characteristics of BJTs
As noted earlier, there are a number of electronic devices available from a bipolar process,
and these may be used either as discrete components or as part of an IC, such as an oper-
ational amplifier or logic switch. Here, a basic discussion of the characteristics of the
bipolar transistor is presented. There are many textbooks that cover different aspects
of the bipolar transistor from the basic (e.g. Sze 1985) through to an advanced treat-
ment of the device physics, the construction of sophisticated models (e.g. Hart 1994),
and bipolar circuitry. Most of this material is outside the scope of this book, because
here we are mainly interested in the technologies that are relevant to the integration
of standard ICs with microtransducers and MEMS devices. However, it is necessary to
include some basic material on bipolar devices for three reasons: First, as a background
material to the readers who are less familiar with electrical engineering topics and who
want to know more about the basic electrical properties of a junction diode and tran-
sistor and the technical terms used, such as threshold voltage or current gain; second,
to serve as a reminder to other readers of the typical characteristics of a bipolar device
for use when designing an IC. Finally, and perhaps most important, to provide back-
ground information on microelectronic devices that can be exploited directly, or within
an 1C, as a microtransducer or MEMS device. For instance, a bipolar diode or bipolar
transistor may be used to measure the ambient temperature (see Chapter 8). Therefore,
for all these reasons, a brief discussion of the properties of the bipolar junction and FETs
is given here.
The basic properties of a semiconducting material have already been discussed in
Section 3.3 and they should be familiar to an electrical engineer or physicist. Therefore,
we start our discussion with the properties of the junction diode before moving on to the
BJT. As shown in the last section, a junction diode can be fabricated from a standard
bipolar process by forming a contacting region between an n-type and p-type material.
