Diodes and Semiconductors

        This section describes the widely used devices known as diodes.
The device's name comes from DI (meaning two) and ODE (from electrode).
Originally, this name was formed to describe the construction of the first
Electron Tube which contained two Electrodes; however, it
is now also applied to Simiconductor units that perform the same
function as the diode tube.

        The term Semiconductor, when linked with diode, refers to
a unit made by joining two different types of semiconductor material that do
not conduct electrons as well as a true conductor, but do conduct better
than an insulator. SEMI means half, so semiconductor is, literally, a HALF
CONDUCTOR. A conductor generally provides a very low resistance to conduction
in either direction, but a semiconductor material exhibits a higher resistance
to electron flow or conduction. The semiconductor material types are joined 
in a special way so that, when a proper polarity voltage is applied, electrons
flow through both materials across the junction (where the materials join)
but flow only very slightly in the opposite directionif the voltage is reversed.

        In AC, the voltage changes and alternates polarity periodically, while
in DC the polarity of the voltage doesn't change. Almost all generating stations
in the USA supply ac power. Many other countries of the world also have ac
power readily available, because ac can be distributed more efficiently than
dc. However electronic equipment frequently requires dc power for proper 
operation and, therefore, some method of converting ac to dc is necessary.
This conversion of ac to dc is called Rectification, and the device
that does the rectifying is called a Rectifier.

Rectifiers:

        The foundation on which the field of electronics is based is the
control of electrons. One of the basic forms of electron control is the 
conversion of ac to dc, by which the alternating current is changed or
rectified to direct current. In a rectifier, the electrons of an alternating
current are prevented from reversing their direction of flow (changing
their polarity) so that they flow in one direction only. Because semiconductor
diodes and electron tube diodes both achieve this rectification action, 
they are both classed as rectifiers, even though they operate in somewhat
different ways. The two terms, diode and rectifier, are often used interchangeably.

        The semiconductor diode used two types of conductive materials that
readily pass electrons from one material to the other but strongly oppose any
electron flow in the reverse direction. In the field of electronics, the diode
is the simplest form of an electron controlling device. 

Semiconductors:

        In order to understand semiconductors, as well as other related items,
we must become familiar with the properties that they possess which make them
behave as they do. Let's consider an atom of conductive material, which is
composed of electrons orbiting around the nucleus. If an electron in outer orbit
of an atom is not strongly attracted to the nucleus, it may move through the
material as a free electron, if sufficient energy is applied. A major factor
that determines the ease of movement is the form of the material or the way
that the atoms group together in nature.

        An atom seldom exists alone because it tends to combine with other
atoms to form the various materials that we know. At room temperature, these
materials may be either solids, liquids, or gases. Semiconductors generally
take the form of solids, so lets examine them more closely.

        Some solid materials can give up electrons more easily than other solid
materials. This is because some of the electrons of the atoms of such materials
are not as strongly bonded, or attracted, to the nuclei of their atoms. These
materials are said to contain free electrons, because they easily give up
electrons, or permit electron flow, when electric energy is applied to them.
A conductor such as copper has more free electrons than does a semiconductor
material such as silicon.

        In some solid materials (such as semiconductors), the atoms form into
definite groups called CRYSTALS. Within a crystal, the arrangement of
the atoms is very orderly and takes the form of a LATTICE. Perhaps you
have seen oranges or apples neatly piled in rows and layers in a grocery store.
This is a lattice arrangement, and it shows how each individual item is linked
to the others in the lattice to make up the whole group. In the crystal lattice
of atoms, the linking between atoms is done by electrons and these, when
sufficient energy is applied to the material, become free electrons that flow 
from atom to atom. This movement of electrons provides the basis for conduction
in the material.

        Several times energy has been mentioned as being a requirement for 
moving electrons from atom to atom in a material. 

Some forms of energy include:

Mechanical
Electrical
Heat
Light

        Materials vary in their reactions to applied energy. If a material
is subjected to a temperature of absolute zero (-459 degrees fahrenheit or
459 degrees below zero) and no other energy is present, there will theoreticlly
be no electron movement. At any temperature higher than this extreme, some
energy (in the form of heat) is being applied to material and some type of
reaction occurs. Generally speaking, the higher the temperature, the greater
the reaction. At room temperature, a conductor will have many free electrons,
while an insulator will not. Semiconductors such germanium and silicon rank
between conductors and insulators in the number of free electrons that provide
them with the property of conductivity.

Conduction:

        At room temperature, the free electrons of a semiconductor are considered
to have relatively high energy levels. To produce a current (which is a flow
of electrons), a voltage must be applied across the semiconductor. This electrical
energy applied to the material provides enough additional energy to the electrons
of the material so that more of them are freed. However, an unusual effect should
be noted.

        As an electron moves from one atom to the next, it leaves a vacancy in
the outer orbit of the atom where the electrons circle the nucleus. This vacancy
is called a HOLE. Every electron that is freed from an atom produces a
hole. Since a hole is the absence of an electron, it can be thought of as having
a positive charge equal to the negative charge of an electron. With this positive
charge as an attractive force and the electrical energy from the applied voltage
an electron from a nearby atom may jump to fill the hole just created, but
it likewise creates another hole in the atom it leaves.

        We can think of this action as negative electrons moving in one direction
or as positive holes moving in the other direction. In this way, a flow of holes
is called a current, just as we call a flow of electrons a current. It may be
handy to think of the holes as though they were particles, similar to electrons
but with positive charges. Remember that the idea of a hole is for convenience
only. Also, it is only in the case of crystals that the absence of an electron
is called a hole.

        Why do we think in terms of hole movement at all? Well, laboratory tests
show that the holes move at about half the speed of free electrons at the same
applied voltage. Due to this difference in speed, conduction by holes differs
a little from that by free electrons.

        The low-energy electrons that move by jumping from atom to atom are
not free electrons. Their low energy prevents them from moving far enough
away to become free of the atoms. They remain bound to the atoms, even while
they are jumping from one to another. Thus, two kinds of electron flow are
formed when a voltage is applied to a semiconductor. One flow consists of the
high-energy free electrons, and the other consists of low-energy atom jumping
electrons. These currents add to produce the total current (I).

        Since both hole conduction and electron conduction occur in semiconductors
the type of conduction being considered should be specified. From this point
on, we will use the term HOLE FLOW for conduction by the atom-jumping
process. The term ELECTRON FLOW will be used for the flow of free
electrons. This will avoid confusion between the two kinds of conduction.

Materials:

Intrinsic material:

        An INTRINSIC semiconductor is one that does not contain atoms of
other materials. In this type of pure crystal, whether or not a voltage is
applied, whenever an electron leaves an atom as a free electron, it always
creates a hole. As a result, the number of holes always equals the number
of free electrons.

        Two major factors that affect conduction in a intrinsic semiconductor
have been mentioned: Temperature and applied Voltage. You will remember that
there is no electron movement at absolute zero; there is also no hole flow
conduction at this temperature.

        At higher temperatures, small amounts of electron flow and hole flow 

take place when voltage is applied. At very high temperatures, many electrons
leave their atoms and create an equally large number of holes. Then, the 
intrinsic semiconductor acts like a conductor. However, it is not convenient
to operate electronic equipment at extremes of temperature. The conductive
ability of semiconductors at normal temperatures must be increased in another
way. Certain impurities are added to the intrinsic semiconductor, so that it 
is no longer pure. It is now called a doped semiconductor. The result is an
increase in one of the two kinds of conduction. As we will explain, either the
holes or the free electrons increase, depending on the kind of impurity used.

N-TYPE MATERIALS:

        An added impurity affects a semiconductor in either of two ways. If
the impurity atoms have more electrons in their outer orbits than there are
in the outer orbits of the semiconductor atoms, the excess electrons become
free electrons. Then, with the impurity material added, the semiconductor has
many more free electrons than it does holes. Because the impurity material
has added free electrons to the semiconductor, the impurity is called a 
DONER.

        Arsenic, antimony and phosphorus are donor-type impurities. Because
electrons have negative charges, a semiconductor with an excess of free
electrons is called anN-TYPE SEMICONDUCTOR.

P-TYPE MATERIALS:

        If there are fewer electrons in the outer orbits of the impurity atoms
than in the outer orbits of the semiconductor atoms, the impurity atoms take
electrons away from some of the semiconductor atoms. Because it takes or
(accepts) electrons, an impurity of this type is called an ACCEPTOR.

        Aluminum, indium, boron and gallium are acceptor-type impurities.
Due to the loss of electrons, the semiconductor atoms are left with more
holes than free electrons. Since holes are considered to have positive charges
a semiconductor with an excess of holes is called a P-TYPE SEMICONDUCTOR.

RECOMBINATION:

        A voltage applied to either an N-type or a P-type semiconductor material
causes current in the circuit. Within the semiconductor, the conduction is a
flow of both free electrons and holes, as explained for the intrinsic semiconductor.
However, in the doped semiconductor, the current is much greater for a given
applied voltage at room temperature.

        The greater conduction is provided by the type of excess current 
carriers the impurity has produced (holes or electrons). The excess current
carriers are called the MAJORITY CARRIERS. In N-type semiconductors,
the majority carriers are free electrons, while in P-type semiconductors, the
majority carriers are holes.

        When the holes reach the negative terminal of the semiconductor, they
meet some of the electrons entering this terminal. When a hole and an electron
meet, they combine with each other, and both cease to be current carriers.
This action is called RECOMBINATION.

        This would soon use up the supply of holes in the material. But new
holes are formed in the semiconductor by electrons that leave their atoms and
these electrons travel as free electrons to the positive terminal of the semi-
conductor.

        The minority carriers add their effect to that of the majority carriers
to produce the total current. However, the current due to the minority carriers
is very small compared to that provided by the majority carriers.

        Conduction is not possible outside the semiconductor material. When
the holes reach the negative terminal, they combine with electrons entering
the material at this terminal. This recombination action is the same as
explained for the minority materials in the N-type material.

        In the P-type material, many holes are formed near the positive 
terminal by electrons leaving the material at this point. These holes
replace the ones lost by recombination at the negative terminal. In this
case, most of the semiconductor current is due to hole conduction. To this
is added the minority current, consisting of the few electrons that flow
in the direction opposite that of the holes.

        In both N-Type and P-Type semiconductors, the total current is the
sum of the flows of free electrons and holes. However, the conduction is mostly
the movement of free electrons in the N-Type materials, and mostly the 
movement of holes in the P-Type materials.

        In any semiconductor material, holes and free electrons are lost due
to recombination within the material. This occurs regardless of whether or
not a voltage is applied across the material. But in every instance, new
free electrons and holes are formed as the result of electrons gaining
enough energy to leave their atoms. A balance is reached when the recombination
rate equals the rate at which the new electron-hole pairs are being formed.

The Semiconductor Junction Diode

        To form a semiconductor diode, a P-Type semiconductor and an
N-Type semiconductor are combined. The junction between the P-Type and
N-Type materials is called a PN JUNCTION. Junction Diodes
are constructed in many ways, but in every diode the two semiconductor
materials are joined to form a single piece. There are two types of 
semiconductor junction diodes. One is called a Grown Junction.
In the grown junction diode, the PN junction is formed by adding the 
proper impurities to the semiconductor material as it is (grown) or
Manufactured. The second type is called a DIFFUSED JUNCTION.
In the diffused junction diode, an impurity gas is allowed to diffuse
into the semiconductor material to form the PN junction.

Light Emitting Diodes

        We know that the free electrons in a semiconductor material
are relatively high energy particles; this energy is a requirement for
them, not only to break free from the atom in the first place, but to
continue their movement from atom to atom in the material. We also know
that this energy may come from various sources and in different forms; 
our earlier discussions mentioned using heat and electrical energy to
free the electrons.

        At the PN junction in semiconductor materials, the process of 
recombination, which was mentioned earlier as the combining of an electron
and a hole, produces an excess of energy because the energy requirement of
the electron drops when it combines with the hole. In some special
semiconductor materials, this excess energy is given off in the form of
light (another type of energy) at the PN junction. These special semiconductor
diodes have the property of emitting light, and this is the reason for
their name, LIGHT EMITTING DIODE, which is abbreviated LED.

Selenium Rectifiers

        Another type of diode or rectifier is the SELENIUM RECTIFIER
This unit was widely used before the PN junction diode was developed.
Although still in use today, many of its applications have been taken over
by PN junction diodes.

        Selenium in crystal form is deposited upon an aluminum plate. A
counter electrode is then deposited over the selenium. The blocking layer
is formed by applying a high reverse voltage to the cell for a period of 
time. The blocking layer has the property of passing electrons in one 
direction and not in another. That is, the blocking layer will allow electrons
to flow from cathode to anode but offers resistance to electron flow from
anode to cathode.

        Several other types of material can be used to form a Metallic
Rectifier Cell. Copper oxide is one such material.

Diode Circuit Action

        Semiconductor diodes work as one way switches (or check valves) to
pass current in only one direction. On this basis, they are used to block
one polarity of the alternations of an ac source.

        Alternating voltages occur very often in electronic systems. For
example, commercial power companies use an alternating voltage which goes
from positive to negative and back to positive again 60 times per second.

        The voltage of an ac generator, starts at zero, and builds to  a
maximum or peak voltage, then falls to zero and reverses polarity and builds
to a maximum or peak voltage in the opposite polarity. A diode changes this
type of alternating voltage to a series of pulses of one polarity only, or
pulsating dc voltage.

        In operation, the alternating voltage, makes the anode of the
semiconductor diode, alternately positive and negative with respect to
the cathode allowing voltage to flow in one direction and not the other.

        In the semiconductor diode, an N-type material and a P-type material
are bound closely together. Electrons flow easily from the N-type material
to the P-type material when an external voltage source of the proper polarity
is applied. However, very few electrons flow if the polarity of the source
voltage is reversed.

        This property allows semiconductor diodes to rectify an alternating
voltage. The diode conducts on one alternation to develope a voltage across
a resistor or load. But, on the other alternation, the diode does not conduct
and no load voltage is developed.
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