|Through the researches
in the new field of solid state Physics scientists discovered that
the atomic structure of certain materials like Germanium and Silicon,
contains few free electrons that could scape leaving holes of vacancies
of charges in the crystal lattice.
In this way, the so called boundary
layer of P-N junction shaped allows current to flow in just one direction
upon application of an alternating voltage between the two contacts
points on the crystal.
|Principle of the semiconductor diode where:
a) Crystal structure
b) P-N junction
c) Curve showing operational functions
|Fig.249 - Cross section of a point contact
Basically, this kind of man-made diode consists in handling the Germanium
atomic structure by adding into its crystal lattice small quantities
of impurities known as DOPE.
Those impurities are atoms of several chemical elements, generally
metals comprising their own electronic configuration. For instance,
Phosphor, Arsenium and Antimony have 5 electrons in their outer orbit
while Aluminium, Indium and Galium or Boron only 3.
Those atoms are called DONORS and ACCEPTORS
respectively due to their capacity o donating or receiving electrons.
|Fig. 250 - The first thermionic diode invented
by Fleming in 1904.
When adding those impurities into the Germanium crystal lattice, two
kinds of crystalline structure are formed: amixture containing surplus
of free electrons, also called NEGATIVE GERMANIUM; another one with
shortage of electrons known as POSITIVE GERMANIUM.
By joinning together a P and
N Germanium, an electrical charge transfer occurs at the boundary
layer of the P-N junction. Thus, P and N crystals have a surplus and
shortage of electrons respectively. Those charges repel each other
preventing further diffusion of electons and holes in the
crystal; therefore a potential difference
is generated at the same time. In reallity this type of crystalline
arrangement operates as a rectifier, i.e. when a positive voltage
refereing to P crystal is applied to N crystal, the voltge is increased
without any flow of current. Therefore, if a negative voltage is applied
to the N crystal, it is neutralized providing a surge of small voltage
in such away that it starts the electrons and holes movements all
over again. Fig 248
|Fig. 251- Pictorial evolution of the crystal
a) Lead Sulphide or Galena
b) Carborundum detector
c) Early type of Germanium diode
In principle the first point contact diode was almost identical to
that earliest types of crystal detectors. Basically it comprises a
pellet of Germanium with a flat polished surface was where it was
soldered soldered to one of the connecting wires; the other wire is
provided with a sharply point springy Tungsten end, similar to the
cat'whisker principle invented by Pickard. Fig 249
The Germanium diode had several advantages when compared with the
diode valve such as: no heater, low cost, small volume and it could
operate either as a detection as well as a switching device. In the
beguinning its main application was in the manufacturing of televisers
and computers. Fig 250
As aforementined, the point-contact diodes were an improved version
of the original crystal detector. Fig 251
In this way, soon they were replaced by whiskerless diodes whose operational
principle was related with the uniform thickness configuration of
the junctions in the semi-conductor material.