From
http://www.electronics-tutorials.ws/diode/diode_1.html
If
Resistors are the most basic passive component in electrical or electronic circuits, then we have to consider the
Signal Diode as being the most basic “Active” component.
However, unlike a resistor, a diode does not behave linearly
with respect to the applied voltage as it has an exponential I-V
relationship and therefore can not be described simply by using Ohm’s
law as we do for resistors.
Diodes are basic unidirectional semiconductor devices that
will only allow current to flow through them in one direction only,
acting more like a one way electrical valve, (Forward Biased Condition).
But, before we have a look at how signal or power diodes work we first
need to understand the semiconductors basic construction and concept.
Diodes are made from a single piece of
Semiconductor
material which has a positive “P-region” at one end and a negative
“N-region” at the other, and which has a resistivity value somewhere
between that of a conductor and an insulator. But what is a
“Semiconductor” material?, firstly let’s look at what makes something
either a
Conductor or an
Insulator.
Resistivity
The electrical
Resistance of an electrical or
electronic component or device is generally defined as being the ratio
of the voltage difference across it to the current flowing through it,
basic
Ohm´s Law
principals. The problem with using resistance as a measurement is that
it depends very much on the physical size of the material being measured
as well as the material out of which it is made. For example, if we
were to increase the length of the material (making it longer) its
resistance would also increase proportionally.
Likewise, if we increased its diameter (making it fatter) its
resistance value would decrease.
So we want to be able to define the
material in such a way as to indicate its ability to either conduct or
oppose the flow of electrical current through it no matter what its size
or shape happens to be.
The quantity that is used to indicate this specific resistance is called Resistivity and is given the Greek symbol of
ρ, (
Rho). Resistivity is measured in Ohm-metres, (
Ω-m ). Resistivity is the inverse of conductivity.
If the resistivity of various materials is compared, they can be classified into three main groups,
Conductors,
Insulators and
Semi-conductors as shown below.
Resistivity Chart
Conductors
From above we now know that
Conductors are materials
that have very low values of resistivity, usually in the micro-ohms per
metre. This low value allows them to easily pass an electrical current
due to many free electrons floating about,within their
basic atom structure. But these electrons will only flow through a
conductor if there is something to spur their movement, and that
something is an electrical voltage.
When a positive voltage potential is applied to the material these “free
electrons” leave their parent atom and travel together through the
material forming an electron drift, more commonly known as a current.
How “freely” these electrons can move through a conductor depends on how
easily they can break free from their constituent atoms when a voltage
is applied. Then the amount of electrons that flow depends on the amount
of resistivity the conductor has.
Examples of good conductors are generally metals such as Copper,
Aluminium, Silver or non metals such as Carbon because these materials
have very few electrons in their outer “Valence Shell” or ring,
resulting in them being easily knocked out of the atom’s orbit and become "free".
This allows them to flow freely through the material until
they join up with other atoms, producing a “Domino Effect” through the
material thereby creating an electrical current. Copper and Aluminium is
the main conductor used in electrical cables.
Generally speaking, most metals are good conductors of
electricity, as they have very small resistance values, usually in the
region of micro-ohms per metre. While metals such as copper and
aluminium are very good conducts of electricity, they still have some
resistance to the flow of electrons and consequently do not conduct
perfectly. Also the conductivity of conductors increases with ambient temperature because metals are also generally good conductors of heat.
Insulators
Insulators on the other hand are the exact opposite
of conductors. They are made of materials, generally non-metals, that
have very few or no “free electrons” floating about within their basic
atom structure because the electrons in the outer valence shell are
strongly attracted by the positively charged inner nucleus.
In other words, the electrons are stuck to the parent atom
and can not move around freely so if a potential voltage is applied to
the material, no current will flow as there are no “free electrons”
available to move and which gives these materials their insulating
properties.
Insulators also have very high resistances, millions of ohms
per metre, and are generally not affected by normal temperature changes. Examples of good insulators are
marble, fused quartz, p.v.c. plastics, rubber etc.
Insulators play a very important role within electrical and
electronic circuits, because without them electrical circuits would
short together and not work. For example, insulators made of glass or
porcelain are used for insulating and supporting overhead transmission
cables while epoxy-glass resin materials are used to make printed
circuit boards, PCB’s etc. while PVC is used to insulate electrical
cables.
Semiconductor Basics
Semiconductors materials such as silicon
(Si), germanium (Ge) and gallium arsenide (GaAs), have electrical
properties somewhere in the middle, between those of a “conductor” and
an “insulator”. They are not good conductors nor good insulators (hence
their name “semi”-conductors). They have very few “free electrons”
because their atoms are closely grouped together in a crystalline
pattern called a “crystal lattice” but electrons are still able to flow,
but only under special conditions.
The ability of semiconductors to conduct electricity can be
greatly improved by replacing or adding certain donor or acceptor atoms
to this crystalline structure thereby, producing more free electrons
than holes or vice versa. That is by adding a small percentage of
another element to the base material, either silicon or germanium.
On their own Silicon and Germanium are classed as intrinsic
semiconductors, that is they are chemically pure, containing nothing but
semi-conductive material. But by controlling the amount of impurities
added to this intrinsic semiconductor material it is possible to control
its conductivity. Various impurities called donors or acceptors can be
added to this intrinsic material to produce free electrons or holes
respectively.
This process of adding donor or acceptor atoms to
semiconductor atoms (the order of 1 impurity atom per 10 million (or
more) atoms of the semiconductor) is called Doping. As the
doped silicon is no longer pure, these donor and acceptor atoms are
collectively referred to as “impurities”.
The most commonly used semiconductor basics material by far is
silicon.
Silicon has four valence electrons in its outermost shell which it
shares with its neighbouring silicon atoms to form full orbital’s of
eight electrons. The structure of the bond between the two silicon atoms
is such that each atom shares one electron with its neighbour making
the bond very stable.
As there are very few free electrons available to move
around the silicon crystal, crystals of pure silicon (or germanium) are
therefore good insulators, or at the very least very high value
resistors.
Silicon atoms are arranged in a definite symmetrical pattern
making them a crystalline solid structure. A crystal of pure silica
(silicon dioxide or glass) is generally said to be an intrinsic crystal
(it has no impurities) and therefore has no free electrons.
But simply connecting a silicon crystal to a battery supply is not
enough to extract an electric current from it. To do that we need to
create a “positive” and a “negative” pole within the silicon allowing
electrons and therefore electric current to flow out of the silicon.
These poles are created by doping the silicon with certain impurities.
A Silicon Atom Structure
The diagram above shows the structure and lattice of a ‘normal’ pure crystal of Silicon.
N-type Semiconductor Basics
In order for our silicon crystal to conduct electricity, we
need to introduce an impurity atom such as Arsenic, Antimony or
Phosphorus into the crystalline structure making it extrinsic
(impurities are added). These atoms have five outer electrons in their
outermost orbital to share with neighbouring atoms and are commonly
called “Pentavalent” impurities.
This allows four out of the five orbital electrons to bond with its
neighbouring silicon atoms leaving one “free electron” to become mobile
when an electrical voltage is applied (electron flow).
As each impurity
atom “donates” one electron, pentavalent atoms are generally known as
“donors”.
Antimony (symbol Sb) as well as
Phosphorus (symbol P), are
frequently used as a pentavalent additive to silicon. Antimony has 51
electrons arranged in five shells around its nucleus with the outermost
orbital having five electrons.
The resulting semiconductor basics
material has an excess of current-carrying electrons, each with a
negative charge, and is therefore referred to as an N-type material with the electrons called “Majority Carriers” while the resulting holes are called “Minority Carriers”.
When stimulated by an external power source, the electrons
freed from the silicon atoms by this stimulation are quickly replaced by
the free electrons available from the doped Antimony atoms. But this
action still leaves an extra electron (the freed electron) floating
around the doped crystal making it negatively charged.
Then a semiconductor material is classed as N-type when its
donor density is greater than its acceptor density, in other words, it
has more electrons than holes thereby creating a negative pole as shown.
Antimony Atom and Doping
The diagram above shows the structure and lattice of the donor impurity atom Antimony.
P-Type Semiconductor Basics
If we go the other way, and introduce a “Trivalent”
(3-electron) impurity into the crystalline structure, such as Aluminium,
Boron or Indium, which have only three valence electrons available in
their outermost orbital, the fourth closed bond cannot be formed.
Therefore,
a complete connection is not possible, giving the
semiconductor material an abundance of positively charged carriers known
as holes in the structure of the crystal where electrons are
effectively missing.
As there is now a hole in the silicon crystal, a
neighbouring electron is attracted to it and will try to move into the
hole to fill it. However, the electron filling the hole leaves another
hole behind it as it moves. This in turn attracts another electron which
in turn creates another hole behind it, and so forth giving the
appearance that the holes are moving as a positive charge through the
crystal structure (conventional current flow).
This movement of holes results in a shortage of electrons in
the silicon turning the entire doped crystal into a positive pole. As
each impurity atom generates a hole, trivalent impurities are generally
known as “Acceptors” as they are continually “accepting” extra or free electrons.
Boron (symbol B) is commonly used as a trivalent
additive as it has only five electrons arranged in three shells around
its nucleus with the outermost orbital having only three electrons.
The
doping of Boron atoms causes conduction to consist mainly of positive
charge carriers resulting in a P-type material with the positive holes being called “Majority Carriers” while the free electrons are called “Minority Carriers”.
Then a semiconductor basics material is classed as P-type when its
acceptor density is greater than its donor density. Therefore, a P-type
semiconductor has more holes than electrons.
Boron Atom and Doping
The diagram above shows the structure and lattice of the acceptor impurity atom Boron.
