Question

to obtain n type semiconductor we dope

Answer 1

When intrinsic semiconductor  is doped with trivalent impurity  it will become n type  semiconductor

Answer 2

  the N-type impurity loses its extra valence electron easily when added to a semiconductor material,and in so doing, increases the conductivity of the material by contributing a free electron. This type ofimpurity has 5 valence electrons and is called a PENTAVALENT impurity. Arsenic, antimony, bismuth,and phosphorous are pentavalent impurities. Because these materials give or donate one electron to thedoped material, they are also called DONOR impurities.When a pentavalent (donor) impurity, like arsenic, is added to germanium, it will form covalentbonds with the germanium atoms. Figure 1-10 illustrates this by showing an arsenic atom (AS) in agermanium (GE) lattice structure. Notice the arsenic atom in the center of the lattice. It has 5 valenceelectrons in its outer shell but uses only 4 of them to form covalent bonds with the germanium atoms,leaving 1 electron relatively free in the crystal structure. Pure germanium may be converted into anN-type semiconductor by "doping" it with any donor impurity having 5 valence electrons in its outershell. Since this type of semiconductor (N-type) has a surplus of electrons, the electrons are consideredMAJORITY carriers, while the holes, being few in number, are the MINORITY carriers.