3.Frenkel Defects Often a vacancy and interstitial occur together an ion is displaces from its site into an interstitial position. This is a Frenkel Defect (common in e.g.AgCl)and charge balance is maintained Frenkel defects can be induced by irradiation of a sample
3. Frenkel Defects Often a vacancy and interstitial occur together - an ion is displaces from its site into an interstitial position. This is a Frenkel Defect (common in e.g. AgCl) and charge balance is maintained. Frenkel defects can be induced by irradiation of a sample
4.Impurities Preparing pure crystals is extremely difficult often foreign atoms enter the structure and substitute for “native”atoms-often by contamination from container This can have a large effect (either detrimental or beneficial)on the properties of the crystal.We can also add impurities (or dopants)deliberately. An important example is that of silicon
4. Impurities Preparing pure crystals is extremely difficult - often foreign atoms enter the structure and substitute for “native” atoms - often by contamination from container This can have a large effect (either detrimental or beneficial) on the properties of the crystal. We can also add impurities (or dopants) deliberately. An important example is that of silicon
Silicon Silicon is a group IV element and,like carbon,bonds to four nearest neighbours: Si Si Si Si Si Si Si Si Si Si Si Si 1 Si Si Si Si Si Si At elevated temperatures bonds are broken to produce a (positive)gap-known as a hole-and a conduction electron. This is known as the intrinsic effect in semiconductors
Silicon is a group IV element and, like carbon, bonds to four nearest neighbours: At elevated temperatures bonds are broken to produce a (positive) gap - known as a hole - and a conduction electron. T This is known as the intrinsic effect in semiconductors
Doped Silicon If we take a group V element (e.g.As)and substitute (at low levels)for Si there is a spare electron for conduction and no positive hole: Si Si Si This process is known as “doping”.As acts as an electron donor to Si,making it Si As Si easier to conduct electricity. Si doped with As is an extrinsic semiconductor and because Si Si Si the electron is negative this is an n-type semiconductor
If we take a group V element (e.g. As) and substitute (at low levels) for Si there is a spare electron for conduction and no positive hole: This process is known as “doping”. As acts as an electron donor to Si, making it easier to conduct electricity. Si doped with As is an extrinsic semiconductor and because the electron is negative this is an n-type semiconductor
Doped Silicon If we take a group Ill element (e.g.B)and substitute (at low levels)for Si there is a positive hole and no conduction electron Boron acts as an electron acceptor to Si. Si Si Si Electrons can move by diffusion-“hopping”into the Si B Si hole leaves behind a new hole. Again this is an extrinsic Si Si Si semiconductor and because the hole is negative this is a p-type semiconductor
If we take a group III element (e.g. B) and substitute (at low levels) for Si there is a positive hole and no conduction electron Boron acts as an electron acceptor to Si. Electrons can move by diffusion - “hopping” into the hole leaves behind a new hole. Again this is an extrinsic semiconductor and because the hole is negative this is a p-type semiconductor