21. Semiconductors - explanation according to band theory of solids, band-gap value, doping - donors and acceptors, n-type and p-type semiconductors. Fermi level in semiconductors. (3p.)

Question 1

Semiconductors - explanation according to band theory of solids

Answer 1

From the band theory of solids, we see that semiconductors have a band gap between the valence and conduction bands. The band structure of a semiconductor is similar to that of an insulator, but the energy gap is much narrower. The size of the band gap has implications for the types of applications that can be made. Its value is for example only 0.7eV in Ge, 1.1 eV in Si and 1.4 eV in GaAs.

Question 2

Semiconductors - doping - donors and acceptors

Answer 2

Doping is the process of adding selected impurities to semiconductors in order to increase the conductivity.

Donor atom – the atom that contributes the electron.

Acceptor atom – the atom that accepts the electron from other sites.

Question 3

Semiconductors - n-type and p-type semiconductors. Fermi level in semiconductors.

Answer 3

Donor impurities donate negatively charged electrons to the lattice, so a semiconductor that has been doped with a donor is called an n-type semiconductor; “n” stands for negative. Free electrons outnumber holes in an ntype material, so the electrons are the majority carriers and holes are the minority carriers.

Because an acceptor donates excess holes, which are considered to be positively charged, a semiconductor that has been doped with an acceptor is called a ptype semiconductor; “p” stands for positive. Notice that the material as a whole remains electrically neutral. In a p-type semiconductor, current is largely carried by the holes, which outnumber the free electrons. In this case, the holes are the majority carriers, while the electrons are the minority carriers.