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the operation, but once started, it completes its cycle of operation with no further input.
semiconductor. A material whose electrical conductivity is between that of an insulator and a conductor.
The electrical properties of a semiconductor are caused by its atomic structure and by the way the material
has been processed.
Valence electrons are the electrons in the outer shell of an atom, and the number of valence
electrons is the important characteristic of the atomic structure of a semiconductor material. Silicon and
germanium, the two chemical elements most often used for semiconductor devices, both have four valence
electrons.
An important step in the production of semiconductor materials is that of “doping,” which is the
addition of minute amounts of other chemical elements, called impurity elements, which have either three
or five valence electrons. Just a few parts per million of these impurity elements cause the semiconductor
material to act as either a conductor or an insulator, depending upon the polarity of the voltage placed
across it.
semiconductor diode. A two-element, solid-state electron check valve made of silicon or germanium,
doped with appropriate impurities. Electrons can either flow through a semiconductor diode or not flow
through it depending upon the polarity of the voltage across it.
When the diode is forward-biased, with the positive voltage connected to the anode, and the
negative voltage to the cathode, electrons flow through it. But, when it is reverse-biased, with the negative
voltage connected to the anode, electrons do not flow through it.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 513
semiconductor integrated circuit. A complex electronic circuit made on a single chip of semi-conductor
material. Passive components, such as resistors and capacitors, and active components such as transistors
are formed by several layers of conducting and insulating materials. These components are connected
together to form such complete circuits as gates and amplifiers.
semiconductor memory (computer memory). A computer memory in which data is stored in
semiconductor devices, such as flip-flop circuits and capacitors. Flip-flops are normally used for static
storage and capacitors for dynamic storage. Static storage is used for ROM (read-only memory), in which
data is programmed when the chip is manufactured. Data can be read from it, but no new data can be
written into it. ROM is nonvolatile, meaning that the data remains in ROM even when all computer power
is turned off.
Dynamic storage is used for RAM (random-access-memory). Data can be both written into RAM
and read from it, and this data can be changed as needed. The data stored in a dynamic RAM (D-RAM) is
volatile. This means that all data is lost when computer power is shut off. The data in a D-RAM must be
continually refreshed (the charge renewed) as the charge (the data) leaks off.
semiconductor transducer. A piezoelectric crystal that converts input energy of one form, such as
pressure, into output energy of another, such as an electrical signal.
semimonocoque structure (aircraft structure). A form of stressed skin structure used in the construction
of aircraft. Most of the strength of a semimonocoque structure is in the skin, but the skin is supported on a
substructure of formers and stringers that gives the skin its shape and increases its rigidity.
semirigid rotor (helicopter rotor). A type of helicopter rotor that allows the pitch of the blades to be
changed by rotating the blades about their feather axis (the axis that extends along the length of the blade).
The blades cannot flap up and down individually, but the entire rotor can flap as a unit. It is free to teeter
(rock back and forth) about the mast.
sender (fuel quantity measuring system). The part of a fuel quantity measuring system located at the tank
to measure the level of the fuel in the tank. In most simple systems, the sender is a variable resistor
mounted outside the tank with a wire arm protruding into the tank. A float mounted on the arm rides on the
top of the fuel, and the arm is connected to the wiper of the variable resistor. As the level of fuel in the tank
changes, the position of the arm on the variable resistor changes.
The fuel quantity gage on the instrument panel measures the current flowing through the variable
resistor and translates this current into the amount of fuel in the tank.
sense antenna (automatic direction finder antenna). A type of nondirectional radio antenna used with an
automatic direction finder. The sense antenna picks up signals with equal strength from all directions. An
ADF system uses two antennas — a loop antenna and a sense antenna. The loop antenna is highly
　
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