history of semiconductor devices starts in 1930's when Lienfed and Heil first
proposed the mosfet. However it took 30 years before this idea was applied to
functioning devices to be used in practical applications, and up to the late 1980
this trend took a turn when MOS technology caught up and there was a cross over
between bipolar and MOS share.CMOS was finding more wide spread use due to its
low power dissipation, high packing density and simple design, such that by 1990
CMOS covered more than 90% of total MOS scale.
1983 bipolar compatible process based on CMOS technology was developed and BiCMOS
technology with both the MOS and bipolar device fabricated on the same chip was
developed and studied. The objective of the BiCMOS is to combine bipolar and CMOS
so as to exploit the advantages of both at the circuit and system levels. Since
1985, the state-of-the-art bipolar CMOS structures have been converging. Today
BiCMOS has become one of the dominant technologies used for high speed, low power
and highly functional VLSI circuits especially when the BiCMOS process has been
enhanced and integrated in to the CMOS process without any additional steps. Because
the process step required for both CMOS and bipolar are similar, these steps cane
be shared for both of them.
System On Chip (SOC) Fundamentals
The concept of system-on-chip (SOC) has evolved as the number of gates available
to a designer has increased and as CMOS technology has migrated from a minimum
feature size of several microns to close to 0.1 µm. Over the last decade,
the integration of analog circuit blocks is an increasingly common feature of
SOC development, motivated by the desire to shrink the number of chips and passives
on a PC board. This, in turn, reduces system size and cost and improves reliability
by requiring fewer components to be mounted on a PC board. Power dissipation of
the system also improves with the elimination of the chip input-output (I/O) interconnect
Superior matching and control of integrated
components also allows for new circuit architectures to be used that cannot be
attempted in multi-chip architectures. Driving PC board traces consume significant
power, both in overcoming the larger capacitances on the PC board and through
larger signal swings to overcome signal cross talk and noise on the PC board.
Large-scale microcomputer systems with integrated peripherals, the complete digital
processor of cellular phone, and the switching system for a wire-line data-communication
system are some of the many applications of digital SOC systems.
of analog or mixed-signal SOC devices include analog modems; broadband wired digital
communication chips, such as DSL and cable modems; Wireless telephone chips that
combine voice band codes with base band modulation and demodulation function;
and ICs that function as the complete read channel for disc drives. The analog
section of these chips includes wideband amplifiers, filters, phase locked loops,
analog-to-digital converters, digital-to-analog converters, operational amplifiers,
current references, and voltage references. Many of these systems take advantage
of the digital processors in an SOC chip to auto-calibrate the analog section
of the chip, including canceling de offsets and reducing linearity errors within
data converters. Digital processors also allow tuning of analog blocks, such as
centering filter-cutoff frequencies. Built-in self-test functions of the analog
block are also possible through the use of on-chip digital processors.
or mixed-signal SOC integration is inappropriate for designs that will allow low
production volume and low margins. In this case, the nonrecurring engineering
costs of designing the SOC chip and its mask set will far exceed the design cost
for a system with standard programmable digital parts, standard analog and RF
functional blocks, and discrete components. Noise issues from digital electronics
can also limit the practicality of forming an SOC with high-precision analog or
RF circuits. A system that requires power-supply voltages greater than 3.6 V in
its analog or RF stages is also an unattractive candidate for an SOC because additional
process modifications would be required for the silicon devices to work above
the standard printed circuit board interface voltage of 3.3 V+- 10%.
a high-performance analog system can be integrated on a digital chip, the analog
circuit blocks must have available critical passive components, such as resistors
and capacitors. Digital blocks, in contrast, require only n-channel metal-oxide
semiconductor (NMOS) and p-channel metal-oxide semiconductor (PMOS) transistors.
Added process steps may be required to achieve characteristics for resistors and
capacitors suitable for high-performance analog circuits. These steps create linear
capacitors with low levels of parasitic capacitance coupling to other parts of
the IC, such as the substrate. Though additional process steps may be needed for
the resistors, it may be possible to alternatively use the diffusions steps, such
as the N and P implants that make up the drains and sources of the MOS devices.
The shortcomings of these elements as resistors, as can the poly silicon gate
used as part of the CMOS devices. The shortcomings of these elements as resistors,
beyond their high parasitic capacitances, are the resistors, beyond their high
parasitic capacitances, are the resistor's high temperature and voltage coefficients
and the limited control of the absolute value of the resistor.
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