Page 49 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

30 H Chapterfour

In the analog world, a transistor can be used as a voltage amplifier, a current
amplifier, or a switch; in the digital world, a transistor is primarily considered to
be a switch. The structure of a transistor between the collector and emitter
terminals is similar to that of two diodes connected back-to-back. Two diodes
connected in this way would typically not conduct; however, when signals are
applied to the base terminal, the transistor can be turned ON or OFF. If the
transistor is turned ON, it acts like a CLOSED switch and allows current to flow
between the collector and the emitter; if the transistor is turned OFF, it acts
like an OPEN switch and no current flows. We may think of the collector and
emitter as datu terminals, and the base as the control terminal.
As for a diode, an individually packaged transistor consists of the silicon,
with connections to external leads, all
encapsulated in a protective package
(the silicon is typically smaller than a
grain of sand). The package protects the
silicon from moisture and other impurities
and helps to conduct heat away from the
silicon when the transistor is operating.
Transistors may be packaged in plastic or
in little metal cans about a quarter of an
Figure 4-8. Individually packaged
transistor inch in diameter with three leads sticking
(photo courtesy of Alan Winstanley) out of the bottom (Figure 4-8).

Metal-Oxide Semiconductor Field-Effect Transistors
Another family of transistors is known as metal-oxide semiconductor
field-effect transistors (MOSFETS)~ of which there are two basic types called
n-channel and p-channel; once again these names relate to the way in which the
silicon is doped (Figure 4-9).6
In the case of these devices, the drain and source form the datu terminals
and the gate acts as the control terminal. Unlike bipolar devices, the control

5 In conversation, the term MOSFET is pronounced as a single word, where “MOS” rhymes
with “boss” and “FET” rhymes with “bet”.
6 Nothing is simple. In fact the MOSFETs discussed in this book are known as enhancement-type
devices, which are OFF unless a control signal is applied to turn them ON. There are also
depletion-type devices, which are ON unless a control signal is applied to turn them OFF.