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Review Notes on BJT
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Presentation of 3 pages for the course ECE 301 at Batangas State University (Review Notes on BJT)
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Review Notes Electronics EngineeringBIPOLAR JUNCTION TRANSISTORS Semiconductor devices have virtually replaced all vacuum tubes nowadays except
for applications that require very high power like in broadcasting and other
Introduction communication facilities. Semiconductor devices have many advantages over
vacuum tubes – they are relatively small and lightweight, require no heater, they
Before the advent of transistors, vacuum tubes were the most commonly used are rugged in construction, more efficient, instantly available for us, requires no
electronic devices. The first diode which is a vacuum tube diode was developed by warm-up period and operates under lower voltages. The development of a family
John A. Fleming in 1904. This device was also called the Fleming valve in honor of transistors has even made possible the miniaturization of electronic circuits.
of its inventor. Two years later, Lee De Forest added a third element called the From small-scale integration, the semiconductor technology of today is already in
control grid in the vacuum tube diode, hence it was called triode. The additional the nanotechnology level!
mechanism served as a control element, which controlled the amount of current
passing through the device. Triode is actually the first amplifier and the After semiconductor diodes, our interest will now be focused on transistors. The
amplifying action is achieved through the additional element, the control grid. word transistor was coined by J.R. Pierce, an engineer from Bell Labs. It is a
Vacuum tubes experienced an evolution. A number of terminals had been added to contraction of the words transfer and resistor. This term was adopted because it
the triode to form tetrode, pentode, sextode, etc. But the vacuum tube has reached best describes the operation of the transistor - the transfer of an input signal
it limitations. current from a low-resistance circuit to a high-resistance circuit.
A transistor is a three-terminal, a solid-state device that amplifies electrical
signals by controlling the current passing through it. Although the basic function
of a transistor is to amplify, it can also be used as a switch.
There are two predominant types of transistors – the bipolar junction transistor
(BJT) and the field effect transistor (FET). In the field of electronics industry,
some people are still accustomed to mention the word “transistor”, but actually,
what he was referring to is a BJT. In this chapter, we would be focusing first on
the bipolar one – the BJT. Bipolar junction transistors are used in two broad areas
– as linear amplifiers and as electronic switches.
The Bipolar Junction Transistor
Image of the Fleming valve - the first vacuum tube diode. One type of transistor is the bipolar junction transistor. A BJT is a transistor that
is constructed with three doped semiconductor regions separated by two pn
junctions This is done by connecting p and n type semiconductors in such a way
Due to the rapid advances in technology, new kinds of electronic devices were
as to form three layers – either pnp or npn. Hence, BJTs are classified as either
developed. In 1940s, the Bell Telephone Laboratories had a team of physicists and
pnp or npn transistors.
researchers aiming to develop a kind of transistor. The team headed by Dr.
William Shockley also included Dr. Walter Brattain and Dr. John Bardeen.
Shockley’s theory is based on the effect of an electric field, and that this field can
be used to control the operation of a semiconductor device. During that time, he
was trying to develop the present-day Field Effect Transistor (FET). But
unfortunately, he wasn’t able to develop a device based on his theories. Instead,
another kind of transistor was developed. On the afternoon of December 23, 1947,
Bardeen and Brattain demonstrated the amplifying action of the first transistor
which is a point-contact transistor. In 1951, Shockley developed the first
junction transistor. Since then, the semiconductor industry has experienced rapid
developments.
The construction of a BJT
The three layers, which correspond to the three terminals of the transistor are
called the emitter, base and the collector. The emitter gives off or “emits” the
charged carriers, which can be electrons or holes; the base controls the flow of the
charged carriers while the collector receives or “collects” the charged carriers.
Two p-n junctions are formed: one between the emitter and the base (BE junction)
and the other, between the base and the collector (BC junction). The standard
schematic symbol for npn and pnp BJTs are shown below. For the npn transistor,
notice that the arrow is not pointing in. Also, the arrow is always at the emitter.
The first transistor, developed at Bell Labs in 1947
Schematic Symbol of (a) npn and (b) pnp transistor
Bell Lab’s germanium junction transistor
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1 Prepared by: Engr. Jefril M. Amboy
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