October 1973 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Mr. Lothar Stern, of Motorola
Semi, published a 3-part series on transistor theory in Popular Electronics
magazine in 1973. This is part 3.
Part 1 introduced
the basics of the bipolar transistor, and
Part 2 addressed
transistor circuit configurations - common emitter, common gate, common collector, Darlington,
differential - as well as presenting gain equations and delving a bit into the physical
construction of the semiconductor elements. Finally, the author talks about the newest
processes in use at the time and what was available for low power and high power RF applications.
In 1973, high power semiconductors were just pushing past the 100 MHz barrier. Gallium
arsenide (GaAs) and gallium nitride (GaN) were still in university and corporate laboratories
being prepared for the amazing devices we have powering today's wireless products.
Here are Part 1,
Part 2, and
(thanks to Jeff, KE5KQJ, for providing a copy of Part 3).
Fig. 15 - Latest process is epitaxial planar with annular ring.
Do You Know Your Bipolar Transistors? - Part 3
Conclusion of a 3-Part Series on Basic Transistor Theory
By Lothar Stern, Motorola Semiconductor Products Inc.
Newer Processes. Among new techniques, the latest process (see Fig.
15) diffuses controlled-geometry base and emitter regions into the collector layer and
covers the entire device with a protective coating of silicon dioxide to eliminate impurity
contamination. It permits operation at extremely high frequency, high voltage, and high
current and provides good reliability at low cost.
For small-signal, low-frequency transistors, the epitaxial planar structure with annular
ring is, by far the most widely used. For high-power applications, however, other processes
are often employed to optimize characteristics needed for special requirements. Figure
16 shows the most common power-transistor processes.
Fig. 16 - Cross sections of the processes employed in making transistors
for power applications, with some characteristics.
EPI-Base Structure, Single-Diffused Structure, Triple-Diffused Mesa
Structure, Double-Diffused Structure
At very high frequencies, power transistors take on very complex geometries to compensate
for current crowding. The latter restricts the emission of charge carriers to the edges
of the emitter at high current levels. With rectangular or round emitters, therefore,
the center portion of the emitter does not contribute to current emission, but it does
add parasitic capacitance which reduces high-frequency response. High-frequency structures,
therefore, have very long, thin emitters which may be folded many times to fit within
a given base area. (Fig. 17) This greatly increases the ratio of emitter periphery to
base area, thereby maximizing the emission-to-capacitance ratio. These extremely complex
structures stress present processing technology to its limits, accounting for the rapid
increases in prices as frequency capabilities of power transistors go beyond 100 MHz.
Typical high-frequency transistor structure has Inter-Digitated geometry. Complex
structure is due to large number of separate emitters all interconnected to form a single
transistor. Resistor depositions equalize current to the individual emitter areas.
Table I - Small-signal transistor capabilities.
Table II - Low-frequency
Transistor Capabilities. For small signal applications, today's transistors
cover virtually every conceivable requirement. Darlington (compound-connected) transistors
offer high input impedances and betas up to 75,000 at audio frequencies. Amplifiers for
r-f, oscillator, and mixer applications run well into the GHz band. For special applications,
low-noise transistors with noise figures around 2 dB are common, and for high-voltage
applications, devices with ratings up to several hundred volts are no longer unique.
And, due to plastic packaging, prices are so low that there are few device capabilities
that can't be purchased for under $1.00 even in unit quantities, (See Table I.)
Low-Frequency Power. Power transistors for the lower frequencies
are also plentiful, but do not yet fill the desired applications as completely as do
the small-signal devices. (See Table II.). Prices are quite low for plastic packaged
devices rated up to 15 A and about 100 W. At currents below 5 A, even some of the metal
packaged devices are inexpensive. But prices rise rapidly at higher current and voltage
levels, particularly if higher frequency operation is required. Considerable room for
further development still exists.
Fig. 17. At very high frequencies, power transistors take on some
very complex geometries to compensate for current crowding. Characteristics of structures
are at right.
High-Frequency Power. At very high frequencies, the power picture
is still more limited. Power outputs of one to five watts are available up to 1 GHz,
with up to 50 watts at 500 MHz and 100 watts in the 150-MHz region. Prices remain high
at the upper limits of power and frequency.
Fig. 18 - Range of transistor power available for given frequency
and supply voltage.
R-F Power. Figure 18 shows the range of transistor power commonly
available for a given frequency and power supply voltage. Specific device types have
been included to suggest possible choices for a particular application. The power supply
voltages given are those most usually encountered in practice. Devices are tailored for
best operation at these voltages. Using a transistor with a higher than necessary voltage
rating can result in performance degradation.
Posted August 29, 2018