Introduction
The invention of the bipolar transistor
in 1948 ushered in a revolution in electronics. Technical feats
previously requiring relatively large, mechanically fragile,
power-hungry vacuum tubes were suddenly achievable with tiny,
mechanically rugged, power-thrifty specks of crystalline silicon. This
revolution made possible the design and manufacture of lightweight,
inexpensive electronic devices that we now take for granted.
Understanding how transistors function is of paramount importance to
anyone interested in understanding modern electronics.
My intent here is to focus as exclusively
as possible on the practical function and application of bipolar
transistors, rather than to explore the quantum world of semiconductor
theory. Discussions of holes and electrons are better left to another
chapter in my opinion. Here I want to explore how to use these
components, not analyze their intimate internal details. I don't mean to
downplay the importance of understanding semiconductor physics, but
sometimes an intense focus on solid-state physics detracts from
understanding these devices' functions on a component level. In taking
this approach, however, I assume that the reader possesses a certain
minimum knowledge of semiconductors: the difference between "P" and "N"
doped semiconductors, the functional characteristics of a PN (diode)
junction, and the meanings of the terms "reverse biased" and "forward
biased." If these concepts are unclear to you, it is best to refer to
earlier chapters in this book before proceeding with this one.
A bipolar transistor consists of a
three-layer "sandwich" of doped (extrinsic) semiconductor materials,
either P-N-P or N-P-N. Each layer forming the transistor has a specific
name, and each layer is provided with a wire contact for connection to a
circuit. Shown here are schematic symbols and physical diagrams of these
two transistor types:
The only functional difference between a
PNP transistor and an NPN transistor is the proper biasing (polarity) of
the junctions when operating. For any given state of operation, the
current directions and voltage polarities for each type of transistor
are exactly opposite each other.
Bipolar transistors work as
current-controlled current regulators. In other words, they
restrict the amount of current that can go through them according to a
smaller, controlling current. The main current that is controlled
goes from collector to emitter, or from emitter to collector, depending
on the type of transistor it is (PNP or NPN, respectively). The small
current that controls the main current goes from base to emitter,
or from emitter to base, once again depending on the type of transistor
it is (PNP or NPN, respectively). According to the confusing standards
of semiconductor symbology, the arrow always points against the
direction of electron flow:
Bipolar transistors are called bipolar
because the main flow of electrons through them takes place in two
types of semiconductor material: P and N, as the main current goes from
emitter to collector (or visa-versa). In other words, two types of
charge carriers -- electrons and holes -- comprise this main current
through the transistor.
As you can see, the controlling
current and the controlled current always mesh together through
the emitter wire, and their electrons always flow against the
direction of the transistor's arrow. This is the first and foremost rule
in the use of transistors: all currents must be going in the proper
directions for the device to work as a current regulator. The small,
controlling current is usually referred to simply as the base current
because it is the only current that goes through the base wire of the
transistor. Conversely, the large, controlled current is referred to as
the collector current because it is the only current that goes
through the collector wire. The emitter current is the sum of the base
and collector currents, in compliance with Kirchhoff's Current Law.
If there is no current through the base
of the transistor, it shuts off like an open switch and prevents current
through the collector. If there is a base current, then the transistor
turns on like a closed switch and allows a proportional amount of
current through the collector. Collector current is primarily limited by
the base current, regardless of the amount of voltage available to push
it. The next section will explore in more detail the use of bipolar
transistors as switching elements.
- REVIEW:
- Bipolar transistors are so named
because the controlled current must go through two types of
semiconductor material: P and N. The current consists of both electron
and hole flow, in different parts of the transistor.
- Bipolar transistors consist of either
a P-N-P or an N-P-N semiconductor "sandwich" structure.
- The three leads of a bipolar
transistor are called the Emitter, Base, and
Collector.
- Transistors function as current
regulators by allowing a small current to control a larger
current. The amount of current allowed between collector and emitter
is primarily determined by the amount of current moving between base
and emitter.
- In order for a transistor to properly
function as a current regulator, the controlling (base) current and
the controlled (collector) currents must be going in the proper
directions: meshing additively at the emitter and going against
the emitter arrow symbol.
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