Page 99 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

80 STANDARD MICROELECTRONIC TECHNOLOGIES

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n,

II n epi

+
n Buried layer
(a)
r°-

n epi
- A
+
{ n Buried layer
(b)

n epi

Figure 4.15 Various kinds of diodes available from a bipolar process: (a) emitter-base; (b)
base-collector; and (c) epi-isolation

+
rather than a lateral p-n-p transistor could be fabricated by leaving out the buried n -
layer. The problem that arises then is that it restricts the possible circuit configurations
(because the collector is connected to the substrate that gives parasitic problems) and,
therefore, the process can no longer be regarded as standard.
Various other components can also be formed using this bipolar process. For example,
Figure 4.15 shows three different types of diode that can be formed, namely, the emitter-
base diode that has a low reverse breakdown voltage of 6 to 60 V and can be used as a
Zener diode; the base-collector diode that has a higher reverse breakdown voltage of 15
to 50 V; and the epi-isolation diode.
Figure 4.16 shows five different types of resistor that can be formed: the base resistor
with a typical sheet resistance of 100 to 500 /sq, the pinched-base resistor with a typical
sheet resistance of 2000 to 10000 /sq, the emitter resistor with a typical sheet resistance
of 4 to 20 /sq, the epi-resistor with a sheet resistance that varies from 400 to 2000 /sq,
and a pinched epi-resistor that has a higher sheet resistance than an epi-resistor of 500 to
2000 /sq and is often used in preference to the latter.
Finally, different capacitors can be formed; Figure 4.17 shows both the dielectric capac-
itor, in which the thermal oxide or thinner emitter oxide is used as the dielectric, and
the junction capacitance, which is suitable when there is no requirement for low leakage