... 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. ...

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