Atomic number of silicon is 14. So the electronic configuration is 2,8,4. So there are 4 electrons in outermost orbit. So to complete the octet, it has to get 4 electrons caus
…ing much electro negativity or to give 4 electrons causing much electro positivity. So it is reluctant to give electrons unless larger force is applied. So for the same reason carbon and germanium having 4 electrons in outer orbit are semiconductors of electricity. So when we add trivalent impurity (like aluminium ) to silicon it promote electrical conductivity and if we add pentavalent impurity ( like phosphorus) we get flow of electricity from trivalent plus silicon to pentavalent plus silicon. This is the basic principle used in electronics. (MORE) 
