As the above sketch shows, at 0K all electrons are bonded to Si atoms and are not free to carry current. At any temperature above 0K, valence electrons start to break bonds due to the thermal energy they receive. As the valence electron becomes free, a positive hole is also created which can carry current as well as the free electron – under the action of an applied voltage, electrons will drift towards the positive side, and holes will drift towards the negative side – but the CONVENTIONAL current will still be in the same direction, i.e. positive to negative.
As the temperature rises more electron/hole pairs are created. However, the number liberated is not sufficient to be of use to us. Thus, we need another way of making free charge carriers available – this we do by DOPING the silicon with another element from a different group of the Periodic Table.
If we dope or substitute some of the Si atoms in the above structure with atoms from group V, say phosphorous, P, 4 of the P valence atoms will be used in the bonding process, leaving one ‘spare’ that is free and can carry current. We have now created an n-type extrinsic semiconductor (n for negative):
If we add 106 P atoms per unit volume (i.e. for every 1m3 of Si), we significantly improve the conduction properties of the silicon. In fact, heavily doped Si can approach metal-like properties. The same is true for doping Si with atoms from group III. In this case, we create a Si structure which has many free holes, and this also improves the conduction properties of the Si.
Doped Silicon in Use
Doped silicon has many uses, least of all in the production of bipolar and field effect transistors which are the heart of microprocessors. However, to give an example of the use of doped silicon, we shall look at the production of the humble p-n junction diode, the heart of the transistor!!
In essence, this is 2 regions of doped silicon (n-type and p-type) which are chemically combined, for example, by diffusion at an elevated temperature:
Upon combination, a small region of the n-type Si loses electrons by diffusion to the p-type where they become minority carriers and combine with charged particles. This leaves a small part of the Si at the junction completely depleted of mobile charge carriers. A similar process occurs for the p-type Si (and holes). The end result is a depleted junction region with a potential distribution as shown above (this distribution is caused by the ionised donors/acceptors which are left behind when the mobile charges in the junction region diffuse to the other side).
If the n-side is made positive and the p-side negative, the potential distribution across the junction increases, effectively stopping any charge from crossing one side to the other of the junction.
If the n-side is made negative and the p-side positive, then this voltage reduces the barrier potential. Thus, electrons on the n-side can DIFFUSE to the p-side. Also, holes on the p-side can diffuse to the n-side. The net result of this diffusion is a current from positive to negative.
Thus, the diode allows current to flow one way only. This is entirely governed by the p-n junction formation.
