Tubes, Discrete Solid State Devices, and Integrated Circuits 335Resistors are made at the same time as transistors.
The resistance is characterized in terms of its sheet
resistance, which is usually 100–200:/square material
for diffused resistors and 50–150:/square material for
deposited resistors. To increase the value of a resistor,
square materials are simply connected in series.
It is very difficult to produce resistors with much
closer tolerance than 10%; however, it is very easy to
produce two adjacent resistors to be almost identical.
When making comparator-type circuits, the circuits are
balanced and are made to perform on ratios rather than
absolute values. Another advantage is uniformity in
temperature. As the temperature of one component
varies, so does the temperature of the other components,
allowing good tracking between components and
circuits so ICs are usually more stable than discrete
circuits.
Capacitors are made as thin-film integrated capaci-
tors or junction capacitors. The thin-film integrated
capacitor has a deposited metal layer and an n+ layer
isolated with a carrier-free region of silicon dioxide. In
junction capacitors, both layers are diffused low-resis-
tance semiconductor materials. Each layer has a dopant
of opposite polarity; therefore, the carrier-free region is
formed by the charge-depleted area at the pn junction.
The MOSFET transistor has many advantages over
the bipolar transistor for use in ICs as it occupies only
(^1) e 25 the area of the bipolar equivalent due to lack of
isolation pads. The MOSFET acts like a variable
resistor and can be used as a high-value resistor. For
instance, a 100 k: resistor might occupy only 1 mil^2 as
opposed to 250 mil^2 for a diffused resistor.
The chip must finally be connected to terminals or
have some means of connecting to other circuits, and it
must also be packaged to protect it from the environ-
ment. Early methods included using fine gold wire to
connect the chip to contacts. This was later replaced
with aluminum wire ultrasonically bonded.
Flip-chip and beam-lead methods eliminate the prob-
lems of individually bonding wires. Relatively thick
metal is deposited on the contact pads before the ICs are
separated from the wafer. The deposited metal is then
used to contact a matching metal pattern on the
substrate. In the flip-chip method, globules of solder
deposited on each contact pad ultrasonically bond the
chip to the substrate.
In the beam-lead method, thin metal tabs lead away
from the chip at each contact pad. The bonding of the
leads to the substrate reduces heat transfer into the chip
and eliminates pressure on the chip.
The chip is finally packaged in either hermetically
sealed metal headers or is encapsulated in plastic, which
is an inexpensive method of producing ICs.
12.3.2 Hybrid Integrated Circuits
Hybrid circuits combine monolithic and thick- and
thin-film discrete components for obtaining the best
solution to the design.
Active components are usually formed as mono-
lithics; however, sometimes discrete transistors are
soldered into the hybrid circuit.
Passive components such as resistors and capacitors
are made with thin- and thick-film techniques. Thin
films are 0.001–0.1 mil thick, while thick films are
normally 60 mils thick. Resistors can be made with a
value from ohms to megohms with a tolerance of 0.05%
or better.
High-value capacitors are generally discrete, minia-
ture components that are welded or soldered into the
circuit, and low-value capacitors can be made as film
capacitors and fabricated directly on the substrate.
Along with being certain that the components will fit
into the hybrid package, the temperature must also be
taken into account. The temperature rise TR of the
package can be calculated with the following equation:
(12-35)
where,
TC is the case temperature,
TA is the ambient temperature,
PT is the total power dissipation,
TCA is the case-to-ambient thermal resistance.
The TCA for a package in free air can be approxi-
mated at 35°C/W/in^2 or a device will have a 35°C rise
in temperature above ambient if 1 W is dissipated over
an area of 1 in^2.
12.3.3 Operational Voltage Amplifiers (Op-Amp)
One of the most useful ICs for audio is the op-amp.
Op-amps can be made with discrete components, but
they would be very large and normally unstable to
temperature and external noise.
An op-amp normally has one or more of the
following features:
- Very high input impedance (>10^6 –10^12 :).
- Very high open-loop (no feedback) gain.
TR TC–= TA
=PTTTCA