1-2.
SOLID-STATE DEVICES
Semiconductor devices include an increasingly large family of solid-state,
current-controlling devices. Two of the most widely known are junction diodes and
Semiconductors require only a low operating current; consequently
battery size can be reduced.
The low current requirements also reduce the heat
problem, and components can be mounted close together.
Therefore, the use of
semiconductors results in smaller and lighter electronic equipment.
a. Conductors and Insulators.
The primary difference in the physical
characteristics of conductors and insulators is the number of electrical charge
carriers (electrons) that are free to move when electrical pressure (voltage) is
applied.
Conductors, having a large number of free electrons, permit current to
flow easily when voltage is applied.
Insulators have fewer free electrons and
therefore seriously impede the flow of current.
b. Semiconductors.
A semiconductor is sometimes defined as a material that
has less resistance than an insulator but more resistance than a good conductor.
(1) In a true semiconductor the atoms and molecules of the crystalline
material are bound together in fixed patterns.
Germanium and silicon
are two of the more important semiconductor materials. Other elements
are often added to the semiconductor material in a process known as
doping. Doping is done to change the electrical characteristics of the
semiconductor material.
The addition of the phosphorus, antimony,
arsenic, or bismuth creates an N-type semiconductor.
It contains a
quantity of negative charges (electrons) that can move freely.
The
addition of indium, gallium, or boron has the effect of inserting
positive charges; in other words, it creates an affinity for free
electrons.
This is known as a P-type semiconductor.
A quantity of
doped semiconductor material has less resistance than an equal amount
of undoped semiconductor material.
(2) When a piece of semiconductor material is connected in series with a
battery, current will flow through the material. If the battery leads
are reversed, the same amount of current will flow in the opposite
direction.
Semiconductor material has a bidirectional current
characteristic.
c. Junction Diodes.
If a piece of P-and a piece of N-type semiconducting
material are pressed together, a mechanical junction is formed and a semiconductor
diode is created. At the Junction, a barrier voltage is created when some of the
electrons from the N-type material join with the positive charges of the P-type
material.
(1) The barrier voltage can be strengthened or weakened by connecting a
battery.
The result will be to either increase or decrease current
through the barrier, depending upon whether the battery opposes or aids
the barrier voltage. When the junction barrier weakens, current flows
freely through the junction diode, impeded only by the natural
resistance of the semiconductor material
309 L1
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