I agree with Shankar Ekkanath Madathil

Hello Sanjay,

I think the easiest way to answer your question is as follows. TO (Tin Oxide) has a modest to large band gap as you mentioned, and you are right to suppose it would conduct electricity poorly if at all. However, doping with impurities can insert donor or acceptor atoms either just below the conduction band (donor atoms) or just above the valence band (acceptor atoms). If these impurity atoms are ionized, by thermal agitation, they can add enough electrons to the conduction band (from the ionized donor atoms, which lose electrons) or holes to the valence band (from the ionized acceptor atoms, which gain electrons from the valence band) to significantly increase the electrical conductivity of the tin oxide; defects - non-stoichiometric amounts of the oxide - can also significantly change the electrical conductivity, for example, copper(I) oxide (cuprous oxide, Cu2O) is such a defect semiconductor: Cu1.9O is non-stoichiometric. For a very helpful diagram showing the location of different types of donor and acceptor atoms in the band gap of silicon and gallium arsenide, see p. 23 of S.M. Sze's book titled "Semiconductor Devices, Physics and Technology"; John Wiley & Sons; 1985. Of course, adding the impurities does not really change the band gap, which is a good thing since a 3.3 eV band gap means that the tin oxide will not absorb infrared and visible light, which means it can be used as a transparent, conductive layer or electrode if it is properly doped: it is transparent due to the band gap and the thinness of the sample, and conductive due to the doping.

Note, the rule of thumb that materials with large band gaps are usually insulators is usually true but not always. For example bromine, which along with mercury is a liquid at room temperature, is an insulator, but under high pressure it assumes a metallic luster and conducts electricity. And even the common metals, which one would assume conduct via delocalized electrons (metallic bonding), do not always fit this assumed model. Many common metals, for example, have a Hall effect coefficient that implies that the charge carriers are holes instead of electrons. Electrical conductivity, like other material properties, can surprise us.

Anyway, I liked your question.

Regards,

Tom Cuff

Dear Thomas,

Thank you for the proper justification along with good examples.

In short, doping (impurity) provides a conduction level for e- and h+ at below the conduction band and above the valence band. therefore, FTO shows very good conductivity.

Thank you once again...