p: 12pt;" CLASS="Normal">1897 marks the discovery
of the electron by J.J. Thompson, a British Physicist. Thompson was
able to explain the so-called Edison effect: In 1883, four years after
Thomas Edison built the first light bulb (and power generator to provide
the electricity!),  he made the following observation:

metal plate

A bluish flicker arises
between the filament and the plate provided the plate has a positive
charge (relative to the filament)

evacuated glass bulb

heated filament

Thompson
showed that electric current is the movement of electrically charged
objects and that there is a single smallest electrical charge, which
we nowadays call the charge of the electron (1.6*10-16
Coulomb). Hence, the Edison effect was evidence of a flow of electrons
from the filament to the plate.

The heating of the filament
energizes the electrons and allows some of them to escape into the vacuum.
If the plate is positive, it attracts the escaped ones; otherwise they
eventually return into the filament.

J.A. Fleming, a British
engineer,  built on this discovery to construct the first diode,
or Fleming tube, patented in 1904.

Diode

vacuum
tube

anode

cathode
filament

filament = cathode = electron
emittor

plate (now in the shape of a cylinder) = anode =
electron collector

The diode acts as a valve
for electric current: electrons can only flow from the cathode to the
anode.

Since radio waves cause
alternating current, and since a diode is a one-way valve, diodes can
be used to detect radio waves.

In 1907, Lee De Forest,
an American engineer, built the first triode:

Triode

plate
: collector = anode

grid
: gate

filament:
emittor = cathode

The electrons flow from
the cathode to the anode if the gate has a positive voltage; they cannot
flow if the gate has a negative voltage.

The most important function
of early triodes was amplification: a small change in gate voltage produces
a large change in electron flow that can be used to produce a large
voltage.

Early applications of triodes:

radio, TV, computers

Modern application of triodes:

X-ray, microwave, photoelectric tubes (light sensors),
electron-gun tubes,

CRT (cathode ray tubes) for TV, Computer monitors
etc.

The culmination of triodes
in electronics was the first computer, ENIAC, built 1946 out of 17,468
triodes. (Electronic Numerical Integrator and Calculator).  The
machine was built by the University of Pennsylvania for the US Army
to compute trajectories.

2. Transistors

In 1947, three scientists
(Bardeen, Brattain, Shockley), working at AT&T Bell Laboratories,
discovered the transistor. A transistor is a device that operates with
the same effect as a triode, but without the complications of manufacturing
and that essentially does not wear out (no moving parts, no heat).

The discovery was made
in the process of trying to understand the electrical properties of
a class of materials known as semiconductors. Basically, all materials
fall into two categories: those that do and those that dont conduct
electricity. Semiconductors are simply extremely poor conductors (they
conduct electricity about 100,000 times poorer than metals, but that
number varied widely). The question was what is the underlying mechanism.
The semiconductors are: Ge (Germanium), Si (Silicon), Sn (Tin). Plus
once could theoretically also include C (Carbon, in its diamond form).
All these are elements that are in the fourth column of the periodic
table, which means that each element has 4 electrons available for chemistry
(bonds). In the solid form they all crystallize in the same way: space
centric tetrahedral: each atom as four neighbors at equal distances
with equal angles in between.

triangular
pyramid with 4 equal sides

one
atom in each corner

those
are the 4 neighbors of an atom

at
the center of the pyramid

If
the material is perfectly pure and the crystal structure perfect (a
perfect diamond) then all electrons are occupied in chemical bonds and
the material does not conduct electricity. However, if there are impurities
and or imperfections, then some electrons can float around, giving
rise to minimal conductivity.

The genius of the discovery
was that by clever introduction of just the right kind and amount of
impurities, unusual conductivity effects could be achieved.

Electronic diode:

metal
contact from wire to diode

n-type
material   silicon doped with Boron  

electron flow      n-p-junction

p-type
material  silicon doped with Phosphorus

Electrons
are largely unable to cross an n-p-junction from p to n, but can flow
fairly freely from n to p.

Boron
has 5 electrons in its outer shell, so when a boron atom is built into
a pure silicon crystal, one of its electron is free to roam. Since electrons
have a negative charge, such material is called n-type.

Phosphorus has 3 electrons
in its outer shell. The effect of a lack of electron, also called a
hole, as the absence of a negative charge, which amounts to a positive
charge, hence silicon doped with phosphorus is called p-type.

The effect of the electronic
diode hinges only on the n-p junction. Therefore, such devices could
be theoretically made very small and thin (just a few atoms thick would
suffice). Herein lies the main significance of the discovery.

Transistor: an electronic
triode.

   
base  emittor    collector

layer
of insulation

p       
n

silicon

n

The
above is a schematic for an early generation n-p-n transistor. Electrons
can flow from the emittor into the ultra thin layer of p material.

The operation of the above
device is by no means obvious. Suffice it to say that by cleverly applying
various voltages, it is possible to achieve a situation where a small
electric current base-emittor causes a much larger current base-collector.
Thus the base acts to control the effect of the collector current.

Over the years since 1947,
many other arrangements of doped material have been found with interesting
effects that could be exploited to build triodes and other electronic
devices. Each one of these discoveries led to a new technology, or
a new era in the history of electronics.

Assignment: find out what the abbreviations MOSFET and CMOS
stand for, and give a reference to a description of these technologies.

The key to the success
of transistors lies in its miniaturization potential: it became possible
to place many logical circuits into a small area.

3. Logic Gates

Internally most electronic
devices run on 5 volt. By definition, a voltage at or near 5 is called
high, and a voltage at or near 0 is called low. With this rule,
any part of a circuit can be in one of two states: high or low, which
could also be thought of as on or off. We could equally well designate
the two possible states as TRUE or FALSE, hence the use of the word
logic gates. Computer engineers prefer the designations of 1 or
0.