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N-Type Material
By doping the silicon or germanium with arsenic or antimony, atoms which have 5 valance electrons in their outer shell are introduced into the lattice structure. The ratio of impurity atoms to original atoms (doping ratio) is in the order of 1:108.
Figure 16.3 N-type material
Four of the five electrons form covalent bonds with the surrounding atoms, the 5th electron, having no such ties, becomes a free electron. Conductivity through the material is thus increased.
We call this type of material N-type because of the surfeit of free electrons which are, of course, negatively charged. However, it should be noted that the material remains electrically neutral; for each free electron there is a fixed positive ion within the material.
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P-Type Material
By doping with impurities such as aluminium or indium, again in the same doping ratio as above, atoms with only three valence electrons in their outer shell are introduced.
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Figure 16.4 P-type material
This time there are only 3 electrons to form the covalent bonds, one is missing. In other words there is a hole in the valent structure.
Electrons from adjacent atoms tend to move into these holes thus creating holes around the donor atoms which in turn ‘steal’ electrons from their neighbours moving the hole on further.
This apparent movement of holes increases the conductivity of the material.
Because of the shortage of electrons the material is classified as P-type.
Again, it should be noted, it possesses no electrical charge, there being an equal number of holes and fixed negative ions.
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Current Flow
Applying an EMF across a piece of N-type material would cause the free electrons to migrate towards the positive terminal.
Any electrons leaving the material at the positive terminal are replaced by electrons entering at the negative terminal, thus the overall balance between free electrons and fixed positive ions is maintained.
In P-type material the situation is more complex, but in general, electrons are attracted into the positive terminal creating holes in this region.
The holes ‘migrate’ towards the negative terminal and are ultimately filled by an electron entering at that point.
Hence in P-type semiconductor material we can consider current flow as the drift of holes in the conventional direction, namely from the positive to the negative terminal. Again, overall balance is maintained between electrons and fixed negative ions.
The P-N Junction
If we fuse two small pieces of N and P-type materials together, by a process similar to welding, some free electrons from the N-type material migrate across the boundary into the P-type and similarly, holes migrate the other way.
This migration produces a charged region known as the Depletion Layer and creates a Barrier Potential restricting further electron/hole movement. This barrier potential may be represented as an imaginary battery, though it should be remembered, the regions of increased positive and negative charge exist only across the junction. The material as a whole possesses no electrical charge.
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Figure 16.5 The P-N junction
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Reverse Bias
If we now connect an external EMF across the P-N material, as shown in Figure 16.6 A, more electrons are drawn across the barrier into the P-type material and more holes are drawn to the N-type.
This deepens the depletion layer and further electron/hole migration is prevented. Apart from a small leak current, in the order of µA, no significant current flows. The junction is said to be reverse biased.
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Figure 16.6 Reverse and forward bias
Forward Bias
By applying the external EMF, as shown in Figure 16.6 B, the direction of the electric field is such as to produce a drift of holes in the P-type material to the right, and of free electrons in the N-type to the left.
In the junction region, free electrons and holes combine, thus the barrier potential is overcome.
The Junction Diode
It can be seen from Figure 16.7 that current can only flow in one direction through a semiconductor formed from P-N type material.
In other words, the material acts as a rectifier and has similar conduction characteristics to a thermionic diode (valve).
It is therefore referred to as a Junction Diode.
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