Page 119 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Digital Circuits and Logic Design

Notes so that the diffusion time that slows the recovery from saturation will not be involved in the logic
delay,[2] the emitter and collector resistances are chosen such that at maximum input voltage
some voltage is left across the transistor. The residual gain is low (K=R /R <1). The circuit is
C E
insensitive to the input voltage variations and the transistor stays firmly in active linear region.
The input resistance is high because of the series negative feedback.
The cut-off transistor breaks the connection between its input and output. As a result, its input
voltage does not affect the output voltage. The input resistance is high again since the base-emitter
junction is cut-off.
Positive emitter-coupled logic (PECL) is a further development of ECL using a positive 5V supply
instead of a negative 5V supply. Low-voltage positive emitter-coupled logic (LVPECL) is a power
optimized version of PECL, using a positive 3.3V instead of 5V supply. PECL and LVPECL are
differential signalling systems, and are mainly used in high speed and clock distribution circuits.

Use of the IC in excess of absolute maximum rating such as the applied voltage
or operating temperature range may result in IC damage.

7.5 Integrated Circuit (IC)

In an integrated circuit, electronic components such as resistors, capacitors, diodes, and transistors
are formed directly onto the surface of a silicon crystal. The process of manufacturing an integrated
circuit will make more sense if one first understands some of the basics of how these components
are formed.
7.5.1 Design

Some integrated circuits can be considered standard, off-the-shelf items. Once designed, there
is no further design work required. Examples of standard ICs would include voltage regulators,
amplifiers, analog switches, and analog-to-digital or digital-to-analog converters. These ICs are
usually sold to other companies who incorporate them into printed circuit boards for various
electronic products.
Other integrated circuits are unique and require extensive design work. An example would be a
new microprocessor for computers. This design work may require research and development of
new materials and new manufacturing techniques to achieve the final design.

7.5.2 The Manufacturing Process
Hundreds of integrated circuits are made at the same time on a single, thin slice of silicon and
are then cut apart into individual IC chips. The manufacturing process takes place in a tightly
controlled environment known as a clean room where the air is filtered to remove foreign particles.
The few equipment operators in the room wear lint-free garments, gloves, and coverings for
their heads and feet. Since some IC components are sensitive to certain frequencies of light, even
the light sources are filtered. Although manufacturing processes may vary depending on the
integrated circuit being made, the following process is typical.
Preparing the Silicon Wafer

• A cylindrical ingot of silicon about 1.5 to 4.0 inches (3.8 to 10.2 cm) in diameter is held
vertically inside a vacuum chamber with a high-temperature heating coil encircling it.
Starting at the top of the cylinder, the silicon is heated to its melting point of about 2550°F
(1400°C). To avoid contamination, the heated region is contained only by the surface tension
of the molten silicon. As the region melts, any impurities in the silicon become mobile. The
heating coil is slowly moved down the length of the cylinder, and the impurities are carried
along with the melted region. When the heating coil reaches the bottom, almost all of the

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