Question

1.Describe the two-step process by which extrinsic semiconductors are created by doping impurities into intrinsic semiconductors.

2.Compare and contrast the two mechanisms of atomic diffusion and comment on which of these occurs more rapidly and why it occurs more rapidly.

Answer 1

1 An intrinsic semiconductor is a pure semiconductor.eg: Silicon (Si) and Germanium (Ge). When we dopped impurities in to the intrinsic semiconductor we can form extrinsic semiconductor. There are two types of extrinsic semiconductor. N type semicoductor and P type semiconductor.

An N type semiconductor is formed by doping pentavalaent impurities such as phosphorous (P), Arsenic (As), Antimony (Sb).The pentavalent impurity atom makes covalent bonds with four silicon atoms and one electron is not bonded with any silicon atom. This cause an increase in electron number. so it is called as an N type semiconductor. Here the number of electrons is greater tha number of holes.

On the other hand p type semiconductor is formed by doping with trvalent impurites such as Galium (Ga) and I ndium (In). when a trivalent impurity added to Si , each atom of the impurity makes three covalent bond with surrounding three Si atom.for the fourth covalent bond there is an electron short. this missing electron is called as holes. this cause an increase in holes. So it is called as P type semiconductor. here the number of holes is greater than number of electrons.

2 The two mechanisms of atomic diffusions are interstitial diffusion and substitutional diffusion.

The interstitial diffusion is occurs when the crystal lattice jumping from one interstital site to the next adjascent site. Here impurity atom does nt replace the Silicon atom, but instead moves into the interstitial voids.

When the temprature increased the atoms in the semiconductor moves out from there lattice site leaving a vaccancy.This impurities occupy the lattice position in the crystal after it is cooled.That is for a substitutional diffusion the Silicon atom is replaced by the impurity atom.

interstitial diffusion is generally faster than substitutional diffusion. This is because bonding of interstitials to the surrounding atoms is normally weaker and there are many more interstitial sites than vacancy sites to jump to.