The Solid State — Chemistry STD 12 Science — Question

For practical use conductivity of a semiconductor is very low conductivity can be increased by adding small impurity, this process is called doping. Doping can be done with an impurity which is electron rich or electron deficient.

1. N -type semiconductors: Silicon or Germanium (group-14) doped with electron rich impurity (group-15 element like P or As) is called n-type semiconductors. Here conductivity is due to the extra electron or delocalized electron.

Explanation for increase in conductivity in n-type semiconductors: When intrinsic semiconductors like Si or Ge are doped with pentavalent elements as P or As, they occupy some of the lattice sites in silicon or germanium crystal. Four out of five electrons are used in formation of four covalent bonds with four neighbouring silicon atoms. The fifth electron is extra and gets delocalised. These delocalised electrons increase the conductivity of doped silicon (or germanium).

2. P -type semiconductors: Silicon or Germanium (group-14) doped with electron deficient impurity (group 13 element like B or Al or Ga) is called p-type semiconductors. Here conductivity is due to positively charged electron holes.

Explanation for increase in conductivity in p-type semiconductors: In this case the doping of intrinsic semiconductors like silicon or germanium with trivalent elements lke B / In / Ga, three out of four electrons in silicon or germanium form bonds with doping impurity ( I.e. B/ In / Ga). The fourth electron remains unbonded. The place where fourth valence electron is missing is called electron hole or electron vacancy. An electron from neighbouring atom can come and fill up the electron hole, but in doing so it would leave an electron hole at its original position. If this happens it would appear that electron hole has moved in a direction opposite to that of the electron that filled it. Under the influence of electric field electrons would move towards the positively charged plate through electron holes, but it would appear as if electrons are positively charged and are moving towards the negatively charged plate.