Page 90 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

MONOLITHIC PROCESSING 71

Silicon microtechnologies

Bipolar Bipolar/MOS Metal oxide semiconductor
(MOS)
BiC MOS

Analogue Transistor- Integrated Emitter RF Silicon on
transistor injection coupled Bulk insulator
logic logic logic processes (SOI)
(TTL) (IIL) (ECL)

CMOS
I

nMOS CMOS CMOS Sapphire Implanted Wafer Unibond
metal polysilicon oxygen bonding
gate gate

P-well N-well Twin (tub) well

Figure 4.8 Common silicon processes used to make standard semiconductor devices and ICs

analogue circuitry with low-power switching digital circuitry. Clearly, this dual process
is more difficult and expensive than a single process.
The bipolar process is used to make many standard analogue components (see
Section 4.3.1), for example, p-n diodes and bipolar junction transistors (BJTs), and to
make some popular logic families, such as TTL, integrated injection logic (IIL), and
emitter coupled logic (ECL) for fast switching. The MOS process can be divided into
those creating an IC within the silicon wafer (i.e. a bulk process) and those creating an
IC on top of an insulator (i.e. an SOI process). The n-type metal oxide semiconductor
(nMOS) process only makes n-channel metal oxide semiconductor field-effect transistor
(MOSFETs) and is simpler than the complementary MOS (CMOS) process that makes
both nMOS and p-type metal oxide semiconductor (pMOS) devices. The lower cost,
higher packing density, and low power dissipation per gate of the CMOS process makes
it one of the most commonly used technologies today.
Figure 4.9 shows the difference between the main technologies in gate-switching speed
and power dissipation per gate. GaAs has traditionally been a niche technology producing
very high-speed devices and switching circuits but at a much higher cost from a lower
yield process. However, the power dissipation of GaAs does compare favorably with
bipolar TTL and ECL, and, in some cases, with CMOS when the cost difference is
diminishing; therefore, GaAs is becoming more competitive.
A summary of the relative performances of basic bipolar and MOS processes is given
in Table 4.2. Most of the devices available for circuit design are described in the following
two sections.
There is the option to use a polysilicon gate instead of a metal gate in a CMOS
process with an N- or P-well (or both) implanted into a p-type or n-type silicon wafer