|Radio and Electronics (DED Philippinen, 66 p.)|
|9. ACTIVE COMPONENTS -1- DIODES|
As already stated in chapter 7.1.3 active components can be valves or semiconducting type. In this script we will deal only with the modern type: the semiconductor active components. To understand the function of these components it is necessary to have some basical knowledge of semiconducting materials and how they are processed in order to produce the type of active component desired.
9.1. CHARACTERISTICS OF SEMICONDUCTORS
Wellknown Semiconducting materials are: germanium, silicon and selenium. All of them have exactly FOUR VALENCE ELECTRONSTRONS.
To purify those materials they are first molten in order to get rid of any other type of atoms. During cooling them down again, the atoms form compounds in which always two neighbours use two of their electrons together. That means: those two electrons could be found on both atoms. This scheme is represented in fig. 86b. In this figure we realize as well: only within this structure the centre atom has eight valence electrons now, and this means in chemical sense it is saturated.
A whole crystal of these atoms would look like fig. 86.c
At cero degrees Kelvin a piece of this type of material has no free electrons, and therefore no free chargecarriers ® infinite resistance (remember the difference at metals: they have the lowest possible resistance at this temperature).
In order to make further drawings concerning semiconductors more easy to understand we will simplify the structure shown in fig. 86d and show only two dimensions in fig. 86e.
As a crystal-structure this looks now like fig. 86f.
If this material is now heated the whole structure is moving - as hotter as faster! This will cause some of the electrons to loose contact to their related atoms and therefore they are released to move through the material. As there are now free chargecarriers there will arise conductivity now and this is called INTRINSIC CONDUCTION. This effect is represented in fig. 86g.
If there is released an electron anywhere there is not only created a free negative chargecarrier but at the same time there is left back in the atomic structure a positive charge - a so called HOLE. The effect through which the hole and the free electron are created is called GENERATION.
The hole now again will attract any electron in its surrounding. Therefore at the same time when generation takes place another opposite effect happens too: the return of an electron to a hole, and this effect is called RECOMBINATION.
SEMICONDUCTORS HAVE INFINITE RESISTANCE AT 0 DEGREES KELVIN.
WHEN HEATED UP, THERE ARE CREATED MORE AND MORE CHARGECARRIERS WHICH MEANS: THE CONDUCTIVITY OF THE MATERIAL IS INCREASING WITH INCREASING TEMPERATURE.
As you might know already: one of the advantages of semiconducting active components is, that they are not depending on heat (as valves do). So for normal components there must be something done, to get them conducting reasonably at normal temperatures. Therefore now some atoms of another type will be implanted by purpose to the pure semiconductor. The process to do this under controlled conditions is called DOPING. It is done with two types of foreign atoms.
THE N-TYPE MATERIAL
Doping with atoms five with valence electrons leaves one of the five atoms of the foreign atom here arsenic free. As this electron is not related to any other part of the structure it is free to move, and it can carry now electricity.
If we look at this doped material from a more general point of view. We can forget about the structure of normal-semiconducting-atoms and only see it as represented in fig. 86h as a material with:
POSITIVE CHARGES which are FIXED here within the atomic structure, and NEGATIVE ELECTRONS which are FREE to move.
THE MATERIAL CREATED BY DOPING WITH ATOMS WITH FIVE VALENCE ELECTRONS IS CALLED:
THE P-TYPE MATERIAL
Doping with atoms with three valence electrons leaves one of the links between the other atoms and the foreign atom (here indium) free.
As always at normal temperatures generation takes place this gap will be filled by such a generated electron. But this leaves back a hole which can move now, in the following way: the next generated electron in the surrounding will leave back another hole and on the other hand fill this hole. In this manner the hole has been moved.
So we have now holes as free chargecarriers.
If we look at this doped material from a more general point of view. We can forget about the structure of normal-semiconducting-atoms and only see it as a material with:
NEGATIVE CHARGES which are FIXED here within the atomic structure, and POSITIVE HOLES which are FREE to move.
THE MATERIAL CREATED BY DOPING WITH ATOMS WITH THREE VALENCE ELECTRONS IS CALLED:
BUT KEEP IN MIND: even after doping the material as a whole is still electrically neutral.