August 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 1 (here is
Part 2). At the time, transistors had pretty much replaced vacuum
tubes in all new electronic products - if you don't count cathode ray tubes (CRTs)
in televisions and some high power applications. Technical schools were still teaching
about vacuum tubes since there was a still a lot of legacy equipment that needed
to be maintained, and the main concentration had switched over to transistor theory.
1973, the year I turned 15, is right around the time I began seriously delving into
the electrical and electronic realms as an eventual vocation. I distinctly remember
being bewildered and amazed the first time I was told, in a USAF classroom, that
a PN junction held to a fairly constant voltage (~0.7 V for silicon) regardless
of the current through it - within the specified operational range.
Here are Part 1,
Part 2, and
Part 3 (thanks to Jeff, KE5KQJ, for providing
a copy of Part 3).
Do You Know Your Bipolar Transistors? - Part 1
Part 1 of a 3-Part Series on Basic Transistor Theory
By Lothar Stern, Motorola Semiconductor Products Inc.
Fig. 1 - Forward and reverse characteristics of typical
The transistor is the most versatile component of the semiconductor family. Its
most important characteristic is current and/or voltage amplification. Because of
this capability, it is the heart of most solid-state electronic circuits involving
signal amplification and switching.
Transistor Types. There are two varieties of transistors: unipolar
transistors, usually referred to as field-effect transistors (FET's), and bipolar
transistors. The FET was invented circa 1930 but was not exploited commercially
at the time. The bipolar transistor was invented around 1948 and rapidly became
a practical product. The FET was finally developed as a commercial product by the
early 1960's, many years after the bipolar transistor had already established itself.
FET's, especially MOSFET's, are now used widely in low-noise front ends for TV and
FM and are gaining importance in digital elements. However, in this article, we
will discuss only bipolars.
Transistor Basics. The basic element of any bipolar transistor
is the pn junction, or diode, formed by chemically uniting a layer of p-doped semiconductor
material with a layer of n-doped material. When the junction is forward biased (p
layer positive, n layer negative), the current through the device increases rapidly
as the voltage is increased. When the junction is reverse biased, an increase in
voltage causes only a very small amount of leakage current, until the reverse voltage
becomes high enough to cause "breakdown" of the junction. (See Fig. 1.)
A transistor is formed by sandwiching a very thin layer of n-doped material between
two layers of p-doped material (pnp transistor) or a thin layer of p-doped material
between two layers of n-doped material (npn transistor). The characteristics of
this 3-layer, 2-junction structure are achieved by varying the thickness and the
resistivities and geometries of the three layers.
Theoretically, an npn transistor should operate at higher frequencies than does
a pnp device because electrons (the principal current carriers of an npn unit) are
more mobile than holes (the principal current carriers of pnp devices). In practice,
however, both types have similar characteristics. The main difference is that pnp
transistors operate with a negative voltage on the collector element while npn transistors
operate with a positive collector voltage. This makes it possible to have complementary
circuits that often provide improved performance over the single-polarity circuits
that can be implemented with only one type of transistor.
The three layers of a bipolar transistor (Fig. 2) are the emitter, the base
and the collector. The emitter represents the current source, where the current
carriers originate. The base is the control element, and the collector is the element
through which the current carriers are transferred to an external circuit.
Fig. 2 - Current flow in a transistor.
Fig. 3 - Three basic transistor circuits and their characteristic
Fig. 4 - Characteristics of a common-emitter circuit showing
the input and output.
In operation, the collector-base junction is always reverse biased. If the emitter-base
junction is forward biased, the emitter-base current is high, as in any forward
biased diode junction. Instead of flowing out, however, most of this current diffuses
through the very thin base region and crosses the collector-base junction, with
only a small fraction flowing out the base lead. This action gives rise to the two
commonly used expressions for current gain in a transistor. The first, a, is the
ratio of IC/IE when the base-emitter junction is forward biased
and is normally called the common-base current gain. It is always slightly less
than unity (usually 0.95 to 0.998). The second gain expression is β, the ratio
of IC/IB , called the common-emitter current gain. It can
be quite high, depending on the value of α, since β = α/(1-α).
Thus, if α = 0.95, β ≈ 20; and if α = 0.99, β ≈
Circuit Configurations. There are three basic transistor circuit configurations:
common emitter, common base, and common collector. They differ principally in the
manner in which the signal is applied to the transistor and where the load is attached.
Figure 3 shows these basic circuits. Since the common-emitter circuit is by far
the most prevalent, most data sheets characterize the transistor in terms of this
Figure 4 shows the characteristics of a typical transistor in a common-emitter
circuit. The input circuit curve in quadrant III shows that a base-emitter voltage
(VBE) of less than approximately 0.5 volt (for silicon; for germanium,
about 0.2 volt) causes virtually no base or collector current to flow, thereby keeping
the transistor cut off. Above 0.5 V, IB rises sharply, limited only by
the ohmic resistance of the base region. Since the latter is very small, a very
small rise in VBE (beyond the threshold voltage) causes a large rise
in IB and IC as seen in quadrant II. From such a plot, the
transistor current gain, IC/IB, can be established. It is
also evident that considerable nonlinearity exists in the IC/IB
curves. This indicates that distortion for large output signals can be quite high.
The collector curves in quadrant I are the most significant. They show, for example,
that the maximum reverse-bias collector voltage that can be applied, before breakdown
of the collector-base junction, is BVCEO. The minimum collector voltage
needed to keep the collector junction reverse biased, thereby sustaining transistor
action, is VCE(sat). The maximum voltage excursion, therefore, is BVCEO
When the transistor is cut off (IB = 0), a residual current, ICEO,,
flows in the collector circuit. This leakage current can be reduced somewhat (but
not to zero) by applying reverse bias to the emitter junction. The limiting value,
with reverse bias applied to the emitter-base junction, is ICBO. This
is equivalent to the collector-base leakage current when the emitter is open-circuited.
Collector current increases rapidly as the base is energized. The maximum IC
is that which would damage the internal transistor structure, This value of collector
current is given as a maximum rating on data sheets. Thus, the output current could
range from IC(max) to ICEO (or ICBO in the event
of reverse bias), but usually IC islimited to a value far less than IC(max).
Power dissipation, PD, is the product of VCE and IC
and it causes the collector junction to heat up. Beyond a critical junction temperature,
TJ(max), the device could be damaged. Thus the PD(max) rating
of a transistor limits the maximum VCE and IC that can be
applied simultaneously. The locus of a PD(max) rating is a parabolic
curve on the VCE/IC plot. A load line, XQY, must be chosen
to prevent steady-state operation to the right of the locus.
From the above it is evident that, when the voltage applied between base and
emitter is less than approximately 0.5 V, there is only a very small current flow
through the transistor. Between collector and emitter, therefore, the transistor
represents a very high resistance, or open switch. When VBE is raised
above the threshold voltage, the internal resistance of the transistor drops to
a very small value so that the device approximates a closed switch. As shown in
Fig. 4, it takes a change of only 7 mA of IB along loadline XQY
to cause VCE to change from VCC to VCE(sat), that
is, from an open switch to a closed switch. This switch action is accomplished by
a VBE of only a few tenths of a volt.
(To be continued.)
Posted August 28, 2017