18WuD2x

618 self-test • semiresonant line
self-test Any arrangement whereby a device or
system determines, without the aid of an external
operator, whether or not it is operating correctly.
self-ventilated motor See OPEN MOTOR.
self-wiping contacts See SELF-CLEANING CON-
TACTS.
selsyn See AUTOSYN and SYNCHRO.
SEM 1. Abbreviation of SINGLE-ELECTRON MEM-
ORY. 2. Abbreviation of SCANNING ELECTRON
MICROSCOPE.
semantic network A reasoning scheme sometimes
used in artificial intelligence. Logical state-ments
or sentences are broken down into nodes (generally
nouns) and relationships (generally verbs
and modifiers). This allows statements to be
mapped in a way that is easy for computers to
store and modify.
semiautomatic key Also called bug. A telegraph
key that mechanically produces a string of
Morse-code dots (short pulses) when its lever is
pressed to one side (usually toward the right),
and continuous circuit closure when the lever is
pressed to the opposite side (usually toward the
left). Dashes are manually sent by the operator.
semiconductor A material whose natural resistivity
lies between that of conductors and insulators
(e.g., GERMANIUM, SILICON, SELENIUM, and
GALLIUM ARSENIDE).
semiconductor capacitor A miniaturized component
that takes advantage of reverse biasing
in a semiconductor P-N junction. When a voltage
source is connected across a diode so that it
does not conduct, the diode acts as a capacitor.
The capacitance varies depending on how much
reverse voltage is applied to the diode. The
greater the reverse voltage, the smaller the capacitance.
Some diodes are especially manufactured
to serve this function. This phenomenon
can be useful in the fabrication of integrated circuits
(ICs). See also INTEGRATED CIRCUIT,
VARACTOR.
semiconductor counter A device for measuring
the intensity of ionizing radiation (such as alpha
particles, beta particles, or gamma rays) using a
photodiode and sensing circuit.
semiconductor device A component (such as a
diode, photocell, rectifier, or transistor) that exploits
the properties of a semiconductor.
semiconductor diode A solid-state diode, as opposed
to a vacuum-tube diode or gas-tube diode.
Examples: germanium diode, selenium diode, and
silicon diode.
semiconductor junction Within a body of semiconductor
material, the area of physical contact
between two regions (usually n and p) having opposite
electrical properties.
semiconductor laser See LASER DIODE.
semiconductor material See SEMICONDUCTOR.
semiconductor-metal junction The area of
physical contact between a metal and a semiconductor.
semiconductor photosensor A semiconductor
photodiode or phototransistor, as opposed to a
phototube.
semiconductor rectifier A heavy-duty semiconductor
diode (or assembly of such diodes) designed
primarily to change alternating current to
direct current in power-supply units. Rectifiers
commonly are made from copper oxide, germanium,
magnesium-copper sulfide, selenium, or
silicon. Also see JUNCTION DIODE and METER
RECTIFIER.
semiconductor resistor A tiny resistor manufactured
from semiconductor material, especially one
that is etched onto the chip of an integrated circuit
(IC). The thickness, and the types and concentrations
of impurities added, determine the resistance
of the component. Such resistors can handle
only a tiny amount of power because of their small
size. But because IC circuits are designed to consume
minimal power, this is not a problem. The
small signals produced by ICs can be amplified,
using circuits made from discrete components, if it
is necessary to obtain higher signal power. See
also INTEGRATED CIRCUIT.
semidirectional Pertaining to a transducer that
exhibits different directional characteristics at
different frequencies.
semiduplex operation A two-frequency communication
system that operates in duplex at one end
of the link, and in simplex at the other end. Also
see DUPLEX OPERATION and SIMPLEX TELEG-
RAPHY.
semilogarithmic graph Also called semilog graph.
A graph in which one axis is logarithmic and the
other axis is linear.
Logarithmic scale
y
10
7
5
3
2
y = x
1
x
0 2 4 6 8 10
linear scale
semilogarithmic graph
semimetal An elemental substance that exhibits
some, but not all, of the properties of a metal (e.g.,
antimony and arsenic). Also called METALLOID.
semiresonant line An open-wire transmission line
cut approximately to resonant length at the frequency
of operation.