Electron-deficit impurities

 Electron-deficit impurities

Silicon or germanium can also be doped with a group 13 element like B. Al or Ga which contains only three valence electrons.

 The place where the fourth valence electron is missing is called electron hole or electron vacancy (Fig.). 

An electron from a neighboring atom can come and fill the electron hole, but in doing so it would leave an electron hole at its original position. 

If it happens. 

It would appear as if the electron hole has moved in the 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 electronic holes, but it would appear as if electron holes are positively charged and are moving towards negatively charged plate.

 This type of semi conductors are called p-type semiconductors.