|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.
The next step of explanation is a totally theoretical one, because in reality a material with two zones - a N-type and a P-type one - is created in practice by doping with from two opposite sides with two different materials.
But in order to understand the effect, it will be imagined here, that both types are ready prepared and then brought together. As soon as they are near to each other the free chargecarriers near to their border will move in direction to each other as shown in fig. 86 j.
Of course will the positive and the negative chargecarriers meeting at the border at once cancel each other.
But the cancellation of chargecarries on each side will leave the opposite chargecarriers belonging to them back. This will lead along the border to a so called SPACE-CHARGE-REGION, which is represented in fig. 86 l (top).
This space-charges prevent now the free charge carriers from both sides to cross the border and to go on cancelling eachother.
At last there will appear a voltage across this border which is depending on the material used. So the intruding of the chargecarriers from one side to the other is called DIFFUSION this voltage is called DIFFUSION POTENTIAL. In technical sense it will be called THRESHOLD VOLTAGE (Ge = 0.2 V/Si = 0.7 V)
As we derived above, the free chargecarriers around the border cancelled eachother. This leaves on the other hand a zone in which we will find no more free chargecarriers a so called DEPLETION ZONE (Depletion means to get poor). From the point of view of conductivity means this:
THIS ZONE HAS NO CONDUCTIVITY AND AN ALMOST INFINITE RESISTANCE.
Up to this point, there was nothing connected to the PN-junction. Now we have to consider, what will happen at this junction if we connect a voltagesource to it.
If we connect the positive pole of the source to the N-type side and the negative pole to the P-type side, we can imagine the positive pole attracting the free electrons of the N-type side, and the negative pole attracting the holes of the P-type side, and so on both sides reducing the number of charge carriers there as shown in fig. 86 n.
This effect extents the depletion layer. Therefore the resistance of the junction is increased and therefore
IN REVERSE DIRECTION THERE WILL FLOW NO CURRENT.
If we connect the negative pole of the source to the N-type side and the positive pole to the P-type side, we can imagine the positive pole repelling the free holes of the P-type side in direction of the border, and the negative pole repelling the electrons of the P-type side in direction of the border.
So the charge carriers are invading the depletion layer it-will get conducting. Therefore the resistance of the junction is tremendously decreased and their depletion layer vanishes. IN FORWARD DIRECTION THERE WILL FLOW CURRENT.
The behaviour of such a junction as explained up to here would look like fig. 89 a. If we look at it from a general point of view we find on the left side (at reverse direction) no current at any voltage (infinite resistance) and on the right side (forward voltage) no voltage necessary for any current (no resistance).
We see: this component behaves like a valve which lets flow current only in one direction. The symbol is shown in fig. 89b. It is called a DIODE and its terminals are called ANODE and CATHODE (these terminals are originated from the valve diode).
But the characteristics shown in fig. 89a is only a theoretical one. We can use this imagination for simple considerations in circuits with diodes.
In reality the characteristics is shown in fig. 89. The difference is:
- in reverse direction exists a voltage-limit.
If we increase the reverse voltage above this limit there will flow suddenly a strong current which will destroy the diode at once. This limitting voltage is called BREAK THROUGH VOLTAGE.
- In Forward direction at first a voltage is needed to get any current flowing, this voltage was mentioned already above, and it is called the: THRESHOLD VOLTAGE.
- Additional the diode still needs some voltage to let flow some forward cur rent which means it has a certain resistance which is called INTERNAL RESISTANCE
There are special diodes which are not destroyed if connected in reverse direction if the voltage is reaching the break down voltage. This type of diodes is called zenerdiode and it is used for stabilization. Its symbol and characteristics is shown in fig. 89d.